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NAME _______________________________ NOTES: UNIT 1 (4): ENERGY & CHEMICAL REACTIONS VS. PHYSICAL CHANGES I)-III) Matter – Sig Figs IV) Energy: * The ability to create a change or to do work on Earth, it is described as: Internal Energy (sum of P.E. and K.E.) can be divided The kinetic energy of chemicals is expressed in the temperature, and motion of the chemicals into 2 really big categories Potential Energy Kinetic Energy can be converted to defined as defined as energy of position of between species relative to an assumed standard energy of motion (possessed by an object in motion) examples of kinetic energy examples of potential energy Chemical Bond Energy P.E. is associated with the chemical energy of bonds. Electromagnetic Spectrum [Light, Thermal, Microwave]; &then there is/are: Sound energy, motion, Between bonded species, there is a bond length or distance and thus the energy possessed by a chemical is associated closely with potential energy … in this case, it’s called, chemical energy Now … Let’s break this down … I want to attack the confusion that surrounds key terms like: work, kinetic energy, potential energy, heat, and temperature. A) Work: W = (force)(distance) or W = (mass)(acceleration) x (distance) B) Introduction: The Electromagnetic Spectrum …a range/listing of kinetic energies associated with the movement of electrons. We will see this again, in our work with the atom … but I wish to introduce it here. 1) Most people do not realize that UV radiation, microwaves, radio waves and visible light (good ol’ ROYGBV…notice I left out the “I” for indigo) …. are actually variations on the same theme. Each is related to the other in that each is a transverse wave which originates, essentially produced from the movement of electrons, and represents the energy transitions of those e-. 106 Spectrum of Electromagnetic Energy NYS Physics Reference Tables The wavelength of visible light energy is quite close to the differences in energy experienced by the electrons of bio-chemicals. Violet: Indigo: Blue: Green: Yellow: Orange: Red: 400 - 420 nm Shortest wavelength of visible light (thus, highest frequency) 420 - 440 nm 440 - 490 nm 490 - 570 nm 570 - 585 nm 585 - 620 nm 620 - 780 nm Longest wavelength of visible light (thus, smallest frequency) 2) There are electromagnetic waves and mechanical waves. But, first understand, all waves transfer energy, not matter. Electromagnetic waves can travel through a vacuum … Mechanical waves require a medium (a system of interacting particles) Mechanical waves may be longitudinal (so called because they are propagated in parallel with the disturbance e.g. p-waves & sound waves) and transverse waves. “When a wave is present in a medium (that is, when there is a disturbance moving through a medium), the individual particles of the medium are only temporarily displaced from their rest position. There is always a force acting upon the particles that restores them to their original position.” http://www.physicsclassroom.com/Class/waves/u10l1b.cfm a) Sound Waves are mechanical and are longitudinal waves. http://www.physicsclassroom.com/class/waves/u10l1c.cfm 107 Check out: http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html for an animation comparing mechanical longitudinal and transverse waves 3) We will concern ourselves with the transverse waves, of electromagnetic energy. These do not require a medium … hence, light, microwaves, radio waves are each transverse (note, sound waves and radio waves are NOT the same) http://cse.ssl.berkeley.edu/lessons/indiv/nellie/em_wave.html Take Home Message: All light waves carry energy. The number of waves per second (Hertz) is the frequency (related to radio stations…). A higher frequency is related to a greater the amount of energy per second, transferred. The electromagnetic transverse wave exhibits two types of fields … http://www.one-school.net/Malaysia/UniversityandCollege/SPM/revisioncard/physics/wave/electromagnetic.html 108 Take Home Message: As an electron moves through “space”, it creates a magnetic field, which will become important in chemistry … 4) Photonic Radiation refers to energy which is emitted and/or travels in the form of waves &/or particles called photons Please understand that light is a form of radiation. Most people think of radiation as nuclear radiation only. For the most part, the only type of electromagnetic radiation that dovetails with nuclear radiation is limited to gamma radiation. …. Okay, now let’s begin to connect this up … with a conversation on… C) POtential Energy = Energy of * POsition POtential Energy is essentially due to a POsition of objects: It is the energy (ability to do work) possessed by an object, due to its position relative to some standard (assumed) second position. (MR #2) Vs. solid phase (or distance between objects, if you wish...) gas phase This is intimately linked to changes in chemical bond energies, phase changes, the production of light, and intermolecular forces of attraction(s) This applies to bond theory, because as a bond is made, the POsition between the atoms, that will bond, must decrease (they must get closer to each other). This “getting closer” must occur, if the positive nuclear forces of one atom are to attract the electron(s) of the other atom. The LCME gives us the understanding that as this POsition (POtential) decreases, the energy must go somewhere. Often it is converted into a form of kinetic energy & is released from the chemicals to the environment. Try a bow and arrow as a metaphor…some neat connections are about to be made! http://clipart.usscouts.org/library/BSA_Cub_Scouts/Cartoons 109 Vs. gravitational potential energy = (m)(gconstant)(height) GIMME ANother METAPHOR!!!! How about trying to connect this major reason with .......NASCAR! http://rubbingsracing.com/rubbingsracingWP/2010/12/15/harraka-prepares-to-head-west-for-showdown/ http://www.marctimesracingnews.com/articles-archive/series-news/1093-nascar-reports/639-nascar-and-its-fans 110 D) Often I encourage students to approach chemical bond energy along the line of POsition (as in, there is a positional distance between species, which changes as atoms bond, or break away from each other). And while this concept does not work at every point, regarding bonding …it works pretty darn well … For instance, as expressed by Wikipedia & Dr. Rod Nave at Georgia State’s Hyperphysics site: Chemical potential energy is a form of potential energy related to the structural arrangement of atoms or molecules. This arrangement may be the result of chemical bonds within a molecule or otherwise. http://en.wikipedia.org/wiki/Chemical_energy In the chemical bonds of a molecule the attractive electrical forces cause bound states to exist. That is, the atoms of the molecule cannot escape the molecule (break away from each other) without a supply of external energy. Bound states imply a negative potential energy (a loss of energy) compared to the free atoms, so any chemical bond has associated with it a negative (loss of) potential energy. http://hyperphysics.phy-astr.gsu.edu/hbase/molecule/boneng.html Take Home Message: Making New Bonds releases energy (it is an exothermic process) … As two free atoms approach each other, position between them clearly decreases, which is linked to a loss in potential energy … This decrease is “lost” by the reacting chemicals, to the surrounding environment often, by a conversion of the net change in potential energy to a form of electromagnetic radiation, such as thermal energy or light Identify two key ideas from the above passage, which surprise you / inform you / or cause you to question. _______________________________________________________________________________ _______________________________________________________________________________ __________________________________________________________________________________________________________ __________________________________________________________________________________________________________ E) Review: Law of the Conservation of Matter and Energy: Matter and Energy can NOT be created nor (MR #1 links to MR #2) destroyed by ordinary chemical means …BUT, energy can be converted into various forms of energy and/or energy can be transferred. 1) As the potential energy between objects decreases, * it cannot just disappear potential energy is often converted into some form of kinetic energy (light, thermal, etc…) And this brings up the nature of a flame… From where, exactly does the energy come? 111 F) Kinetic Energy: The ability to create a change or to do work, due to motion (Energy of motion) 1) Generally, the examples of energy with which we are familiar are forms of K.E (each due to the "motion" or change in position of particles … light, radio waves, thermal energy… ) 2) When particles collide the K.E. can be transferred (as in billiards…) 3) Unit for Energy: joules (J) and kilojoules (kJ) http://commons.wikimedia.org/wiki/File:Billard.JPG a) Conversion factor: 1,000 Joules = 1 kJ Convert: 250.9 kJ = * 250,900 Joules Convert: 309.4 J = kJ * 0.3094 4) Heat: (Latin: calor) Heat is effectively the physical equivalent of work. James Prescott Joule demonstrated that mechanical work, electrical work, and chemical work each produces a transfer of energy (heat). In Joule’s words: “The amount of heat (the transfer of energy) produced by friction is proportional to the work done and independent (regardless) of the nature of the rubbing surfaces”(The Extraordinary Chemistry of Ordinary Things Snyder 4th ed. p 179) & http://en.wikipedia.org/wiki/Infrared What most of us don’t get readily is that we need to deal with the Second Law of Thermodynamics, which has 2 major ideas wrapped up in it. a) Idea 1: It is impossible to extract all of the energy of a system for work. Some is irrevocably transferred from the chemicals as heat. That is, when energy is transferred, there will be less energy available at the end of the transfer process than at the beginning. translation: There is no such thing as 100% conversion of chemical energy into useable energy (work). implication: We can’t win… In every reaction energy is conserved (First Law of Thermodynamics) … but some of that energy is heat. As we burn fossil fuels, some of that converted chemical energy is heat, and essentially transferred to a different system and lost to us as “useable” work energy … This is because: b) Idea 2: Energy flows away from its source, naturally, thus, energy cannot flow from a colder body to a hotter body, without the addition of even more energy. translation: The universe is constantly losing usable energy and never gaining. We can force the reverse of the natural flow of energy …but that means we must use up even more energy to do it. implication: A working refrigerator won’t keep working, unless plugged in! Energy won’t flow from cold to warm, spontaneously c) Conclusion: We age. 112 Thus, heat is the sibling of work, and heat is often referred to as a form of energy (but “heat” per se, is not on the electrochemical spectrum [infrared energy is … but not, heat] Heat really is equivalent to the amount of energy that is transferred between the reacting chemicals and surroundings. This reinforces the idea that heat is the transfer of an amount of energy between two areas of different energy content. The transfer is from HIGH to LOW (source to sink). I think the confusion evolves, because in chemistry we are always measuring the heat transfer and calling it energy (as in exothermic & endothermic reactions). Essentially we do that, because the measurements are made so that the “work” portion is limited to virtually zero …encouraging all released energy to be found in the transfer (heat). Chemists limit the work produced during a chemical reaction, by measuring the progression of the reaction in rigid, non-flexible containers (a calorimeter). This limits the work by limiting the movement of the container atoms, over a distance. [Think about when I pushed against the teacher’s station and it didn’t move, a rigid system] So, in our measurements (calorimetry) we tend to refer to the energy release only as heat … (which represents the change in internal energy) …and this is at the heart of the confusion. Intrinsically we recognize that “heat” is a transfer (a verb) not a specific type of energy. e.g. We need to "heat up" a cup of coffee. It’s really heating up out here”. And, some experts don’t even like talking about it as a verb. See: http://hyperphysics.phy astr.gsu.edu/hbase/thermo/heat.html …. According to the late Dr. Mark Zemansky: Don't refer to the "heat in a body", or say "this object has twice as much heat as that body". The First Law of Thermodynamics identifies both heat and work as methods of energy transfer which can bring about a change in the internal energy of a system. After that, neither the words work or heat have any usefulness in describing the final state of the system - we can speak only of the internal energy of the system. Okay, be aware, that released energy has 2 parts [work and heat], but we, as chemists tend to focus on the exchange or transfer of energy [the heat, the change in internal energy]. This exchange is often what we list in a chemical reaction (as in exothermic / endothermic), with a unit of Joules (J) or kilojoules (kJ). b) the transfer occurs naturally from areas of * high energy to areas of * relatively lower energy or from source to sink: hot to colder c) And while we’re at it…there is no such thing as cold energy i) So what can you say to someone when they walk into the chemistry lab and ask: “Why is this room so cold?” 113 Now, listen to me very carefully …. There is another term that confuses us & it too is not really a form of energy (it’s not on the EMS!!) 4) Temperature Energy (Temperature is a measurement, not a type of energy) a) When I say, temperature, you think: * measure of average kinetic energy (& vice versa) i) Think about the idea that temperature heat nor internal energy What is/are the unit(s) for measurements of temperature? * K , °C What is the unit for the measurement of energy, in our course? *joules Conclusion? _______________________________________________ b) Which has a higher temperature 1 gram of iron at 200°C or 1,000 grams of iron at 200°C? Which has a greater amount of energy? This is a bit tricky … We commonly mix up terms like: Temperature, Thermal Energy and Heat… "Temperature reflects the average total kinetic energy of particles in matter. Or, we could look at temperature as: “a measure of the ability of a substance, or more generally of any physical system, to transfer heat energy to another physical system. The higher the temperature of an object is, the greater the tendency of that object to transfer heat. The lower the temperature of an object is, the greater the tendency of that object to be on the receiving end of the heat transfer.” http://www.physicsclassroom.com/class/thermalP/u18l1d.cfm Heat is the transfer of thermal energy; it flows from regions of high temperature to regions of low temperature. Thermal energy is stored as kinetic energy in the random modes of translation in monatomic substances, and translations and rotations of polyatomic molecules in gases. Additionally, some thermal energy may be stored as the potential energy associated with higher-energy-modes of vibration, whenever they occur in interatomic bonds in any substance. Translation, rotation, and the two types of energy in vibration (kinetic and potential) represent the degrees of freedom of motion which classically contribute to the heat capacity of a thermodynamic system." http://en.wikipedia.org/wiki/Heat_capacity Think About It: When you feel a "draft" in your house, which starts the process? cold air moving into the house, or warm air moving out of the house? * Most probably it is do to warm air moving out of the house, first, then being replaced by cold air moving into the house. This is most likely due to the rising of warm air, and its lost through a roof, or cooling up against uninsulated walls or windows. 114 TRY THIS! 1) A 50.0 gram block of copper at 10ºC is carefully lowered into 100.0 grams of water at 90.0ºC in an insulated container. Which statement describes the transfer of energy in this system? 6) A person with a body temperature of 37 ºC holds an ice cube with a temperature of -5.0 ºC in a room where the air temperature is 20 ºC. The direction of heat flow is from the: (1) The water loses heat to the block until both are at 10.0ºC. (1) person to the ice, only (2) person to the ice and air and from the air to the ice (2) The block gains heat from the water until both are at 90.0 ºC. (3) ice to the person, only (3) The water loses heat and the block gains heat until both are at the same temperature that is between 10.0 ºC and 90.0 ºC. (4) ice to the person and air, and from the air to the person 7) A cold pack is placed on an injured leg. (4) The water gains heat and the block loses energy until both are at the same temperature which should be between 10.0 ºC & 90.0 ºC. a) Indicate the direction of the flow of energy between the leg and the cold pack ____________ _______________________________________ 2) The average kinetic energy of water molecules is greatest in which of these samples? STOP! Any key (1) 10 g of water at 35ºC (2) 10 g of water at 278 K (3) 100 g of water at 288 K (4) 100 g of water at 45 ºC terms you should interpret??? Energy should apply to the energy exchange that occurs between the cold pack and injured leg (edited June ‘02). 3) Two samples of gold that have different temperatures are placed in contact with one another. Energy will flow spontaneously from a sample of gold at 60°C to a sample of gold that has a temperature of (1) 50°C (2) 60°C b) Describe how the Law of Conservation of 8) Using the concepts of potential energy, explain how a gas becoming a liquid, releases energy. You can write or use (a) diagram(s) (3) 70°C (4) 80°C 4) An iron bar at 325 K is placed in a sample of water The iron bar gains energy from the water when the temperature of the water is (1) 65 K (2) 45 K (3) 65°C (4) 45°C 5) Which term represents some form of energy? (1) heat (2) degree (3) kilojoule (4) temperature 9) As the temperature of a gas increases from 0°C to 10°C at constant pressure, the volume of the gas will (1) decrease by 1/273 (2) decrease by 10/273 selected answers: 1) 3 2) 4 3) 1 4) 3 5) 3 (3) increase by 1/273 (4) increase by 10/273 6) 2 ...... 9) 4 115 How can a poor first year student tell the difference between a chemical reaction and a physical change? V) CHEMICAL REACTIONS (involve breaking AND making new bonds &/or involving changes in the e- clouds) reduce / reduction oxidize / oxidation combust / burn / flammable rust / corrode react / bond ionize / ionized neutralize decompose cook / grill synthesize precipitate flash point polymerize change in oxidation state versus Physical Changes / Properties (May involve breaking bonds, but no new bonds are made Physical changes may occur with chemical reactions) dissolve / stir (so as to make an aqueous solution) mix / mixture melt / fuse vaporize / boil / evaporate sublime or sublimate distill freeze condense filter / separate density (when calculating, comparing samples) measuring / comparing masses measuring / comparing volumes color liquefied, solidified, (change in phase) smell NOT EVERYTHING THAT HAPPENS IS A CHEMICAL REACTION, but many chemical reactions may involve a physical change in the matter. eg. forming an (aq) and a new solid... 2 ways to determine when a chemical reaction has occurred On A Test In Your Life When there’s a re-arrangement of at least 2 of the 5 visual cues element symbols so that new when there is a bold color change (not pre-existing) substances are produced. when a new solid (precipitate) is produced when a new gas is produced (fizzing, bubbles) when a new liquid is produced (hard to tell when this happens) when energy is absorbed or released (found in most interactions) TAKE HOME MESSAGE: Bond Breaking (alone) does NOT mean a chemical reaction has occurred. In reality a chemical reaction occurs only when * new bonds are made (When bonds are just broken ... it is not a chemical reaction … it could imply melting or dissolving in water, for example). A chemical property involves the activity (loss / gain or sharing) of electrons. 116 VI) Reading a reaction equation A) Start at the reaction arrow (), which can be thought of as an equals (=) sign B) Summary of a reaction: coefficient subscript Fe2O3(s) + 2 Al(s) 2 Fe(s) + Al2O3(g) + 4,000 J Reactants Products tells you some sort of change has occurred (equivalent to an = sign) Note: no subscript or no coefficient means "1" the “+” sign is not arithmetic, it means the word, AND 1) Chemical Formula: a series of letter symbols representing the elements that comprise a compound, and the specific ratio between these elements represented by small subscripts a) Subscript: Applies only to the element(s) it “modifies” … inside ( ) next to Na2S Ca3(PO4)2 b) Coefficient: Used to represent the amount of the chemical (formula) … it applies to the entire substance 2) Often, energy (kJ or J) is included on one side or another of the reaction. a) If: * reactant(s) product(s) and kJ i) vocabulary word: *exothermic (energy is released, and is on the product side) ii) Meaning: the chemicals release to the environment more energy due to new bonding, than is gained to break bonds. The environment (water in a beaker or the air of the room) gets hotter. b) If: * kJ + reactant(s) products Students do not always know how to classify a cooling cup of coffee. In terms of exothermic and endothermic ... how would you classify the process? i) vocabulary word: *endothermic (energy is on the reactant side) ii) Meaning: more energy is absorbed by the reacting chemicals to break the bonds, than is released to the environment as new ones are made. * The environment (the water in a beaker of the air) * supplies the energy and thus the environment becomes cooler. Think About This: When trying to classify a process in lab, as exothermic or endothermic, watch how the temperature of the surroundings changes. An exothermic process releases energy, causing the temperature of the surroundings (air / water) to rise. The chemicals of an endothermic process absorb energy from the environment and the temperature of the surroundings drops. 117 On Problem Solving / Discovery Learning and Exothermic Reactions: A Historical Sidelight: Ira Remsen on Copper and Nitric Acid Ira Remsen (1846-1927) founded the chemistry department at Johns Hopkins University, and founded one of the first centers for chemical research in the United States; saccharin was discovered in his research lab in 1879. Like many chemists, he had a vivid "learning experience," which led to a heightened interest in laboratory work: While reading a textbook of chemistry I came upon the statement, "nitric acid acts upon copper." I was getting tired of reading such absurd stuff and I was determined to see what this meant. Copper was more or less familiar to me, for copper cents were then in use. I had seen a bottle marked nitric acid on a table in the doctor's office where I was then "doing time." I did not know its peculiarities, but the spirit of adventure was upon me. Having nitric acid and copper, I had only to learn what the words "act upon" meant. The statement "nitric acid acts upon copper" would be something more than mere words. All was still. In the interest of knowledge I was even willing to sacrifice one of the few copper cents then in my possession. I put one of them on the table, opened the glass vial marked nitric acid, poured some of the liquid on the copper and prepared to make an observation. But what was this wonderful thing which I beheld? The cent was already changed and it was no small change either. A green-blue liquid foamed and fumed over the cent and over the table. The air in the neighborhood of the performance became colored dark red. A great colored cloud arose. This was disagreeable and suffocating. How should I stop this? I tried to get rid of the objectionable mess by picking it up and throwing it out of the window. I learned another fact. Nitric acid not only acts upon copper, but it acts upon fingers. The pain led to another unpremeditated experiment. I drew my fingers across my trousers and another fact was discovered. Nitric acid acts upon trousers. Taking everything into consideration, that was the most impressive experiment and relatively probably the most costly experiment I have ever performed. . . . It was a revelation to me. It resulted in a desire on my part to learn more about that remarkable kind of action. Plainly, the only way to learn about it was to see its results, to experiment, to work in a laboratory. from F. H. Getman, "The Life of Ira Remsen"; Journal of Chemical Education: Easton, Pennsylvania, 1940; pp 9-10; quoted in Richard W. Ramette, "Exocharmic Reactions" in Bassam Z. Shakhashiri, Chemical Demonstrations: A Handbook for Teachers of Chemistry, Volume 1. Madison: The University of Wisconsin Press, 1983, p. xiv: 1) Write the names of the reactants involved in Remsen’s first experiment. * nitric acid + copper 2) Identify 2 physical characteristics of the products of that chemical reaction. *green-blue color, gas, liquid odor 3) Will nitric acid react with cotton? * yes Defend your answer by citing the reading: * his trousers reacted 4) Will nitric acid react with glass? * No Defend your answer by citing the reading: * It was kept in a bottle. 5) Will nitric acid chemically react with skin? * Yes 6) What might you infer about the composition of cotton and skin? *They have a similar composition 118 TRY THIS! …Remember, the loss or gain of energy is from the “point of view” of the chemicals … When chemical systems lose energy, the environment (water or air) becomes warmer. When chemical systems gain energy, the environment (water or air) supplies the energy gained by the chemicals, and thus the environment becomes cooler in temperature. ___1. I am providing you with "before and after" diagrams for this question. Analyze them to see if they help you. A student took the temperature of 150.0 mL of water. She then dissolved 5.00 grams of NH4Cl(s) into the water according to the equation H2O(𝓁) NH4Cl(s) + 14.7 kJ just 150 mL water NH4+1(aq) + Cl-1(aq) 150 mL of water plus the dissolving NH4Cl http://www.wpclipart.com/science/beaker/beaker.jpg THINK! Is this dissolving process, endothermic or exothermic? Is the chemical NH4Cl absorbing energy from the water, OR releasing energy into the water, as NH4Cl dissolves? If the chemical were absorbing energy from the water, what should happen to the temperature of the water? If the chemical were releasing energy to the water, what should happen to the temperature of the water? She took the temperature of the resulting solution. Using the above equation and her knowledge of thermal energy, she could predict that the dissolving process was: a) b) c) d) ___2 endothermic & the temperature of the water increased endothermic & the temperature of the water decreased exothermic & the temperature of the water increased exothermic & the temperature of the water decreased Given the reaction: A(s) + B(aq) C(s) + D(aq) + 170 kJ Assuming that the reaction occurs in water, as the reaction ends, the temperature of the resulting solution have: a) increased b) decreased c) remained the same Think: Is this chemical reaction endothermic or exothermic? Are the chemicals, as they react, absorbing energy from the environment or are the chemicals, as they react, releasing energy into the environment? If the chemicals were absorbing energy, what should happen to the temperature of the water? If the chemicals were releasing energy, what should happen to the temperature of the water? 119 For questions 3 - 5 use the following choices. A choice may be used once, more than once or not at all. a) endothermic b) exothermic 52.4 kJ + 2 C(g) + 2 H2(g) C2H4(g) ___3) H2O ___4) NaOH(s) Na+1(aq) + OH-1(aq) + 44.3 kJ ___5) 2 HCl(aq) + Fe(s) FeCl2(aq) + H2(g) + 598 kJ ___6) A student took the temperature of 150.0 mL of water. She then dissolved 30.00 grams of KClO3(s) into the water according to the equation: H2O (𝓁) KClO3(s) + 41 kJ K+1(aq) + ClO3-1(aq) She took the temperature of the resulting aqueous solution. Using the above information and her knowledge of chemistry she could infer that the reaction was : a) b) c) d) endothermic exothermic exothermic endothermic & & & & the temperature of the surrounding water increased the temperature of the surround water increased the temperature of the surrounding water decreased the temperature of the surrounding water decreased 7) Imagine that you and a friend are sitting in front of a wood fire. The reacting chemicals in this case, are the orgainic gases of wood and dioxygen (O2(g)) gas from the air. Is the reaction exothermic or endothermic? Wood + O2 CO2(g) + H2O(g) 8) Consider liquid water freezing into solid water-ice. Is this exothermic or endothermic? H2O(𝓁) H2O(s) 9) What confuses you or what do you think you now know? ______________________________________ __________________________________________________________________________________ Ans: 1) b energy is absorbed by the reactants. the source of energy is most probably from the water, thus energy would move from the water to the chemicals, and the temperature of the resulting solution would be lower than the water's temperature. 2) a it is an exothermic reaction (energy is on the product side) ... the chemicals release more energy than absorbed and thus the water gains that energy. 3) a 4) b 5) b 6) d 7) exothermic ... energy is being released from the reacting chemicals ... far more than was added to get the fire going... 8) exothermic ... this is a cooling process and yes, also exothermic ... the chemical (water) must lose energy to the environment... 120 VI) Put It All Together: A Recall: A chemical reaction has at least 2 of the following 5 observable changes. 1) thermal energy or light is released / absorbed 2) a new substance is produced: a new solid (s), or precipitate is produced (easily seen) Ba(NO3)2(aq) + Na2SO4(aq) 2 NaNO3(aq) + BaSO4(s) this is a bright white solid that settles out of solution a new gas (g) or bubbling/fizzing is produced (easily seen) Zn(s) + 2HCl(aq) H2(g) + ZnCl2(aq) bubbles out of solution l a new liquid ( ) is produced NOT (aq) (hard to identify this one visually, so it is not used, often) 3) there is a bold color change Pb(NO3)2(aq) + 2 KI(aq) PbI2(s) + colorless colorless 2 KNO3(aq) bright yellow precipitate colorless 4) A simple physical change generally has only 1 of the aforementioned changes a) Note: Ice melting to water … is NOT creating a new liquid (substance) H2O(s) + 334 kJ → H2O(l) Note: Soda pop fizzing (releasing the trapped CO2(g) is NOT creating a new gas … The CO2(g) is already present in the soda...it is just now escaping.) PRACTICE: Use the reasoning of the re-arrangement (new bond making), of atoms / ions or lack thereof, to identify each of the following as a chemical reaction or physical change. 1) Identify the following as being a physical change or a chemical reaction and then defend your answer using the concept of new bond formation (or the lack thereof). very often, water breaks or disrupts the ionic bond Al3+(aq) + 3 Cl-1(aq) really the new bonds are in H2 2 Al(s) + 6 HCl(aq) 2 AlCl3(aq) + 3H2(g) + 134 kJ This represents a _____________________________ because_______________________________ _________________________________________________________________________________ 2) Identify the following as being a physical change or a chemical reaction and then defend your reasoning. Be sure your defense uses a “because” statement which quotes the theory studied in class. Fe(s) + CuSO4(aq) Cu(s) + FeSO4(aq) + 30 kJ This represents a _____________________________ because_______________________________ _________________________________________________________________________________ 121 3) Identify the following as being a physical change or a chemical reaction and then defend your reasoning. Be sure your defense uses a “because” statement which quotes the theory studied in class. H2O(l) 50 kJ + NaCl(s) or: Na+1(aq) + Cl-1(aq) H2O(l) 50 kJ + NaCl(s) NaCl(l) This represents a _____________________________ because_______________________________ _________________________________________________________________________________ 4) Identify the following as being a physical change or a chemical reaction and then defend your reasoning. Be sure your defense uses a “because” statement which quotes the theory studied in class. 100 kJ + CH3OH(l) CH3OH(g) This represents a _____________________________ because_______________________________ _________________________________________________________________________________ 5) Identify the following as being a physical change or a chemical reaction and then defend your reasoning. Be sure your defense uses a “because” statement which quotes the theory studied in class. 2 Al(s) + 3 I2(s) 2 AlI3(s) + 80kJ This represents a _____________________________ because_______________________________ _________________________________________________________________________________ For questions 6 -15 use the underlined boldfaced verb as your clue. Let me know if you think the situation describes a chemical reaction / property or a physical change or property. 6) ____________________ Pat dissolved 10.0 grams of NaOH(s) in water. 7) ____________________ Jordan quickly grilled the steak for every late guest. 8) ____________________ Terry melted the butter and flour together to create a rue for the sauce 9) ____________________ Taylor saw that the metal door handle had oxidized and it needed to be replaced 10) ___________________ Alex was careful to distill the solution to isolate a sample of pure water. 11) ___________________ Perry used baking soda to neutralize the tomato sauce making the taste less tangy 12) ___________________ Sam shredded all the documents listing a social security number. 13) ___________________ Ryan burned the marshmallows in the campfire. 14) ____________________Sidney determined the melting point of the solid to be 58.3C 15) ___________________Shaun noted that a can of Coke was denser than Diet Coke, in water 16) ___________________Aidan filtered the coffee, using unbleached, paper filters. 122 VII) States of Matter (solid, liquid, gas) and the Kinetic Molecular Theory of Gases A) State or Phase: a property of matter, based upon the arrangement and/or “spacing” between the the species. The phase of a substance is dependent upon pressure and temperature, for these two factors affect the attractive forces between species, and thus the arrangement of those species. The term, “state” of matter is generally referred to as solid, liquid or gas. 1) Solid Phase: Classically, true solids are classified as *crystals or as crystalline, exhibiting a repeating lattice structure (a specific geometry building the matter), with a fairly specific melting point From: http://shell.cas.usf.edu/~mccolm/research/RetGeometry.html The reticular (net-like) repeating structure of a fullerene or: http://www.diamondstamps.eu/res/Default/clip_image005.jpg A generalized view of the structure of a diamond. a) the species of making the matter in the solid phase, vibrate in position b) Crystals & powders are examples of matter in the solid phase. i) many students become confused with powders, because they can “be poured” but they are definitely classified as a solid! ii) Macromolecules like diamond & quartz are also solids c) Over recent decades the concept of amorphous solid has developed. These are “hard”, but not classically classified as “true solids” … These are solids but they lack an extended geometric lattice pattern. i) plastics, rubber, wax, and glass are often classified as amorphous solids. d) Regardless of the classification, solids have a fixed volume (it does not change when moved from one beaker to another of a different size, for example) e) by definition … solids are * frozen!!!! 2) Fluid Phases: BOTH the Liquid and Gaseous phases are described as fluids. a) Liquid Phase: The “melted” phase of matter … Liquids have a relatively constant volume, yet the species are separated from each other, sufficiently to allow a “flow” or movement around and over each other. b) Gaseous Phase: The vaporized form of matter, in which the distance between the species is maximized, & the arrangement of the species is at its most random. 123 B) Phase Changes are physical changes. …. No new bonds are made. It requires energy to change from solid to liquid to gas (endothermic) Energy is released to the environment (exothermic) as matter cools from gas to liquid to solid C) Heating (and/or Cooling Curves) and Calorimetry http://en.wikipedia.org/wiki/Supercritical_fluid vaporization 1) We can use q = mcΔT whenever there is a *change in temperature a) ΔT = Tfinal – Tinitial b) 100.0 grams of water were heated from 35.0°C to 55.0°C, calculate the energy absorbed by the water. 2) We can use q = mHf when a sample is *melting … where Hf is a constant giving the number of Joules required per gram to melt at a constant temperature 3) We can use q = mHv when a sample is * boiling or vaporizing at a constant temperature 4) See Table C of your reference charts for constants 124 VIII) Applying Energy: Calorimetry: The measurement of the thermal energy (Joules) contained by chemicals. By using calorimetry calculations, we can determine the change in enthalpy (q or ∆H) of a chemical reaction or of a physical change. This is important for most chemical reactions but also for many engineering issues, such as the integrity of metals used to make pots and pans to the ceramic heat tiles (heat shield) designed to protect the Space Shuttle on re-entry. There are three DIFFERENT calorimetric calculations. Equation q = mHf Key Words and / or Clues melting / freezing / solid liquid q = mHv boiled / vaporized/ condensed / liquid gas change in temperature q = mc∆T Requirements Use the equation & constant for "Hf" Use the equation & constant for "Hv" Use the equation & constant for "c" Calling all Historians and Engineers! According to the Science Channel's 2008 Moon Machines: Lunar Rover, each metal mesh wheel of the first rover, was run by its own electric motor, powered by its own battery system. Fear of becoming too cold or too hot and a maximum 10 lbs. weight limit for the cooling system for the batteries had engineers getting creative. They used a series of cells, containing paraffin (wax!), which, as the batteries released energy, could absorb the released energy and melt, yet maintain a relatively constant temperature, of the system while in use. When not in operation, a series of radiator panels were raised, allowing the paraffin to solidify in the cold of space, becoming ready for the next use of the rover....!!!!! To quote a Kellogg’s® Pop Tart commercial…. That is so “cool”… It’s “hot”… So “hot”, it’s “cool” 1) The reading suggests that as batteries “work” or are used, energy is released. Given all of our work regarding reactions and bonding to date … What must be happening in the battery, that results in released energy? *new bonds (of more stable products) are being produced …. bond production releases energy &/or a chemical reaction is occurring, in which work and heat [the definition of released energy] 125 A) Calorimetry problems: when the temperature changes. 1) The changes in chemical energy can’t be determined directly (…like using a machine for a measurement) … But we can use the change in temperature between unequal matter, to determine the change in energy (q or ∆H) …. This idea gets us back to measuring “heat” by limiting the work. 2) q = mc∆T Key ideas : Joules and a temperature change Where: q = Joules (J) or the heat exchange / change in enthalpy m = grams or the mass of material ΔT = K or ºC which is the change in temperature c = J or J g-1 K-1 specific heat (a different constant for each substance & g•K phase of the substance) 3) Calorimeter: An instrument which helps to measure the q of a substance by allowing a known mass of water to absorb its energy. Fact1: We can’t get the temperature change of a burning substance directly Fact2: The environment surrounding a burning object absorbs the released energy Fact3: Due to its construction (e.g. stainless steel) no work is done on the calorimeter. Thus, we can assume that the joules absorbed by the surroundings = the joules released by a burning substance, en toto. Fact4: Using mwatercwaterTwater NOW, we http://tinyurl.com/l2ugtu8 can now calculate the qwater absorbed Fact5: qwater absorbed = the qreleased by the burning sugar 126 3) A little about specific heat: a) every substance has a different specific heat …. even the specific heat of the phases of each substance can be different values. e.g.) H2O(l) = 4.18 J g-1 K-1 & H2O(s) = 2.03 J g-1 K-1 b) specific heat (c) = the # of joules required to change the temperature of 1.00 g of substance by 1.0 K (or change it by 1.0 ºC) … it is an intensive property i) There is a general rule about specific heat and changes in temperature, the greater the specific heat, the slower temperature change occurs. ★★Hence: Slow to heat up, Slow to cool down. ii) Check out: http://tinyurl.com/l2ugtu8 for more on specific heat and calorimetry The high specific heat of liquid water has important implications for life on Earth. A given mass of water releases more than five times as much heat for a 1°C temperature change as does the same mass of limestone or granite. Consequently, coastal regions of our planet tend to have less variable climates than regions in the center of a continent. After absorbing large amounts of thermal energy from the sun in summer, the water slowly releases the energy during the winter, thus keeping coastal areas warmer than otherwise would be expected (see the diagram below). Water’s capacity to absorb large amounts of energy without undergoing a large increase in temperature also explains why swimming pools and waterbeds are usually heated. Heat must be applied to raise the temperature of the water to a comfortable level for swimming or sleeping and to maintain that level as heat is exchanged with the surroundings. Moreover, because the human body is about 70% water by mass, a great deal of energy is required to change its temperature by even 1°C. Consequently, the mechanism for maintaining our body temperature at about 37°C does not have to be as finely tuned as would be necessary if our bodies were primarily composed of a substance with a lower specific heat. Title: The High Specific Heat of Liquid Water Has Major Effects on Climate Regions that are near very large bodies of water, such as oceans or lakes, tend to have smaller temperature differences between summer and winter months than regions in the center of a continent. The contours on this map show the difference between January and July monthly mean surface temperatures (in degrees Celsius). ****************************************************** 127 Selected Examples of Specific Heat Values Substance or Mixture Water (liquid) Water (gaseous) Aluminum Ethanol Iron Lead Mercury Gold Diamond (C(s)) Graphite (C(s)) Silicon Dioxide (quartz) Calcium Carbonate Ammonia (liquid) Specific Heat (J/g∙K) 4.18 2.06 0.895 2.4 0.46 0.130 0.14 0.129 0.509 0.709 0.742 0.915 4.7 Check out the specific heat values for liquid water, water vapor (gaseous water). Can you suggest a reason for what you observe? Check out the value for liquid ammonia (NH3(𝓁) What’s up with that? QUESTIONS: For questions 1-5 use the table of specific heat values. 1) Which sample would increase in temperature faster? Explain your answer. (Be sure to have a “because statement” in your answer. This “because statement” should include a quote from notes, a reading, a metaphor or an equation.) 20 grams of ethanol (alcohol) or 20 grams of water? ________________________________________________________________________________ ________________________________________________________________________________ _________________________________________________________________________________ 2) Which would make a more sensitive fluid for a thermometer: Mercury or Ethanol? ______________ 3) A saucepan is often made of stainless steel, which is an alloy made primarily of iron. When you put a pan full of water on the stove which heats up faster, the pan or the water in the pan? _______________Defend your answer (Be sure to have a “because statement” in your answer. This statement should include a quote from notes, a reading, a metaphor or an equation.) Why? ____________________________________________________________ __________________________________________________________________ Answers: 1) 20 grams of ethanol will heat up faster because ethanol has a lower specific heat of 2.4 J/g K, compared to water’s 4.18 J/g K. This means that every gram of ethanol requires only 2.4 J of absorbed energy to increase it’s temperature. Since it requires a lesser amount of energy, it will heat up faster. 2) Mercury. 3) The pan. The pan is mostly iron, so the specific heat is roughly close to 0.46 J/g K. The specific heat of water is 4.18 J/g K. The metal pan will heat up faster than the water, since it requires less energy per gram to change temperature (has a lower specific heat value). Keep going… 128 4) You are camping and you wrap up some fish in aluminum foil and cook it over a campfire. (As with most vertebrates, it has a lot of water) After 8 minutes or so, the foil-wrapped fish is taken off the fire. One minute later, the aluminum foil is basically cool enough to touch so you can unwrap your dinner ... but the fish is still hot. (Be sure to have a “because statement” in your answer. This statement should include a quote from notes, a reading, a metaphor or an equation.) What’s up with that? _____________________________________________________________ _______________________________________________________________________________ __________________________________________________________________________ 5) Identify 1 material more sensitive to changes in energy than diamond, but is not as sensitive as gold. (There are at least 3 on the table of specific heat values)… _________________________ 4) Aluminum has a much lower specific heat than water (0.895 J/g K compared to 4.18 J/g K). This means that because it requires less energy per gram, to change in temperature by 1 K (or °C), the aluminum will cool (change temp) much more quickly than the mostly water containing fish. 5) iron, lead or mercury ************************************************ B) A little bit more about T 1) as written, the T value probably is not one of the listed temperatures, in the given problem … but rather it is the change or difference between the final temperature of the matter and the initial temperature. a) this leads to a second equation: T = Tfinal -Tinitial Using this equation, you can determine the final temperature of a sample, when you know the initial temperature and the change in temperature, or some other variation. i) you can assume, that when energy is absorbed, then the final temperature will be greater than the initial, assuming there is no change in phase. you can assume, that if the system were cooled, then the final temperature will be less than the initial temperature, assuming there is no change in phase. ii) Theoretically, the temperatures should be in K, but the change in the Kelvin temperature = the change in temperature in C. Huh?? .... Essentially: ∆C = ∆K Consider the following: Temperature Scale Celsius Kelvin (equivalent) Tinitial 10C 283K Tfinal 15C 288K Difference (∆T) 5C 5K 129 2) Problem: Calculate the energy absorbed in joules when 254.00 grams of water are heated from 320.0 K to 355.0 K Equation: Substitution: * (254.00g) (4.18 J ) (35.0 K) g•K Answer: * 37200 J PRACTICE: Use your understanding of calorimetry to complete each of the following problems. I URGE you to use E.S.A.. Watch those sig. figs!! ___1) How many joules are absorbed by 50.0 g of water as they are heated from 335.0 K to 355.0 K? a) 4,180 joules E: b) 10.5 joules S: 6 c) 2.10 X 10 joules _ d) 2,100 joules A: ___2) Calulate the change in temperature experienced by 70.00 g of water cooled by the release of 8,820 joules. E: a) b) c) d) 40.0 °C 10.6 °C 30.1 °C 50.8 °C S: A: ___3) What mass of water was heated when 2.10 x 103 joules were absorbed between the temperatures of 90.0 °C and 95.0°C? (Do you need to convert to Kelvin?) a) 10. grams _ b) 100 grams E: S: c) 5.0 grams d) 72 grams A: 130 ___ 4) Calculate the change in temperature which occurs when 100.0 grams of water are cooled by the release of 2.1 x 103 joules of heat energy. _ E: a) 50 K c) 5,000 K b) 5.0 K d) 1.0 K S: A: ___ 5) Calculate the change in enthalpy, in joules, that are absorbed when 75.602 grams of water are heated from a temperature of 288.0 K to a temperature of 298.0 K. E: a) 31.6 joules b) 15.6 joules c) 3.16 x 103 joules S: d) 1.96 x 104 joules A: ___6) Calculate the mass, in grams, of water cooled from 100.0°C to 60.0°C assuming a release of 6.720 x 103 joules. a) b) c) d) 80.1 g 60.0 g 30.7 g 40.2 g ___7) What is the temperature change that results when 6.3 x 104 joules of heat are added to 300.0 g of water? a) b) c) d) 50. °C 52 °C 48 ° C 10. °C ___8) Calculate the change in enthalpy required to change the temperature of 300.g of water by 40.0°C a) 5.88 x 104 joules b) 5.02 x 104 joules c) 6.30 x 104 joules d) 7.00 x 104 joules ___9). Challenge Problem 25.000 grams of water at a temperature of 45.0°C absorbed 3,360 joules of energy. Calculate the FINAL temperature of the water. a) b) c) d) 45.8 77.2 13.4 89.0 °C °C °C °C 131 ___10) Challenge problem 60.000 g of water in a beaker were at a temperature of 35.0C. What would be the FINAL temperature of the water, were it to absorb 2,016 joules? a) 8.00 °C b) 27.0 °C c) 43.0 °C d) 103.0 °C ___11) Challenge problem: 248.000 g of water were in a beaker. The water was heated and absorbed 3.1878 x 103 joules. If the final temperature of the water were 95.0 °C calculate the INITIAL temperature of the water. a) 2 °C b) 111.0 °C 12) c) d) 3.03 °C 91.9 °C a) The apple filling of an apple pie is mostly water. By how much will 200 g of apple filling cool if it loses 8.360 x 103 J of energy? b) The mixture, of pie crust has a specific heat of 0.836 J/gK. By how much will 200 g of crust cool if it loses 8.360 x 103 J of energy? c) When I bite into a heated piece of apple pie, I often burn the roof of my mouth. I don’t get that. The pie crust was cool to the touch … why do I get burned? __________________________________________________________________ __________________________________________________________________ Answers : 1) a 2) c 3) b 4) b 5) c 6) d 7) a 8) b 9) b 10) c 11) d 12 a) 10 K or 10°C 12 b) 50 K or 50°C 12c) It has to do with the differences of specific heat. Because the crust’s specific heat is about 5 times lesser than that of water’s, the pie crust will cool off far more quickly …. slow to heat up, slow to cool down … and vice versa 132 5) Changes in potential and average kinetic energy vaporization a) POtential energy = energy of POsition i) It is the energy associated with chemical bonds ii) As species move closer to each other, PE decreases iii) As species move farther away from each other PE increases iv) question: When something such as wax melts or fuses to liquid, what is occurring to the potential energy of the molecules of paraffin? b) Average Kinetic Energy = the *temperature of a chemical system 133 Practice: 1) How many Joules of energy are required to melt 25.0 grams of ice completely at a constant temperature of 0C? (ans: 8,350 J) vaporization a) What is happening to the PE? * increasing 2) 14.0 grams of ice were melted completely at a constant temperature of 0˚C. How many Joules of energy were required to melt the ice completely at 0C? (ans:4680 J) vaporization a) What is happening to the average KE? * remains the same 3) A student melted 50.0 grams of an unknown compound, at a constant temperature of 73.2 C. She calculated the number of Joules required for the fusion to be 1.34 x 104 J. What is the heat of fusion constant for this unknown substance ? vaporization 4) A student vaporized a mass of water at a constant temperature of 100.0C. If 4.50 x 105 J were absorbed by the water, what was the mass? (ans: 199 grams) vaporization 134 5) What mass of aluminum solid was completely liquefied at its melting point of at 660.4 C, if 17,800 Joules were absorbed? (For aluminum, Hf = 396.9 J/g) (ans: 44.8 grams of Al) 6) Calculate the Joules of energy released as 45.0 grams of water vapor condensed to liquid at 100. C (ans: 102,000 J) 7) Calculate the number of Joules absorbed by 50.0 g of water as they are heated from 335.0 K to 355.0 K? (ans : 4,180J) vaporization 8) What was the change in temperature when 70.00 g of water were cooled by the release of 8,820 Joules? (ans: 30.1 °C) 9) 25.000 grams of water at a temperature of 45.0°C absorbed 3,360 Joules of energy. Calculate the FINAL temperature of the water. (ans: 77.2 °C) vaporization 135 10) 60.000 g of water in a beaker were at a temperature of 35.0C What would be the FINAL temperature of the water, were it to absorb 2,016 Joules? (ans: 43.0 °C) 11) What mass of water was heated when 2.10 x 103 Joules were absorbed between the temperatures of 90.0 °C and 95.0°C? (ans: 1.0 x 102 grams) vaporization 12) 248.000 g of water were in a beaker. The water was heated and absorbed 3.1878 x 103 J of energy. If the final temperature of the water were 95.0 °C, calculate the INITIAL temperature of the water . (ans: 91.9 °C) 13) Calculate the change in temperature which occurs when 100. grams of water are cooled by the release of 2.1 x 103 Joules of heat energy. (5.0 K or 5.0°C ) vaporization a) Why can the answer be in K or in °C? 136 Extra Practice: HEATING / COOLING CURVES AND PHASES __1) Which graph best represents the change of phase of a substance from a gas to a solid, using the axes of temperature as a function of time ? For questions 2 and 3 use the following graph and information: The graph below represents a graph of temperature as a function of time of as a substance is heated uniformly, starting at a temperature below the substance's melting point. ___ 2) Which portions of the graph represent times when energy is absorbed and the potential energy increases, while the average kinetic energy remains constant ? a) A B b) B C c) C D d) E F ___3) Between which two time periods are the liquid and the gas phase both present? a) t0 t2 b) t2 t3 c) t3 t4 d) t4 and beyond 137 For questions 4-6 one or more of the responses given is (are) correct. Using your understanding of chemistry decide which of the responses is (are) correct. Then choose: a) b) c) d) e) if only I is correct if only II is correct if only I and II are correct if only II and III are correct if I, II, and III are correct Questions 4-6 each deal with the following cooling curve of a substance as it uniformly cools from a gas to below its melting point. Be sure that you refer to this diagram for each question. ___ 4) Given the above cooling curve : I) from C to D, the potential energy of the substance decreases II) from C to D, the average kinetic energy of the substance decreases III) from E to F, both liquid and the solid phase are present ___ 5) Given the above cooling curve : I) it is a graph of temperature as a function of time. II) the melting point of the substance is approximately 60. ˚C III) the substance is probably water, based on the melting point. ___ 6) Using the above cooling curve, from point B to C, I) potential energy is decreasing (-∆S) II) the average kinetic energy is constant III) the temperature is decreasing 138 ___ 7) Which change of phase is exothermic? a) NaCl(s) NaCl(𝓁) c) H2O(𝓁) H2O(s) b) CO2(s) CO2(g) d) CCl4(𝓁) CCl4(g) ___ 8) What is the total number of Joules of energy absorbed by 15.0 grams of water when it is heated from 303.0 K to 313.0 K? a) 627 J c) 89.9 J b) 6,260 J d) 630.J ___ 9) Given the following heating curve, of a substance as it uniformly heats from a solid below its melting point to beyond its boiling point, which single choice is most correct ? a) The melting point of the solid approximately equals 50 ˚C b) Vaporization occurs at approximately 80 ˚C c) It takes more time to melt the substance than it does to boil it away d) Only liquid is present between points B and C F D B E C A 139 Special Directions: For questions 10-14 write down the most appropriate equation. You do not need to solve the following problems. Simply write down the equation you should use were you to solve the problem. 10) Calculate the number of Joules absorbed by 20.00 grams of ice as the ice melts at 0.0˚C. 11) The temperature of 110 g of water rises from 25.0°C to 26.2°C when a chemical reaction occurs in water. How many Joules of energy are absorbed? 12) As 19.00 grams of water are converted to water vapor at 100 C, calculate the Joules of energy absorbed. 13) If 0.500 gram of substance, H2CO3, is dissolved in a calorimeter with 150.0 grams of water, and the temperature of the bath changes from 24.0 to 39.2C, how many Joules of energy were absorbed by the water? 14) Calculate the constant for the heat of vaporization when 10.0 grams of ethanol are vaporized at its normal boiling point. Answers: 1) c 2) b 3) c 4) b 5) c 6) c 7) c 8) a 9) b 10) q = mHF 11) q = mc∆T 12) q = mHV 13) q = mc∆T 14) q = mHV 140 D) Phase diagram: A graph which traces the change in entropy and enthalpy, during the physical changes the substance undergoes with pressure as a function of temperature Where heating / cooling curves generally illustrate temperature as a function of time (or time of heating /cooling), the figure below shows an example of a phase diagram, which summarizes the effect of temperature and pressure on a substance in a closed container. Every point in this diagram represents a possible combination of temperature and pressure for the system. 1) The diagram is divided into 4 areas, which represent the solid, liquid, gaseous states of the substance, and a supercritical zone … beyond the critical point. Conditions = high pressure & low temperature Supercritical Fluid Conditions = low pressure & low temperature One good way to recall which area corresponds to each of these states is to remember the conditions of temperature and pressure that are most likely to be associated with a solid, a liquid, & gas. Low temperatures and high pressures favor the formation of a solid. Gases, on the other hand, are most likely to be found at high temperatures and low pressures. Liquids lie between these extremes. Common components of a phase diagram are lines of equilibrium or phase boundaries, which refer to lines that mark conditions under which multiple phases can coexist at equilibrium. Phase transitions occur along lines of equilibrium. Regardless of the substance, most phase diagrams are similar in construction and interpretation. The conditions of the solid phase tend to be in the lower to upper left, the gaseous phase is represented by the conditions of the lower right, and the liquid phase is in the middle. 141 http://library.thinkquest.org/C006669/media/Chem/img/Graphs/Phase.gif The triple point: * the temperature and pressure at which all three phases exist simultaneously The critical point is that set of conditions above which distinct phases (e.g. liquid and gas) do not exist. As the critical temperature is approached, the properties of the phases approach one another, resulting in only one phase at the critical point, often referred to as a homogeneous supercritical fluid. A supercritical fluid can effuse through solids like a gas, and dissolve materials like a liquid. close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a supercritical fluid to be "fine-tuned". Supercritical fluids are suitable as a substitute for organic solvents in a range of industrial and laboratory processes. Carbon dioxide and water are the most commonly used supercritical fluids, being used for decaffeination and power generation, respectively. http://en.wikipedia.org/wiki/Supercritical_fluid Phase Diagram of CO2 Phase Diagram of H2O 142 Notice in the following diagram, that point “A” has been added and that there are two arrows. When we follow “A” across the solid to gas boundary line, we can see, that there are two ways by which a solid can sublimate. A Questions: 1) What is the most likely phase of the substance, at point A? * gas phase 2) What is the phase of the substance represented by this phase diagram at STP? *liquid 143 3) What would happen to the state of a substance represented by this phase diagram if the pressure changed from Point A to Point B (holding temperature constant)? * It would sublimate (undergo sublimation) 4) What phase change would be associated with moving directly from Point A to Point B on this phase diagram? *It melts (fuses / goes from solid to liquid) 5) What phase change would be associated with moving directly from Point B to Point A on this phase diagram? * The phase change is condensation 144 6) At what approximate temperature will all 3 phases exist in equilibrium with each other? * approximately 150K 7) The horizontal line on this phase diagram occurs at 1 atmosphere of pressure. What is the normal freezing point of this substance? *approximately 200 K 8) The horizontal line on this phase diagram occurs at 1 atmosphere of pressure. What is the normal boiling point of this substance? *approximately 425°K 145 E) The Kinetic Molecular Theory of Matter (KMT) 1) Matter consists of extremely tiny particles (atoms, molecules, ions = “species”) that are in constant motion. 2) In solids the species are paced closely together. Particles vibrate back and forth, essentially “in place” 3) In liquids the species are arranged randomly, and as in gases, the species can move past and around each other … thus each phase is described as a fluid. 4) Under normal conditions, the species of a gas are far apart. The gaseous particles (often molecules), move rapidly in straight-line motion. The gas molecules collide with each other and with the interior walls of the container holding the gas (creating a pressure). a) The random motion of the gaseous molecules allows the gas molecules to occupy the entire volume of the container. Hence, the volume of the gas sample is the volume of the container!!! 5) A very important extension of the KMT is : Increasing temperature (average kinetic energy) corresponds to faster and faster motions of the species. … This means that a greater number of energetic collisions occur at higher temperatures, as as rule. A Generalized Maxwell-Boltzmann Energy Diagram: With a Constant Number of Gas Molecules At lower temperature, only a few molecules under this curve have enough energy to collide effectively to make bonds (shaded area in the lower right.) When temperature is increased, a greater number of molecules possess sufficient average K.E. to collide effectively and to bond (larger shaded area) Note: Point A represents the necessary activation energy needed to break the reactant species’ bonds and thus initiate a reaction. Check Out: https://www.youtube.com/watch?v=66AxPceP5QY (up to 2:45 only) Check out: For the shift in graph: https://www.youtube.com/watch?v=YnHIfqUZi48 146 More Practice 1) An example of a physical property of a compound, is it’s ability to: 6) Explain how freezing can be considered to be an exothermic physical change. Be sure to address both terms in your response. 1) react with an acid 2) react with oxygen 3) bond with with chlorine 4) dissolve in water 2) Which statement describes a chemical property of the element magnesium? 1) Magnesium is malleable. 2) Magnesium conducts electricity. 3) Magnesium reacts with an acid. 4) Magnesium has a high boiling point. 3) Which property could be used to identify a specific substance in the laboratory? 1) melting point 2) mass 7) Explain how boiling can be considered endothermic? 3) temperature 4) volume 4) Which statement describes a chemical property of hydrogen gas? 1) 2) 3) 4) Hydrogen gas is colorless Hydrogen gas has a density of 0.00009 g/cm3 at STP Hydrogen gas has a boiling point of 20. K at 1 atm Hydrogen gas burns in air 5) Which sample could occupy the volume of a 100 mL flask? 1) 25 cm3 Cu(s) 2) 30 cm3 CO2(g) 3) 50 cm3 SiO2(s) 4) 75 cm3 Fe(s) Go on to the next page… Multiple Choice Answers: 1) 4 2) 3 3) 1 4) 4 5) 2 147 For questions 8 -10 use the following passage and your grasp of chemistry. Carbon forms molecular compounds with some elements from Group 16. Two of these compounds are carbon dioxide, CO2 and carbon disulfide CS2. Carbon dioxide is a colorless, odorless gas at room temperature. At standard temperature and pressure, CO2(s) changes directly to CO2(g), with a change in phase called sublimation. Carbon disulfide is formed by a direct reaction of carbon and sulfur. At room temperature, CS2(s) is a colorless liquid with an offensive odor. Carbon disulfide vapors are flammable. 8) Identify one physical property and one chemical property of CS2 at STP * colorless, liquid, offensive odor / flammable 9) State what happens to the potential energy of CO2 molecules as they undergo the sublimation change in phase. * During sublimation (solid to gas), the potential energy increases 10) Is sublimation an exothermic or endothermic? * endothermic Is sublimation a physical change or chemical reaction? *physical change For questions, 11-14 use the following passage, your reference tables, and your understanding of chemistry. Archimedes (287-212 BC), a Greek inventor and mathematician, made several discoveries important to science today. He was related to Hiero of Syracuse, who ruled in Sicily, under the Romans. According to legend, King Hiero, asked Archimedes to determine if a golden crown given as a gift, was indeed pure gold. The king suspected that the crown consisted of a mixture (an alloy) of gold, with silver, tin, or copper. Archimedes was completely flummoxed by the task. He could determine the mass, but the irregular shape of the crown made the volume determination difficult. While getting into a bath, he noticed that his body forced the overflow of water. He immediately realized his volume, displaced an equal volume of water …hence he had a means of determining volume… Supposedly he ran naked through the streets yelling “Eureka” (I have found it!). The story continues that he repeated the displacement procedure using the crown and equal masses of individual samples, of gold, a sample of silver, one of tin, and one of copper. He then determined the ratio of mass to volume (density) of each sample and the crown. Archimedes was able to determine that the crown was not made entirely of gold without damaging it. In fact, it supposedly was equal masses of gold and silver. 11) Identify one physical property that Archimedes used in his comparison of the metal samples. *density 12) If Archimedes melted the crowns, would he have been testing a physical or chemical property? *physical 13) If Archimedes had rusted the crowns, would he have been testing a physical or chemical property? *chemical 14) Determine the volume of a 75.0 gram sample of gold at STP. Show your work, using E.S.A. * Equation: Density = Mass/Volume 19.3 g/cm3 = 75.0 g x x = 3.89 cm3 148