* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Packet 1 - Kentucky Community and Technical College System
Marcus theory wikipedia , lookup
Chemical bond wikipedia , lookup
Chemical thermodynamics wikipedia , lookup
Water splitting wikipedia , lookup
Oxidation state wikipedia , lookup
Hypervalent molecule wikipedia , lookup
Determination of equilibrium constants wikipedia , lookup
Inorganic chemistry wikipedia , lookup
Nucleophilic acyl substitution wikipedia , lookup
Photoredox catalysis wikipedia , lookup
Physical organic chemistry wikipedia , lookup
Transition state theory wikipedia , lookup
Process chemistry wikipedia , lookup
Liquid–liquid extraction wikipedia , lookup
Chemical equilibrium wikipedia , lookup
Chemical reaction wikipedia , lookup
Rate equation wikipedia , lookup
Click chemistry wikipedia , lookup
Acid strength wikipedia , lookup
Strychnine total synthesis wikipedia , lookup
Bioorthogonal chemistry wikipedia , lookup
Acid dissociation constant wikipedia , lookup
Equilibrium chemistry wikipedia , lookup
Electrolysis of water wikipedia , lookup
Stability constants of complexes wikipedia , lookup
Lewis acid catalysis wikipedia , lookup
Debye–Hückel equation wikipedia , lookup
Metalloprotein wikipedia , lookup
Electrochemistry wikipedia , lookup
Nanofluidic circuitry wikipedia , lookup
Acid–base reaction wikipedia , lookup
Ionic compound wikipedia , lookup
Stoichiometry wikipedia , lookup
Evolution of metal ions in biological systems wikipedia , lookup
Packet 4 Chemical Quantities and Aqueous Reactions “Nothing in the world is as soft and yielding as water. Yet for dissolving the hard and inflexible, nothing can surpass it.” Tao Te Ching, ca. 500 B.C. “Divide, and conquer.” Phillip of Macedon last modified: 5/25/2017 CHE 170 Packet 4 - 1 Concept Area I: Terminology stoichiometry stoichiometric coefficients/factors limiting reactant/reagent theoretical yield actual/experimental yield percent yield solute solvent solution concentrated dilute Molarity, M electrolyte (strong & weak) nonelectrolyte acid (strong & weak) base (strong & weak) soluble insoluble slightly soluble precipitate precipitation reaction molecular or complete formula equation complete ionic equation net ionic equation spectator ions acid-base reaction hydronium ion salt oxidation reduction oxidation-reduction, redox oxidation state or numbers oxidizing agent reducing agent oxidizer reducer CHE 170 Packet 4 - 2 The Mole Refresher Remember, one mole is the amount of substance that contains as many particles (atoms, molecules, whatever) as there are in 12.0 g of 12C. And remember that there is Avogadro’s number of particles in a mole of any substance. What is Avogadro’s number? CHE 140 Packet 8 - 3 Solution Concentration: Molarity To indicate how much solute we have in a solution, Molarity is commonly used. It is abbreviated with a capital “M”. It is defined: mols of solute M 1 L of solution “Molarity” is usually used as a noun. If an adjective is needed to mean the same thing, chemists use the term “Molar”. I will usually always capitalize “Molarity” and “Molar” since the symbol is a capital “M”. CHE 170 Packet 4 - 4 A “road map” between grams, moles, number of particles and Molarity! mass of element or compound molar mass(es) from periodic table number of particles Avogadro’s number 6.022×1023 moles of element or compound Volume Molarity CHE 170 Packet 4 - 5 mols of solute M 1 L of solution This picture shows very clearly that a one liter of solution (in the volumetric flask) is quite different than one liter of solvent (the water in the volumetric flask and the graduated cylinder). CHE 170 Packet 4 - 6 Let’s do an example for M! What concentration is the solution we made on the previous slide? We took 25.0 g of CuSO4·5 H2O and made a 1.00 liter solution. Do the homework (problems 4.25—4.34) and make sure comfortable with calculating Molarity because we will be adding it CHE 170 Packet 4 - 7 to stoichiometry at the end of these notes! Molarity Handout Let’s Calculate Molarity! 1. Sodium bicarbonate is also known as baking soda. If 38.9 g of it is dissolved in enough water to make 500 mL of solution, what is the Molarity of the solution? 2. If 25.3 g of sodium carbonate is dissolved in enough water to make 250 mL of solution. What is the Molar concentration of the sodium ions? 3. How many grams of sodium phosphate do we need to make 1.50 L of a 0.125 M sodium phosphate solution? CHE 170 Packet 4 - 8 Dilute vs. Concentrated Solutions can be dilute or concentrated. dilute: concentrated: The concentration of a solution is dependent only on how much solute is present in the solvent (not whether the solute is strong or weak). 0.1 M NaCl is a dilute salt solution 5.0 M NaCl is a concentrated solution CHE 170 Packet 4 - 9 Strong vs. Weak Acids and bases can be classified as either strong or weak. Salts are either strong or not, never weak. Whether a solute is strong or weak has nothing to do with the concentration (dilute or concentrated). What determines if strong or weak? CHE 170 Packet 4 - 10 Concept Area II: Stoichiometry a. You should be able to interconvert between grams and moles of reactants and products in a balanced chemical equation. b. You should be able to calculate theoretical yield, actual yield, and percent yield. CHE 170 Packet 4 - 11 Roadmap meets Stoichiometry worksheet Stoichiometry! Stoichiometry is the study of the aspects of chemical reactions. It rests on the principle of conservation of matter. Balanced chemical reactions are very important in stoichiometry. It is also still very important, or even more important, to write down all units in any stoichiometric calculation! . CHE 170 Packet 4 - 12 General Plan for Stoichiometric Calculations Step 1) write a balanced chemical reaction. Step 2) write down what we know Step 3) write down what we want to know Step 4) solve the problem using our new and improved roadmap! Mass reactant Mass product molar mass(es) from periodic table molar mass(es) from periodic table Moles reactant use stoichiometric factors from balanced chemical equation Moles product CHE 170 Packet 4 - 13 Some Problems using: 2 NH4NO3 → 2 N2 + 4 H2O +O2 Mole ratio problem: If 37.2 mol NH4NO3 explode, how many mols of H2O are produced? Mass problem: If we want to produce 12.0 g oxygen, how many grams of NH4NO3 do we need? CHE 170 Packet 4 - 14 Let’s try another one! Using 5.00 g of H2O2, what mass of O2 and H2O can be obtained when it completely decomposes? Step 1: Write a balanced chemical equation! Step 2: Write down what we know! Step 3: Write down what we want to know. Step 4: Solve problem using roadmap. CHE 170 Packet 4 - 15 Using 5.00 g of H2O2, what mass of O2 and of H2O that can be obtained when it completely decomposes? So, first all roads lead to moles! Calculate moles of H2O2 from grams given. Then, use stoichiometric coefficients to calculate moles of O2 and H2O. Finally, calculate mass of O2 and H2O from moles calculated. See notes page for step by step worked out. CHE 170 Packet 4 - 16 We have been assuming that we get 100% yields (reaction always goes to completion). Do we really? No! Just like most things (like popcorn), reactions usually do not produce as much as they theoretically could. So, we take our actual yield (our experimentally measured quantity) and compare it to our theoretical yield (the amount we’d get if everything reacted). To tell us our percent yield. CHE 170 Packet 4 - 17 Theoretical & Actual Yields We started with 20 popcorn kernels. What is our theoretical yield of popped popcorn? What was our actual or experimental yield of popped popcorn? CHE 170 Packet 4 - 18 Percent Yield Handout So, to summarize and continue… Theoretical yield is what we get if all reacts. Actual yield is what we or get. To calculate percent yield: actual yield 100 % theoretical yield So, what is our percent yield in our popcorn example where only 16 of the 20 kernels popped? Experimental yield should always be less than theoretical yield. Why? CHE 170 Packet 4 - 19 Concept Area III: Limiting Reagents and Excess Reagents a. You should be able to identify the limiting reactant in a reaction mixture. b. You should be able to calculate the amount of product produced based on the limiting reactant. c. You should be able to calculate the amount of excess reactant left over from a reaction mixture. CHE 170 Packet 4 - 20 Limiting Reactant We all know: 1 hot dog + 1 bun → 1 hot dog “sandwich” So, let’s go to the store hot dog package has hot dogs hot dog bun package has buns Hmmmm… how many hot dog sandwiches can we make if we only buy one package of each? How many of what would be left over? Determining the limiting reactant in a chemical reaction works the same way! CHE 170 Packet 4 - 21 Limiting Reactant So, what will we look at to see which limits in a reaction? The number of of the ! The chemicals leftover are the excess reactants or reagents. For this reaction: N2(g) + 3 H2(g) → 2 NH3(g) If we start with the top flask, which of the bottom flasks shows the reaction at completion? Tro page 122 CHE 170 Packet 4 - 22 Limiting Reactants React solid Zn with 0.100 mol HCl(aq): Zn(s) + 2 HCl(aq) ZnCl2(aq) + H2(g) So, how many moles of Zn are needed to completely react with the HCl? 1 mol Zn 0.100 mol HCl 0.0500 mol Zn Let’s look at data for 2 mol HCl three reactions: Rxn 1 Rxn 2 Rxn 3 mass Zn (g) 7.00 mol Zn 0.107 mol HCl 0.100 mol HCl/mol Zn 0.93/1 Lim. Reactant 3.27 0.0500 0.100 2.00/1 1.31 0.0200 0.100 5.00/1 CHE 170 Packet 4 - 23 Let’s try this one! SiO2 + HF → SiF4↑ + 2 H2O Suppose we have 6.0 moles of silicon dioxide and 11 moles of HF. Determine excess and limiting reagents, and how much SiF4 gas and water are theoretically formed. What steps should we take? First, write a balanced chemical equation! Now, what do we know and want to know? Solve! Correct mol:mol ratio? If no, which one do we have too much of? too little? Use limiting reagent to calculate theoretical amount of products produced. Let’s continue on next slide… CHE 170 Packet 4 - 24 SiO2 + 4 HF → SiF4↑ + 2 H2O Need 1 mol SiO2 for every mols HF Have 6 mol SiO2, so would need mols HF We have 11 mols of HF, is that enough? . So, the limiting reagent is? Thus, theoretical mol amounts that can be produced: CHE 170 Packet 4 - 25 Of course, in a lab we can’t directly measure moles, so let’s try a more realistic problem! Calculate the theoretical yield of zinc sulfide, in grams, that can be made from 0.488 g zinc and 0.503 g of S8. If the actual yield is 0.606 g of zinc sulfide, what is the percent yield? Those numbers are difficult to compare, so let’s get whole numbers by dividing both by the smaller number. CHE 170 Packet 4 - 26 Calculate the theoretical yield of zinc sulfide, in grams, that can be made from 0.488 g zinc and 0.503 g of S8. If the actual yield is 0.606 g of zinc sulfide, what is the percent yield? Okay, we have a 8 Zn + S8 → 8 ZnS 1 mol Zn –3 7.46 10 – 3 mol Zn 1.96 10 3.81 4 65.39 g Zn 1 mol S8 0.503 g S8 1.96 10 – 3 mol S8 1.96 10 –3 1.00 1 256.5 g S8 0.488 g Zn ratio of 4 mols Zn for each mol S8. We need 8 mols Zn for each mol S8. Therefore, since 4<8, Zn is limiting! This is the what? CHE 170 Packet 4 - 27 Limiting Reactant Practice Handout Calculate the theoretical yield of zinc sulfide, in grams, that can be made from 0.488 g zinc and 0.503 g of S8. If the actual yield is 0.606 g of zinc sulfide, what is the percent yield? 8 Zn + S8 → 8 ZnS 7.46 10 – 3 mol Zn 8 mol ZnS 97.46 g ZnS 0.727 g ZnS 8 mol Zn 1 mol ZnS How do we calculate percent yield again? CHE 170 Packet 4 - 28 Concept Area IV: Introduction to Solutions a. You should understand the difference between a solute, solvent and solution. b. You should understand the term electrolyte and be able to classify a compound as a strong electrolyte, weak electrolyte or a nonelectrolyte. c. You should know what ions are formed when an electrolyte dissolves in water. d. You should know the basic solubility rules and how to read a solubility table for more advanced rules. CHE 170 Packet 4 - 29 Water, water everywhere… Most of the solutions we form are water based. Why? Well, ¾ of the Earth is covered by water. And, 70-80% of us is water. Because water is so abundant it is very useful to use as a solvent. The fact that so many ionic and molecular chemicals are soluble in it makes it even more useful. Many chemists are even trying to do all their experiments in water since it is so much more environmentally friendly, green chemistry. Since water is so common as a solvent, we have a special name for solutions that use water as the solvent, aqueous. Remember that it can be abbreviated “aq” when labeling species in a chemical CHE 170 Packet 4 - 30 reaction. Solution Composition solute: ; it is normally the component of a solution present in the smaller amount Tro page 127 solvent: ; it is normally the component of a solution present in the greater amount solution: a mixture of substances that has a uniform composition; a homogeneous mixture CHE 170 Packet 4 - 31 Different kinds of Solutions A solution is a homogeneous mixture of two or more substances in a single phase: solid, liquid, gas. Rubies are a solution of red chromium compounds with transparent aluminum oxide. Fillings are also a metal solution as are all metal alloys! When we blow up a balloon, we are filling it with a gaseous solution of O2, CO2 and other gases. Mixing sugar into water makes a sugar water solution once the sugar dissolves. CHE 170 Packet 4 - 32 Solubility Solubility is how well a solute dissolves in a solvent to form a . The more soluble or miscible something is in a solvent, the it is to make a solution. Sometimes something is so soluble we could put large amounts in. Others are so insoluble that maybe one molecule dissolves in a swimming pool full of solvent. What determines a compound’s solubility in a particular solvent? CHE 170 Packet 4 - 33 Aqueous Solutions of Ionic versus Molecular Compounds Although many solutes are soluble/miscible in water, there are two different ways they can dissolve in it. 1) They can separate into ions. What are some examples? 2) The solute can be attracted to the solvent. (Preview of intermolecular forces we’ll learn later.) What are some examples (no ionic compounds!)? Once again we have special terms; compounds of type 1 are called electrolytes and type 2 are called nonelectrolytes. CHE 170 Packet 4 - 34 What ions do we get? Remember how we learned to put together ions using charge balance? Pb2+ and S2– would form? Ca2+ and F– would form? Al3+ and O2– would form? Let’s now reverse that and break them apart! What ions would these soluble compounds form in water? NaOH Pb(NO3)2 Al2(SO4)3 CHE 170 Packet 4 - 35 How does water dissolve something? First, the shape of water makes one side of it partially negative and the other side partially positive. Ions are either positive or negative, right? So, the “negative” side of water surrounds cations, and the “positive” side of water surrounds the anions as we see in this bottom picture. Tro page 133 CHE 170 Packet 4 - 36 Ionic Compounds Let’s look at an example of an ionic compound, potassium permanganate, dissolving/dissociating in water: What’s the formula? So, the ions formed in the solution would be? Ionic compounds are also electrolytes, let’s examine what this means… CHE 170 Packet 4 - 37 Electrolytes & Nonelectrolytes Electrolytes when put into an aqueous solution form what? Do nonelectrolytes form them? Pure water does not conduct electricity. Water with only nonelectrolytes in it does not conduct electricity. Water with electrolytes present does conduct electricity. Why? Hint: what do electrolytes do that pure water and nonelectrolytes don’t? CHE 170 Packet 4 - 38 Here’s a picture that represents what happens to the ions when electricity is put through a solution of ions. The movement of the ions allows for the electrons to flow. If there weren’t ions, electrons would be unable to move through the solution. CHE 170 Packet 4 - 39 Notice each picture below represents what is happening at the submicroscopic level. We can see the water molecules interacting with the ions or molecules present. 2+ 2+ 2+ CHE 170 Packet 4 - 40 These pictures are also trying to represent what is happening at the submicroscopic level, but the water molecules have been omitted for clarity. CHE 170 Packet 4 - 41 Electrolytes, Weak Electrolytes and Nonelectrolytes Okay, so we now know the difference between electrolytes and nonelectrolytes. But, what makes one electrolyte strong and another weak?!?! Strong electrolytes dissociate (break apart) or ionize , and thus, are . conductors of electricity. Experimental proof: . Weak electrolytes dissociate or ionize and thus, are .conductors of electricity. Experimental proof: Nonelectrolytes dissociate or ionize and thus, are .conductors of electricity. Experimental proof: , . , . Remember! Strong and weak do not refer to concentration in this context! CHE 170 Packet 4 - 42 Electrolytes Handout Strong Electrolytes – ions in solution 1) All soluble salts (even if only slightly soluble). 2) Strong bases – Group 1A and 2A hydroxides 3) Strong acids – HCl, HBr, HI, HNO3, HClO4, H2SO4 Weak Electrolytes – ions & molecules in solution 1) None (salts are either soluble or they’re not) 2) Weak bases – NH3, etc. 3) Weak acids – CH3CO2H, etc. Nonelectrolytes – no ions, just molecules (or nothing) in sol’n 1) Insoluble salts 4) All other compounds not already covered. CHE 170 Packet 4 - 43 Solubility Rules Tro page 136 CHE 170 Packet 4 - 44 Solubility Rules handout Solubility Rules Learn how to use solubility tables! A table similar to the one from our book (shown on previous slide) will be provided on exams (a copy of it is on the notes page). However it will be missing the… Key rule – memorize! Salts of alkali metals, ammonium, nitrate, and acetate are almost always soluble (no common exceptions). SOLUBLE COMPOUNDS Almost all salts of Na+, K+, NH4+ Salts of nitrate, NO3– chlorate, ClO3– perchlorate, ClO4– acetate, CH3CO2– EXCEPTIONS Almost all salts of Cl–, Br–, I– Halides of Ag+, Hg22+, Pb2+ Compounds containg F– Fluorides of Mg2+, Ca2+, Sr2+, Ba2+, Pb2+ Salts of sulfate, SO42– Sulfates of Mg2+, Ca2+, Sr2+, Ba2+, Pb2+ INSOLUBLE COMPOUNDS EXCEPTIONS All salts of carbonate, CO32– phosphate, PO43– oxalate, C2O42– Salts of NH4+ and the alkali metal cations chromate, CrO42– Most metal sulfides, S2– Most metal hydroxides and oxides See notes page for the solubility table provided during exams. Ba(OH)2 is soluble CHE 170 Packet 4 - 45 Concept Area V: Writing Different Types of Chemical Reactions a. You should be able to write a complete balanced equation for the following types: molecular/complete formula, complete ionic, and net ionic. b. You should be able to recognize a precipitation forming reaction and predict the identity of the precipitate that forms. c. You should be able to complete a precipitation forming reaction. CHE 170 Packet 4 - 46 AB + CD → AD + CB This type of reaction can be called an exchange reaction, double displacement or metathesis. There are three basic types: a.) precipitation, b.) acid-base, and c.) gas-forming. All involve the exchange of ions to form a new . As my high school chemistry instructor liked to say, “Switch partners and dance! ” CHE 170 Packet 4 - 47 a.) Precipitation Reactions Example: 2 KI(aq) + Pb(NO3)2(aq) → 2 KNO3(aq) + PbI2↓ A precipitate is an insoluble compound formed by a reaction in solution, usually aqueous. The precipitate forms a , that then falls to the bottom due to gravity. We use solubility rules to predict what will happen when two soluble salts are combined. The formation of a precipitate will drive a reaction to completion. Why? Well, why would the following not really happen. Think submicroscopically! NH4OH(aq) + NaCl(aq) → NaOH(aq) + NH4Cl(aq) Tro page 137 CHE 170 Packet 4 - 48 a.) Precipitation Reactions 2 AgNO3(aq) + CaCl2(aq) → Ca(NO3)2(aq) + 2 AgCl ↓ CHE 170 Packet 4 - 49 How to predict precipitates when solutions of two ionic compounds are mixed Step 1 Write the reactants as they actually exist before any reaction occurs (the complete ionic equation). Remember that when a salt dissolves, its ions completely separate. Step 2 Consider the various solids that could form. To do this, simply exchange the anions (or the cations) of the added salts. Step 3 Use the solubility rules to decide whether a solid forms and, if so, to predict the identity of the solid. CHE 170 Packet 4 - 50 Writing Chemical Equations, Part II Molecular or complete formula – previously used when we learned how to write chemical equations. Note that ionic reactions should use the term complete formula, but most people just call it molecular. Complete Ionic – just like above, only all soluble ionic compounds are written as separate ions as they actually exist, not together Net Ionic –just like complete ionic only ions that don’t change (spectators) aren’t written Beginning on the next slide, let’s work an example CHE 170 Packet 4 - 51 to help us learn how to use! Molecular or Complete Formula Equations AgNO3(aq) + CaCl2(aq) → This is a complete formula equation; it’s not technically a molecular equation because we have ions and not molecules. But, remember that most times people will go ahead and call it a molecular equation anyway. Note that we have written everything together as formulas. We’ve even shown what state the chemicals are in to make it complete. CHE 170 Packet 4 - 52 2 AgNO3(aq) + CaCl2(aq) → 2 AgCl(s)+ Ca(NO3)2(aq) Complete Ionic Equations Note that here we wrote everything how it actually exists in the solution. The soluble salts ionize and are written as ions. The insoluble salts don’t ionize and are written with the ions together. The states may still be put in; we didn’t have room here. So in a complete ionic reaction, separate all the ionic compounds that are “aq” and leave together all the “s”, “ℓ” and “g”! Could we write one of these for a molecular equation? CHE 170 Packet 4 - 53 2 Ag+ + 2 NO3– + Ca2+ + 2 Cl– → 2 AgCl + Ca2+ + 2 NO3– Net Ionic Equations Note here we only wrote those species that were different in products and reactants. The rest did not participate, they watched. So, they are called spectator ions. CHE 170 Packet 4 - 54 Your Turn! Write all three kinds of equations for the following: 1. Al(NO3)3(aq) + 3 NaOH(aq) → Al(OH)3(s) + 3 NaNO3(aq) complete ionic net ionic 2. When aqueous solutions of copper(II) nitrate and potassium carbonate are mixed, a precipitate forms. complete formula complete ionic net ionic CHE 170 Packet 4 - 55 Helpful Tips How do we know what state something is when the problem doesn’t say and we’re expected to know? Remember, that many periodic tables color-code to indicate what state the elements are typically found in. Most pure covalent compounds are gases or liquids. Most pure ionic compounds are solids. If the reaction is done in water (may say aqueous or solution to indicate), most of the compounds will be aqueous except… insoluble salts which will be a solid precipitate. covalent gases which will bubble away as a gas. CHE 170 Packet 4 - 56 Concept Area V: Acids & Bases a. You should be able to recognize acids and bases and the reactions involving them. b. You should be able to write a molecular, complete ionic and net ionic reaction for acid-base reactions. c. You should be able to complete an acid-base reaction. d. You should know the difference between strong/weak acids and bases and give common examples of each. e. You should be able to recognize a gas-forming reaction. CHE 170 Packet 4 - 57 Arrhenius Acid & Base Definitions base: acid: CHE 170 Packet 4 - 58 Acids and Bases Handout Some Common Acids & Bases Acids HCl HNO3 H2SO4 H2CO3 H3PO4 CH3CO2H hydrochloric acid nitric acid sulfuric acid carbonic acid phosphoric acid acetic acid Bases NaOH KOH Ca(OH)2 Mg(OH)2 Ba(OH)2 NH3 sodium hydroxide potassium hydroxide calcium hydroxide magnesium hydroxide barium hydroxide ammonia Which ones are strong? Which ones are weak? CHE 170 Packet 4 - 59 Diagram that shows relative strengths of a few acids and bases CHE 170 Packet 4 - 60 b.) Acid-Base Reactions Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(ℓ) ACID + BASE YIELDS SALT & “WATER”! Since the final product is neither acidic or basic (if perfect mol:mol ratio), this reaction can also be called a reaction. The driving force of this reaction is the formation of a covalent compound, water – a liquid. Acid-base reactions are common, and once we recognize are much easier to solve! CHE 170 Packet 4 - 61 2 H2O H3O+ + OH– Water is in constant equilibrium between hydronium ions and hydroxide ions. A proton, H+, never exists alone in an aqueous solution, it is always a hydronium ion, H3O+. However, chemists will write simply H+ in an aqueous solution as a shorthand. We may also use this shorthand when presence of the hydronium ion itself is not important. Just remember when writing H+, it’s really H3O+! – CHE 170 Packet 4 - 62 Acids & Bases We’ve mentioned strong acids/bases and weak acids/bases. What determines which type is which again? Complete the chemical equations for the following: HCl + H2O b) CH3CO2H + H2O c) NaOH + H2O d) NH3 + H2O a) CHE 170 Packet 4 - 63 Completing Acid-Base Reactions Complete the following acid-base reactions. Remember, acid + base yields salt + “water”! 1. HCl(aq) + NaOH(aq) → 2. NH3(aq) + HBr(aq) → 3. CH3CO2H(aq) + KOH(aq) → CHE 170 Packet 4 - 64 Strong vs. Weak versus Concentrated vs. Dilute Science uses these terms differently than non-science uses. Do you know how to use scientifically? Label the following strong/weak and concentrated/dilute appropriately. 1. 2. 3. 4. 5. 6. 7. 0.001 M NaCl (aq) 12.0 M HCl (aq) 6.0 M CH3COOH (aq) 0.05 M NaOH (aq) 8.0 M RbOH (aq) 0.0001 M CaSO4(aq) 0.10 M NH3 (aq) CHE 170 Packet 4 - 65 Practice! Complete the equations below. Then, for each equation, label strong, weak, or non electrolyte, and draw a picture of the solution. The first is done as an example. H2O K3PO4(s) → 3 K+(aq) +PO43–(aq) soluble salt → strong electrolyte H2 O HNO3(ℓ) → H+(aq) +NO3–(aq) H2O C2H3OH(ℓ) → C2H3OH (aq) CHE 170 Packet 4 - 66 And, more practice! 1. Which is the best electrical conductor? Why? a. b. c. d. 0.10 M NaCl 0.10 M CH3CH2OH 0.10 M CH3COOH 0.10 M C5H12O6 2. Which has the highest H+ concentration? Why? a. b. c. d. 0.10 M HCl 0.10 M H2SO4 0.10 M CH3COOH 0.10 M NH3 CHE 170 Packet 4 - 67 c.) Gas-Forming Reactions Example: CuCO3(s) + 2 HNO3(aq) → Cu(NO3)2(aq) + H2CO3(aq) H2CO3(aq) CO2(g) + H2O(ℓ) Notice that one of our products (H2CO3) was able to continue reacting to form a gas. The driving force for this reaction is the gas being evolved in the second step. What would the net ionic equation be? a) CO32– + 2 H+ CO2 + H2O b) CuCO3 + 2 H+ Cu2+ + CO2 + H2O CHE 170 Packet 4 - 68 Examples of Gas-Forming Reactions Tro page 144 CHE 170 Packet 4 - 69 Concept Area VI: Redox a. You should be able to recognize an oxidationreduction, redox, reaction b. You should be able to calculate oxidation numbers for each atom in a chemical species. c. You should be able to identify: what’s oxidized what’s reduced what gains electrons what loses electrons the oxidizing agent the reducing agent CHE 170 Packet 4 - 70 Oxidation-Reduction Reactions, Redox Remember how some elements, especially transition metals, can form differently charged ions? Well, sometimes they change because of a chemical reaction! Those charges are called oxidation states. Why? Well an atom/ion can change its oxidation state and does not have to ever be an ion by itself. Plus, it doesn’t make sense to refer to the charge on an ion in a molecule! When an atom/ion changes oxidation states, what is happening to it? CHE 170 Packet 4 - 71 Oxidation-Reduction Reactions, Redox So, when atoms/ions change their oxidation number by gaining or losing electrons they become reduced or oxidized, respectively. If one of the reactants loses electrons, the other reactant must gain them, right? We’ll be sorting out all this vocabulary in a bit….. But, Can oxidation occur without reduction? Can something lose electrons when nothing gains electrons? Can reduction occur without oxidation? Can something gain electrons when nothing loses electrons? CHE 170 Packet 4 - 72 Common Oxidizing and Reducing Agents CHE 170 Packet 4 - 73 Assigning Oxidation Numbers So far, we’ve always treated polyatomic ions as a unit. However, in a redox reaction that unit can be broken apart. So, we have to be able to determine the oxidation numbers/states on the atoms within the polyatomic ions. While we’re at it how do we assign these numbers to anything? CHE 170 Packet 4 - 74 Assigning Oxidation Numbers Handout Rules for Assigning Oxidation Numbers 1) The oxidation number of any free, uncombined element is zero. Thus the atoms in Cl2, O3, S8, C and Xe all have the oxidation number of zero. 2) The oxidation number of monatomic ions is the same as the charge on that ion. So, you already know the oxidation numbers for alkali and alkaline earth metals, halogens and others you memorized. 3) Hydrogen’s oxidation number is almost always +1. The only exceptions are when hydrogen is in its elemental state, and when hydrogen combines with a metal to form a hydride (where it would be –1). 4) Oxygen’s oxidation number is almost always –2. If you have a peroxide the charge would be –1, and when it’s in its CHE 170 Packet 4 - 75 elemental state the charge would be zero. Rules for Assigning Oxidation Numbers continued 5) Fluorine always has an oxidation number of –1 in compounds. 6) The sum of all the oxidation numbers in a species must equal the charge on that species. So, in an uncharged species, the sum of all the oxidation numbers must be zero; in an ion, the sum of all the oxidation numbers must equal the charge on that ion. 7) The most electronegative element in a compound has a negative oxidation number. 8) Use “charge balance” to obtain oxidation numbers of all other atoms in the species. CHE 170 Packet 4 - 76 An example of a redox reaction… molecular (or complete formula) equation CuSO4(aq) + Mg(s) → MgSO4(aq) + Cu(s) complete ionic equation Cu2+ + SO42– + Mg → Mg2+ + SO42– + Cu Oxidation net ionic equation number of … Cu2+ + Mg → Mg2+ + Cu Now, let’s complete the table below! oxidation number as reactant copper sulfur oxygen magnesium oxidation number as product CHE 170 Packet 4 - 77 Cu2+ + Mg → Mg2+ + Cu Vocabulary time! Copper’s oxidation number went from +2 to 0, so it gained electrons. Therefore, it was reduced. Which makes sense since copper’s oxidation number got smaller (was reduced in size). Since copper(II) ion was reduced, it underwent the process of reduction. Copper(II) ion had to get those electrons from somewhere; so, it caused something else to be oxidized or lose electrons. Thus, it is also known as the oxidizing agent or the oxidizer! Fun, eh? Now you try with Magnesium! CHE 170 Packet 4 - 78 Cu2+ + Mg → Mg2+ + Cu Your turn with the vocabulary: Magnesium’s oxidation number went from , so it . Therefore, it was . Since magnesium was , it underwent the process of . Magnesium electrons didn’t just disappear, they went to another species; so, it caused something else to and get . Thus, magnesium is also the or the . ! Get it? CHE 170 Packet 4 - 79 Useful mnemonic device! LEO the lion goes GER CHE 170 Packet 4 - 80 Or we can use this mnemonic device! OIL RIG CHE 170 Packet 4 - 81 Redox Reactions Handout Answer the following: 1. SnO2 + 2 C → Sn + 2 CO oxidized? reduced? gained electrons? lost electrons? oxidizing agent? reducing agent? 2. Cu + 2 AgNO3 → Cu(NO3)2 + 2 Ag oxidized? reduced? gained electrons? lost electrons? oxidizing agent? reducing agent? Proper practice makes perfect for these! CHE 170 Packet 4 - 82 Summary of Reaction Types CHE 170 Packet 4 - 83 Combustion Burning something is also redox: CH4 + 2 O2 → CO2 + 2 H2O + energy Normally, burning requires oxygen and we produce carbon dioxide and water. Carbon monoxide, CO, is formed when combustion is incomplete – there is not enough oxygen present. CHE 170 Packet 4 - 84 Summary of Common reaction Types with their Driving Force! Reaction Type Driving Force Precipitation Formation of an insoluble compound (see section 4.6) Acid-base / neutralization Formation of water and/or proton transfer (see section 4.8) Gas-forming Evolution of a water-insoluble gas such as CO2 (see section 4.8) Oxidation-reduction Electron transfer (see section 4.9) CHE 170 Packet 4 - 85 Concept Area VIII: Solution Stoichiometry a. You should be able to interconvert between grams, moles, Molarity and volume of solution of reactants or products in a balanced chemical equation. b. You should be able to calculate dilutions. c. You should be able to use titration data to calculate the Molarity of an unknown solution. d. You should be able to use and understand the pH scale (this includes calculating pH from H+ and H+ from pH). CHE 170 Packet 4 - 86 Expanded General Plan for Stoichiometric Calculations Step 1) write a balanced chemical reaction. Step 2) write down what we know Step 3) write down what we want to know Step 4) solve the problem using our new and improved roadmap! Mass reactant Mass product molar mass(es) from periodic table molar mass(es) from periodic table Moles reactant Volume Molarity reactant use stoichiometric factor from balance chemical equation Moles product Volume Molarity product CHE 170 Packet 4 - 87 Let’s try our newest roadmap out! Metallic zinc reacts with hydrochloric acid to produce hydrogen and ZnCl2. What volume of 2.50 M HCl (in mL) is required to completely consume 11.8 g of zinc? Which of these solutions is correct? 1 2 1.00 11.8 0.144 mL 65.39 1 2.50 1 mol 1 mol 1.00 L 1000 mL 11.8 g 36.1 mL 65.39 g 2 mol 2.50 mol 1L 11.8 g Zn 1 mol Zn 2 mol HCl 1.00 L HCl sol' n 1000 mL HCl 144 mL 65.39 g Zn 1 mol Zn 2.50 mol HCl 1 L HCl CHE 170 Packet 4 - 88 Dilutions: M1V1=M2V2 How do we make 500 mL of a 0.00100 M solution of K2Cr2O7 if we only have a 0.100 M K2Cr2O7 and water to make it from? M1V1 M 2 V2 (0.100 M)(V1 ) (0.00100 M)(500 mL) (0.00100 M)(500 mL) V1 (0.100 M) V1 5.00 mL Ah, so we used 5.00 mL of the 0.100 M solution and diluted it to 500 mL (with approx. 495 mL of water) to make our solution! Note: This formula is only to be used with dilutions! If mixing an acid and a base (a reaction), this formula CHE 170 Packet 4 - 89 cannot be used! How to do a dilution in the lab: CHE 170 Packet 4 - 90 How do we determine the concentration of an acid using a base of known concentration or vice versa? Known volume of a base of known concentration slowly added to the acid. Known volume of an acid of unknown concentration. By doing a titration! Titrations are used to determine the concentration of an unknown compound (usually an acid or base) using a compound of known concentration. Usually, a titration is where we take a known volume of an unknown concentration of acid and slowly react it with a known volume and concentration of a base (or vice versa). In lab, we use a buret to deliver the base into an Erlenmeyer flask or beaker holding the acid. CHE 170 Packet 4 - 91 A titration in a lab… CHE 170 Packet 4 - 92 A titration in a lab explained. 1. Add solution from the buret, usually base. 2. The reagent (base) reacts with compound (acid) in solution in the flask. 3. Indicator shows (by changing colors) when the exact stoichiometric reaction has occurred if correct indicator chosen. 4. Net ionic equation: H+ + OH– → H2O. 5. At the equivalence point we have a perfect mol:mol ratio. So, moles H+ = moles OH –. CHE 170 Packet 4 - 93 Titration Problem: So, if 38.55 mL of HCl is used to titrate 2.150 g of Na2CO3 according to the following equation, what is the Molarity of the HCl? Na2CO3(aq) + 2 HCl(aq) → 2 NaCl(aq) + CO2(g) + H2O(ℓ) CHE 170 Packet 4 - 94 Some Problems for Practice! 26 g 1. If we react 1.0 L of 0.44 M HCl with 30 g NaOH how many grams of NaCl can we make? 2. What is the Molarity of sulfuric acid if it takes 20.03 0.267 M mL of 0.100 M NaOH to neutralize 15.00 mL of the acid? 3. If we react 3.25 g of zinc with excess 5.00 M 0.100 g hydrochloric acid at STP, how much hydrogen gas (1.11 L) will be produced in grams (and liters)? Note, the other product formed is ZnCl2. 4. What is the Molarity of a solution formed when 15.0 1.50 M mL of 5.00 M HCl is diluted to 50.00 mL? See notes page for these problems worked out. CHE 170 Packet 4 - 95 The End of Packet 4 Any Questions? CHE 170 Packet 4 - 96