Chapter 4 Aqueous Reactions and Solution Stoichiometry Chemistry, The Central Science, 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten Ch4 Hmwk # 7, 8, 11, 15, 19, 23, 25, 27, 29, 31, 37, 39, 41, 45, 47, 51, 53, 59, 63, 67, 69, 75, 96 Chapter 4 Aqueous Reactions and Solution Stoichiometry John D. Bookstaver St. Charles Community College St. Peters, MO  2006, Prentice Hall, Inc.

Aqueous Reactions and Solution Stoichiometry What is aqueous? What is an aqueous reaction? What are some types of aqueous reactions? Why might these be important? Give examples. Aqueous reactions cannot take place without water. What do you already know about water that will help us understand aqueous reactions?

Solutions: Homogeneous mixtures of two or more pure substances. The solvent is present in greatest abundance. All other substances are solutes.

Dissociation When an ionic substance dissolves in water, the solvent pulls the individual ions from the crystal and solvates them. This process is called dissociation.

Electrolytes Substances that dissociate into ions when dissolved in water. Example: NaCl A nonelectrolyte may dissolve in water, but it does not dissociate into ions when it does so. Example: C12H22O11 Notice, solubility does not imply that something is or is not an electrolyte.

Electrolytes and Nonelectrolytes Soluble ionic compounds tend to be electrolytes.

Electrolytes and Nonelectrolytes Molecular compounds tend to be nonelectrolytes, except for acids and bases.

SAMPLE EXERCISE 4.1 Relating Relative Numbers of Anions and Cations to Chemical Formulas The diagram below represents an aqueous solution of one of the following compounds: MgCl2 , KCl, or K2SO4. Which solution does it best represent?

SAMPLE EXERCISE 4.1 Relating Relative Numbers of Anions and Cations to Chemical Formulas The diagram below represents an aqueous solution of one of the following compounds: MgCl2 , KCl, or K2SO4. Which solution does it best represent? Solution   Analyze: We are asked to associate the charged spheres in the diagram with ions present in a solution of an ionic substance. Plan: We examine the ionic substances given in the problem to determine the relative numbers and charges of the ions that each contains. We then correlate these charged ionic species with the ones shown in the diagram. Solve: The diagram shows twice as many cations as anions, consistent with the formulation K2SO4. Check: Notice that the total net charge in the diagram is zero, as it must be if it is to represent an ionic substance.

Electrolytes A strong electrolyte dissociates completely when dissolved in water. A weak electrolyte only dissociates partially when dissolved in water.

Strong Electrolytes Are… Strong acids

Strong Electrolytes Are… Strong acids Strong bases

Strong Electrolytes Are… Strong acids Strong bases Soluble ionic salts

Precipitation Reactions When one mixes ions that form compounds that are insoluble (as could be predicted by the solubility guidelines), a precipitate is formed.

Metathesis (Exchange) Reactions a.k.a. Double Replacement Rxns Metathesis comes from a Greek word that means “to transpose” AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)

Metathesis (Exchange) Reactions a.k.a. Double Replacement Rxns Metathesis comes from a Greek word that means “to transpose” It appears the ions in the reactant compounds exchange, or transpose, ions AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)

Metathesis (Exchange) Reactions Metathesis comes from a Greek word that means “to transpose” It appears the ions in the reactant compounds exchange, or transpose, ions AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)

How did we know that silver chloride was a solid??? AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq) How did we know that silver chloride was a solid??? We checked the solubility table.

Figure: 04-T01 Title: Table 4.1 Caption: Solubility Guidelines for Common Ionic Compounds in Water

Predict the products of the following reactions BaCl2 (aq) + K2SO4 (aq) Fe2(SO4)3 (aq) + LiOH (aq) Mg(NO3)2 (aq) + CaS (aq) NaCl (aq) + KNO3 (aq)

Predict the products of the following reactions BaCl2 (aq) + K2SO4 (aq) → BaSO4 (s) + 2KCl (aq) Fe2(SO4)3 (aq) + 6LiOH (aq) → 2Fe(OH)3 (s) + 3Li2SO4 (aq) Mg(NO3)2 (aq) + CaS (aq) → MgS (s) + Ca(NO3)2 (aq) NaCl (aq) + KNO3 (aq) → NaNO3 (aq) + KCl (aq) or NR

Solution Chemistry It is helpful to pay attention to exactly what species are present in a reaction mixture (i.e., solid, liquid, gas, aqueous solution). If we are to understand reactivity, we must be aware of just what is changing during the course of a reaction.

AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq) Molecular Equation The molecular equation lists the reactants and products in their molecular form. AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)

Ionic Equation In the ionic equation all strong electrolytes (strong acids, strong bases, and soluble ionic salts) are dissociated into their ions. This more accurately reflects the species that are found in the reaction mixture. Ag+ (aq) + NO3- (aq) + K+ (aq) + Cl- (aq)  AgCl (s) + K+ (aq) + NO3- (aq)

Net Ionic Equation To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq)  AgCl (s) + K+(aq) + NO3-(aq)

Ag+(aq) + Cl-(aq)  AgCl (s) Net Ionic Equation To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. The only things left in the equation are those things that change (i.e., react) during the course of the reaction. Ag+(aq) + Cl-(aq)  AgCl (s)

Net Ionic Equation To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. The only things left in the equation are those things that change (i.e., react) during the course of the reaction. Those things that didn’t change (and were deleted from the net ionic equation) are called spectator ions. Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq)  AgCl (s) + K+(aq) + NO3-(aq)

Writing Net Ionic Equations Write a balanced molecular equation. Dissociate all strong electrolytes. Cross out anything that remains unchanged from the left side to the right side of the equation. Write the net ionic equation with the species that remain.

Writing Net Ionic Equations Write the net ionic equation for each reaction Pb(NO3)2 (aq) + KI (aq) Li2S (aq) + BaBr2 (aq) NH4Cl (aq) + Sr(OH)2 (aq)

Writing Net Ionic Equations Write the net ionic equation for each reaction Pb(NO3)2 (aq) + 2KI (aq) → PbI2 (s) + 2K(NO3) (aq) Pb2+ + 2I- → PbI2 (s) Li2S (aq) + BaBr2 (aq) → LiBr (aq) + BaS (aq) 2Li+ (aq) + S-2 (aq) + Ba2+ (aq) + 2Br- (aq) → 2Li+ (aq) + 2Br- (aq) + Ba2+ (aq) + S-2 (aq) Notice ….all spectator ions. They all cancel and therefore, no reaction occurs. 2NH4Cl (aq) + Sr(OH)2 (aq) → SrCl2 (aq) + 2NH4OH (aq) 2NH4+ (aq) + 2Cl- (aq) + Sr2+ (aq) + 2OH- (aq) → Sr2+ (aq) + 2Cl- (aq) + 2NH4+ (aq) + 2OH- (aq)

Acids: Substances that increase the concentration of H+ when dissolved in water (Arrhenius). Proton donors (Brønsted–Lowry).

Acids There are only seven strong acids: Hydrochloric (HCl) Hydrobromic (HBr) Hydroiodic (HI) Nitric (HNO3) Sulfuric (H2SO4) Chloric (HClO3) Perchloric (HClO4) These substances are consider strong acids because they do not just dissolve in water, they will completely dissociate.

Bases: Substances that increase the concentration of OH− when dissolved in water (Arrhenius). Proton acceptors (Brønsted–Lowry).

Bases The strong bases are the soluble salts of hydroxide ion: Alkali metals Calcium Strontium Barium

Acid-Base Reactions In an acid-base reaction, the acid donates a proton (H+) to the base.

Neutralization Reactions Generally, when solutions of an acid and a base are combined, the products are a salt and water. HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l)

Neutralization Reactions When a strong acid reacts with a strong base, the net ionic equation is… HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l) H+ (aq) + Cl- (aq) + Na+ (aq) + OH-(aq)  Na+ (aq) + Cl- (aq) + H2O (l)

Neutralization Reactions When a strong acid reacts with a strong base, the net ionic equation is… HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l) H+ (aq) + Cl- (aq) + Na+ (aq) + OH-(aq)  Na+ (aq) + Cl- (aq) + H2O (l) H+ (aq) + Cl- (aq) + Na+ (aq) + OH- (aq)  H+ (aq) + OH- (aq) → H2O (l)

Neutralization Reactions What does this have to do with you??? What happens when you have a stomach ache? What do you do about it? Why?

Neutralization Reactions What does this have to do with you??? Many antacids contain magnesium hydroxide. It undergoes a neutralization reaction with stomach acid. What acid constitutes the largest quantity of stomach acid? Write a net ionic equation for the neutralization reaction between magnesium hydroxide and hydrochloric acid. Mg(OH)2 (s) + H+ (aq) → Mg2+ (aq) + H2O (l)

Neutralization Reactions Observe the reaction between Milk of Magnesia, Mg(OH)2, and HCl. Can you overdoes on antacid?? What do you think would happen?

Gas-Forming Reactions These metathesis reactions do not give the product expected. The expected product decomposes to give a gaseous product (CO2 or SO2). CaCO3 (s) + HCl (aq) CaCl2 (aq) + CO2 (g) + H2O (l) NaHCO3 (aq) + HBr (aq) NaBr (aq) + CO2 (g) + H2O (l) SrSO3 (s) + 2 HI (aq) SrI2 (aq) + SO2 (g) + H2O (l)

Gas-Forming Reactions This reaction gives the predicted product, but you had better carry it out in the hood, or you will be very unpopular! Just as in the previous examples, a gas is formed as a product of this reaction: Na2S (aq) + H2SO4 (aq)  Na2SO4 (aq) + H2S (g)

A Carbonated Beverage is Another Example How is a beverage carbonated? What happens to the dissolved gas? H2O (l) + CO2 (g) ↔ H2CO3 (aq) H2O (l) + CO2 (g) ↔ 2H+ (aq) + CO3-2 (aq)

Oxidation-Reduction Reactions An oxidation occurs when an atom or ion loses electrons. (It becomes more positive) A reduction occurs when an atom or ion gains electrons. (It becomes more negative)

Oxidation-Reduction Reactions One cannot occur without the other.

Oxidation Numbers To determine if an oxidation-reduction reaction has occurred, we assign an oxidation number to each element in a neutral compound or charged entity.

Oxidation Numbers Elements in their elemental form have an oxidation number of 0. The oxidation number of a monatomic ion is the same as its charge.

Oxidation Numbers Nonmetals tend to have negative oxidation numbers, although some are positive in certain compounds or ions. Oxygen has an oxidation number of −2, except in the peroxide ion in which it has an oxidation number of −1. Hydrogen is −1 when bonded to a metal, +1 when bonded to a nonmetal.

Oxidation Numbers Nonmetals tend to have negative oxidation numbers, although some are positive in certain compounds or ions. Fluorine always has an oxidation number of −1. The other halogens have an oxidation number of −1 when they are negative; they can have positive oxidation numbers, however, most notably in oxyanions.

Oxidation Numbers The sum of the oxidation numbers in a neutral compound is 0. The sum of the oxidation numbers in a polyatomic ion is the charge on the ion.

Oxidation Numbers Find the oxidation numbers for each of the following species. CO32- H2S S8 SCl2 SO42- Cr2O72-

CO32- C = +4 and O = -2 H2S H = +1 and S = -2 S8 S = 0 SCl2 S = +2 and Cl = -1 SO42- S = +6 and O = -2 Cr2O72- Cr = +6 and O = -2

Examine each Reaction below and Identify the oxidized and reduced reagents 2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g) Fe2O3 (s) + 2Al (s) → 2Fe (l) + Al2O3 (s) Si (s) + 2Cl2 (g) → SiCl4 (l)

2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g) Examine each Reaction below and Identify the oxidized and reduced agents 2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g) K (s) → K+ (aq), it was oxidized; Hydrogen was reduced in water to form hydrogen gas. Fe2O3 (s) + 2Al (s) → 2Fe (l) + Al2O3 (s) Aluminum metal is oxidized; iron is reduced Si (s) + 2Cl2 (g) → SiCl4 (l) Si is oxidized while Cl is reduced

Which three of the following reactions are Red-Ox Which three of the following reactions are Red-Ox? What is oxidized and what is reduced? Zn (s) + 2NO3- (aq) + 4H+ (aq) → Zn+2 (aq) + 2NO2 (g) + 2H2O (l) Zn(OH)2 (s) + H2SO4 (aq) → ZnSO4 (aq) + 2H2O (l) Ca (s) + 2H2O (l) → Ca(OH)2 (s) + H2 (g) 4Fe(OH)2 (s) + 2H2O (l) + O2 (g) → 4Fe(OH)3 (aq)

Which three of the following reactions are Red-Ox Which three of the following reactions are Red-Ox? What is oxidized and what is reduced? Zn (s) + 2NO3- (aq) + 4H+ (aq) → Zn+2 (aq) + 2NO2 (g) + 2H2O (l) Red-ox: zinc is oxidized, nitrogen is reduced Zn(OH)2 (s) + H2SO4 (aq) → ZnSO4 (aq) + 2H2O (l) Not Red-ox Ca (s) + 2H2O (l) → Ca(OH)2 (s) + H2 (g) Red-ox: calcium is oxidized, hydrogen is reduced 4Fe(OH)2 (s) + 2H2O (l) + O2 (g) → 4Fe(OH)3 (aq) Red-ox: Iron is oxdized, oxygen is reduced

Displacement Reactions a.k.a. Single Replacement Reactions In displacement reactions, ions oxidize an element. The ions, then, are reduced.

Displacement Reactions In this reaction, silver ions oxidize copper metal. Cu (s) + 2 Ag+ (aq)  Cu2+ (aq) + 2 Ag (s)

Displacement Reactions The reverse reaction, however, does not occur. Cu2+ (aq) + 2 Ag (s)  Cu (s) + 2 Ag+ (aq) x

Activity Series

Al (s) + HBr (aq) Fe(s) + KI (aq) Zn (s) + Ni(NO3)2 (aq) Use the Activity Series to predict the products of the following reactions. If the reactions does take place, write the net ionic equation for the reaction. Lastly, identify the species being oxidized and reduced. Al (s) + HBr (aq) Fe(s) + KI (aq) Zn (s) + Ni(NO3)2 (aq)

2Al (s) + 6HBr (aq) → 2AlBr3 (aq) + 3H2 (g) 2Al (s) + 6H+ (aq) → 2Al3+ (aq) + 3H2(g) Aluminum is oxidized and Hydrogen is reduced. Fe(s) + KI (aq) No Reaction Zn (s) + Ni(NO3)2 (aq) → Zn(NO3)2 (aq) + Ni (s) Zn (s) + Ni2+ (aq) → Zn2+ (aq) + Ni (s) Zinc is oxidized and Nickel is reduced.

volume of solution in liters Molarity Two solutions can contain the same compounds but be quite different because the proportions of those compounds are different. (For example, it would be much more dangerous to spill a high concentration of hydrochloric acid on your hand than a low concentration) Molarity is one way to measure the concentration of a solution. moles of solute volume of solution in liters Molarity (M) =

Mixing a Solution How many grams of silver nitrate are required to make 100 mL of a 0.5 M solution? How many grams of NaOH are required to make 50 mL of an 8.2 M solution?

Mixing a Solution How many grams of silver nitrate are required to make 100 mL of a 0.5 M solution? 9 grams How many grams of NaOH are required to make 50.0 mL of an 8.2 M solution? 16 grams

Dilution M1V1 = M2V2

Dilution You have 1L of a 1.00 M stock solution of hydrochloric acid. You need to make 500 mL of a 0.20M solution. How much of the stock solution is needed? You need to make 100 mL of 0.50 M nitric acid. The storage bottle reads 6.0M. How much of the 6.0M nitric acid will you need?

Dilution You have 1L of a 1.00 M stock solution of hydrochloric acid. You need to make 500 mL of a 0.20M solution. How much of the stock solution is needed? 100. mL or 0.100 L You need to make 100 mL of 0.50 M nitric acid. The storage bottle reads 6.0M. How much of the 6.0M nitric acid will you need? 8.3 mL

Using Molarities in Stoichiometric Calculations How many grams of Ca(OH)2 are needed to neutralize 20.0 mL of a 0.150 M H2SO4 solution? 0.222 g (see board for work)

Using Molarities in Stoichiometric Calculations How many liters of 0.500 M HCl are needed to react completely with 0.100 mol Pb(NO3)2, forming the precipitate PbCl2? 0.400L (see board for work)

Titration The analytical technique in which one can calculate the concentration of a solute in a solution.

Titration

Titration Example A 1.034-g sample of impure oxalic acid (H2C2O4) is dissolved in water and an acid-base indicator is added. The sample required 34.47 mL of 0.485 M NaOH to reach the equivalence point. What is the mass of oxalic acid and what is its mass percent in the sample? 0.752 g H2C2O4 and 72.7% H2C2O4 (see board for work)

Titration A 0.263-g sample of sodium carbonate requires 28.35 mL of aqueous HCl for titration to the equivalence point. What is the molarity of the HCl? 0.175 M HCl

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