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Dear 3EFG, Refer to your notes for the formula and other data. But read the supplements here. I am sure that these will be helpful. Try answering the problems first. If you cant arrive at a sound answer, confer and discuss with your classmates first. I’ll post the answer key once I knew that you’ve discussed it with your classmates. Sometimes you learn by discussing concepts with your classmates. I made some questions in the long test that will require you to think critically and convert. With the many conversions that we had, I am confident that you can derive them all. Please do read your notes and your oh so helpful textbook. Good luck and God bless! Sir G I. Enthalpy Energy Matters – Heat Changes of State If we supply heat to a solid, such as a piece of copper, the energy supplied is given to the molecules. These start to vibrate more rapidly and with larger vibrations – the molecules gain Kinetic Energy(Ek). The heat supplied has been converted to molecular kinetic energy. If the solid is cooled, the reverse happens (the molecules vibrate more slowly and these vibrations become smaller). The molecules have lost kinetic energy and this lost Ek is converted back into heat energy that is lost to the surroundings. If we heat the solid to a high enough temperature it will eventually melt to form a liquid. If we monitor the temperature of the material during heating, we would see that it varies with time during heating as shown in the graph below. Notice that, in general, the temperature goes up the longer the heating continues. However, there are two horizontal flat parts to the graph. These happen when there is a change of state. The plateaus are also called phase changes. The first change of state is melting (changing from a solid to a liquid). The temperature stays the same while a substance melts. For water, this temperature is 0°C because the melting point for water is 0°C. The second change of state is boiling (changing from a liquid to a gas). The temperature stays the same while a substance boils. For water, this temperature is 100°C because the boiling point for water is 100°C. Different substances have different melting points and boiling points, but the shapes of their heating curves are very similar. For example, this is the heating curve for iron, a metal that melts at 1538°C and boils at 2861°C. Sample problem: 1. Calculate the amount of heat required to completely convert 50 g of ice at -10 ºC to steam at 120 ºC. 2. The molar heat of combustion of a compound is 1,350 kJ/mol. If 0.875 moles of this compound was burned in a bomb calorimeter containing 1.70 L of water, what would the increase in temperature be? 3. How much energy is required in total to change 1.9kg of ice at -10 ̊C to steam at 100 ̊C? II. Nuclear Chemistry Two types of nuclear reactions 1) Radioactive decay - process in which a nucleus spontaneously disintegrates, giving off radiation. 2) nuclear bombardment reactions - nuclear reaction in which a nucleus is bombarded or struck by another nucleus or nuclear particle. Here fission or fusion may occur. An example of nuclear reactions 1) A sample of Uranium-238 decays spontaneously over a period of billions of years. After about 30 billion years it is nearly gone. Strontium -90 formed by nuclear reactions that occur in nuclear weapons testing is essentially gone after several hundred years. 2) Example of a nuclear bombardment reaction is the fusion that goes on in the sun which is essentially four protons and electrons combining to make He. Radioactivity comes out of the nucleus of atoms. The nucleus is radioactive because it is unstable. Like electrons in an excited state dropping back down to ground state and releasing a photon, nuclei need an outlet for their excited state. This outlet is radiation or nuclear reactions. Nuclear reactions, radioactivity, are spontaneous decays, there is no way to tell when it will occur, but eventually they will decay. These radioactive decays occur in any atom with more than 83 protons. Also, in any atom with an exceptionally small or large proton to neutron ratio will be radioactive. Atomic Notation: 3 Types A = Atomic Mass = protons + neutrons Z = Atomic Number = protons X = Atomic Symbol = the elements letter designation Type 1 Type 2 Type 3 A Z A X 12 6 X 12 X–A C C C -12 This type came first and gives the most information This type came second as the first type is redundant. You do not need to tell me carbon has an atomic number of 6, all carbons atomic numbers are 6. This type came last and is the easiest to type, and still relays all the info you need. This symbol is spoken, “carbon twelve.” 3 Main Types of Radioactive Decay: alpha decay – a helium nucleus beta decay – a high speed electron gamma decay – electromagnetic radiation – occurs in an excited nucleus, like excited electrons Other Types of Nuclear Reactions: Positron Emission – A positive beta particle Electron Capture – Electron in orbit around nucleus is drawn into the nucleus combining with a proton to produce a neutron. The mass of the nucleus remains constant but the atomic number drops by 1. Neutron Emission – a neutron is emitted from the nucleus cause the mass to drop by 1. Fusion and Fission Nuclear Binding Energy Puzzling fact that the mass of an atom is always less than the sum of the masses of its constituent particles. For instance the mass of a helium-4 atom is 4.00260 amu Mass of 2 electrons = 2 x 0.000549 amu = 0.00110 amu Mass of neutron is 1.675x10- 2 4 g so 2 x 1.00867 amu = 2.01734 amu Mass of proton is 1.673x10- 2 4 g so 2 x 1.00728 amu = 2.01456 amu Sum of mass = 4.03300 amu -0.03040 amu. This mass difference is explained by the fact that when the nucleons come close together they bind so energy must be lowered. This is related to the binding energy. The binding energy is the energy required to break a nucleus into its individual protons and neutrons. Example The 1 9 9 F isotopes has an atomic mass of 18.9984 amu Nucleus has 9 protons and 10 neutrons, 19 nucleons proton mass - 1.007825 amu neutron mass - 1.008655 amu 9 x 1.007825 + 10 x 1.008655 amu = 19.15708 amu This is larger than the measured mass of 1 9 9 F This is a difference between the mass of the atom and sum of the masses of the nucleons is called the mass defect. Convert mass defect into energy ac - 19.15708 = -0.1587 -0.1587amu x (3.00x108 m/s)2 = -1.43 x 101 6 amu m2 /s2 1 amu = 1.066 x 10- 2 7 kg 1 J = 1 kg m2 /s2 2 -1.43 x 101 6 amu m2 /s2 x 1.066 x 10- 2 7 kg / amu = -2.37 x 10- 1 1 J This is the energy released when on Fluorine-19 nucleus is formed from 9 protons and 10 neutrons. Half-life is the time required for half of the atoms of a radioactive isotope to undergo decay. Some isotopes are very stable, undergo decay very slowly, and have extremely long half-lives. Uranium-238 has a half-life of 4.46 billion years! Other isotopes are extremely unstable, and have short half-lives. The isotope francium-233 has a half-life of 22 minutes. That means that if you possessed 10 grams of francium-233, after only 22 minutes you would have 5 grams of francium-233, while the remainder of the atoms would have been converted by some decay processes to other elements. An exponential decay process can be described by any of the following three equivalent formulas: where 1. N0 is the initial quantity of the substance that will decay (this quantity may be measured in grams, moles, number of atoms, etc.), 2. N(t) is the quantity that still remains and has not yet decayed after a time t, 3. t1/2 is the half-life of the decaying quantity, 4. τ is a positive number called the mean lifetime of the decaying quantity, 5. λ is a positive number called the decay constant of the decaying quantity. The three parameters , , and λ are all directly related in the following way: Nuclear Chemistry problems: 1. The alpha decay of iridium-174 2. The beta decay of platinum-199 3. Positron emission from sulfur -31 4. Krypton-76 undergoes electron capture 5. Write the symbols for an alpha particle, beta particle, gamma ray, and positron. 6. If the half-life for the radioactive decay of zirconium-84 is 26 minutes and I start with a 175 gram sample, how much will be left over after 104 minutes? 7. What is the mass defect for 3Li6, the lithium isotope with 3 neutrons and an atomic weight of 6.01513? Compute for the binding energy 8. Calculate the mass defect for 17Cl35 whose mass is 34.9689 AMU. Afterwards compute for the binding energy per nucleon III. Chemical Reactions Types of Chemical Reactions Consider for a moment the number of possible chemical reactions. Because there are millions of chemical compounds, it is logical to expect that there are millions of possible chemical reactions. It would be very difficult to memorize the equations for all the different chemical reactions that occur so chemists have grouped them according to the similarities in the way they react. It is not quite as simple as this though because some chemical reactions can belong to more than one type. One method groups nearly all the chemical reactions into five main types. 1. synthesis (or combination) 2. decomposition 3. single-replacement (or single displacement) 4. double replacement (or double displacement) 5. combustion 1. Synthesis reactions When two reactants combine to form one product. For example: Word equation Potassium + Chlorine Potassium Chloride Chemical equation K(s) + Cl2(g) 2 KCl(s) 2. Decomposition reactions When one reactant breaks down into two or more products or two of more of the same molecules. It is hard to predict the products of decomposition reactions. For example: 1. 2. Word equation Calcium carbonate Calcium Oxide + Carbon dioxide Chemical equation CaCO3(s) CaO(s) + CO2(g) Word equation Mercury oxide Mercury + Oxygen Chemical equation 2 HgO(s) 2 Hg(l) + O2(g) 3. Single Replacement reactions Occur between a metal element and a compound. The metal element replaces an element in the compound. Common elements that swap are two metals or a metal and hydrogen. For example: 1. 2. Word equation Magnesium + Zinc nitrate Magnesium nitrate + Zinc Chemical equation Mg(s) + Zn(NO3)2(aq) Mg(NO3)2(aq) + Zn(s) Word equation magnesium + Hydrochloric acid Magnesium chloride + Hydrogen Chemical equation Mg (s) + 2 HCl (aq) MgCl2 (aq) + H2(g) In this reaction the element magnesium is replaced by the element hydrogen, H in hydrochloric acid. Hydrogen gas, H2 becomes the product. 4. Double Replacement reactions Occur between two compounds. The two positive metal ions in each reactant swap places with one another. For example: 1. 2. Word equation Potassium Carbonate + Barium Chloride Potassium Chloride + Barium carbonate Chemical equation K2CO3(aq) + BaCl2(aq) 2 KCl (aq) + BaCO3(s) ions K+, CO32- Ionic Equation CO32-(aq) + Word equation Sodium sulfide + Magnesium nitrate Sodium nitrate + Magnesium sulfide Chemical equation Na2S(aq) + Mg(NO3)2(aq) NaNO3 (aq) + MgS(s) ions Na+, S2- Ionic Equation S2-(aq) Ba2+, ClBa2+(aq) K+, Cl Mg2+, NO3+ Mg2+(aq) BaCO3(s) Na+, NO3 Ba2+, CO32- Mg2+, S2- MgS(s) In these reactions the two positive metal ions swap places with one another to form two new compounds. In most double displacement reactions the reactants are aqueous solutions. The products formed are a solid and another aqueous solution. The solid product formed when two aqueous solutions react is called a precipitate. Reactions between acids and bases are also double displacement reactions. They are also called neutralization reactions. For example the reaction between nitric acid and the base sodium hydroxide. Word equation Nitric acid + Sodium hydroxide Sodium nitrate + water Chemical equation HNO3 (aq) + NaOH (aq) NaNO3 (aq) + H2O(l) 5. Combustion reactions Reactions that involve the reactant reacting with oxygen are called combustion reactions by scientists. We commonly called these reactions burning. For example paper burning in oxygen in the air is a combustion reaction. Combustions reactions are all exothermic because they release large amounts of heat energy. Energy because it is produced is written on the product side of the chemical reaction. For example if an element is burned in oxygen the element and oxygen combine to form on product like in a synthesis reaction. Word equation Carbon + Oxygen Carbon dioxide + Heat energy Chemical equation C(s) + O2(g) CO2(g) + Heat energy If a compound burns in oxygen the products are either carbon dioxide and water or carbon monoxide and water. Word equation methane + Oxygen Carbon dioxide + water + Heat energy Chemical equation CH4(s) + 2 O2(g) CO2(g) + 2 H2O(g) + Heat energy Stoichiometry involves quantitative relationships in a chemical reaction. Stoichiometric factors—also called mole ratios—are based on the coefficients in the balanced equation and are used to relate moles of one reactant or product to another. Molar masses and stoichiometric factors, together with other factors, are used to determine information about one reactant or product in a chemical reaction from known information about another. The strategy for reaction stoichiometry calculations can be outlined diagrammatically, as suggested below. I. Mass-mass problems can be solved by following these steps: 1. Write a balanced equation for the chemical reaction involved. 2. Change the mass given into moles. 3.Compare the moles of the substance given to the moles of the substance for which the mass is asked. 4. Calculate the number of moles required. 5. Change moles asked for into mass asked for. Example: The balanced equation for the synthesis of ammonia is 3 H2(g) + N2(g) --> 2 NH3(g). Calculate the mass in grams of NH3 formed from the reaction of 64.0 g of N2 From the balanced equation, it is known that: 1 mol N2 ∝ 2 mol NH3 Use the periodic table to look of the atomic weights of the elements to calculate the weights of the reactants and products: 1 mol of N2 = 2(14.0 g) = 28.0 g 1 mol of NH3 is 14.0 g + 3(1.0 g) = 17.0 g These relations can be combined to give the conversion factors needed to calculate the mass in grams of NH3 formed from 64.0 g of N2: mass NH3 = 64.0 g N2 x 1 mol N2/28.0 g NH2 x 2 mol NH3/1mol NH3 x 17.0 g NH3/1 mol NH3 mass NH3 = 77.7 g NH3 Exercises: Balance the following reactions and indicate which of the six types of chemical reaction are being represented: 1) ____ NaBr + ____ Ca(OH)2 ___ CaBr2 + ____ NaOH Type of reaction: _____________________________ 2) ____ NH3+ ____ H2SO4 ____ (NH4)2SO4 Type of reaction: _____________________________ 3) ____ C5H9O + ____ O2 ____ CO2 + ____ H2O Type of reaction: _____________________________ 4) ____ Pb + ____ H3PO4 ____ H2 + ____ Pb3(PO4)2 Type of reaction: _____________________________ 5) ____ Li3N + ____ NH4NO3 ___ LiNO3 + ___ (NH4)3N Type of reaction: _____________________________ 6) ____ HBr + ___ Al(OH)3 ___ H2O + ___ AlBr3 Type of reaction: _____________________________ Mass to Mass problem 1. Calculate the mass of the sulfate compound produced if 47.5 g of Potassium Iodide reacted with Aluminum Phosphate.