The Molecular Nature of Matter and Change Lecture PowerPoint Chemistry The Molecular Nature of Matter and Change Seventh Edition Martin S. Silberberg and Patricia G. Amateis Copyright  McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.

Chapter 4 Three Major Classes of Chemical Reactions

The Major Classes of Chemical Reactions 4.1 Solution Concentration and the Role of Water as a Solvent 4.2 Writing Equations for Aqueous Ionic Reactions 4.3 Precipitation Reactions 4.4 Acid-Base Reactions 4.5 Oxidation-Reduction (Redox) Reactions 4.6 Elements in Redox Reactions 4.7 The Reversibility of Reactions and the Equilibrium State

Water as a Solvent Water is a polar molecule since it has uneven electron distribution and a bent molecular shape. Water readily dissolves a variety of substances. Water interacts strongly with its solutes and often plays an active role in aqueous reactions. 4

Electron distribution in molecules of H2 and H2O. Figure 4.1 Electron distribution in molecules of H2 and H2O. B. Electron charge distribution in H2O is asymmetrical. A. Electron charge distribution in H2 is symmetrical. C. Each bond in H2O is polar. D. The whole H2O molecule is polar.

Figure 4.2 An ionic compound dissolving in water.

Figure 4.3 The electrical conductivity of ionic solutions.

Using Molecular Scenes to Depict an Ionic Compound in Aqueous Solution Sample Problem 4.1 PROBLEM: The beakers shown below contain aqueous solutions of the strong electrolyte potassium sulfate. Which beaker best represents the compound in solution? (H2O molecules are not shown). If each particle represents 0.10 mol, what is the total number of particles in solution?

Sample Problem 4.2 Determining Amount (mol) of Ions in Solution PROBLEM: What amount (mol) of each ion is in each solution? (a) 5.0 mol of ammonium sulfate dissolved in water (b) 78.5 g of cesium bromide dissolved in water (c) 7.42x1022 formula units of copper(II) nitrate dissolved in water PLAN: Write an equation for the dissociation of 1 mol of each compound. Use this information to calculate the actual number of moles represented by the given quantity of substance in each case.

Concentration of Solutions & Molarity Many reactions occur in solution. A solution consists of one or more solutes dissolved in a solvent. The concentration of a solution is given by the quantity of solute present in a given quantity of solution. Molarity (M) is often used to express concentration. liters of solution moles solute Molarity = L soln mol solute M =

Sample Problem 4.3 Calculating the Molarity of a Solution What is the molarity of an aqueous solution that contains 0.715 mol of glycine (H2NCH2COOH) in 495 mL? PROBLEM:

Summary of mass-mole-number-volume relationships in solution. Figure 4.4 Summary of mass-mole-number-volume relationships in solution.

Sample Problem 4.4 Calculating Mass of Solute in a Given Volume of Solution PROBLEM: How many grams of solute are in 1.75 L of 0.460 M sodium monohydrogen phosphate buffer solution?

Sample Problem 4.5 Determining Amount (mol) of Ions in a Solution PROBLEM: What amount (mol) of each ion is in 35 mL of 0.84 M zinc chloride?

Laboratory preparation of molar solutions. Figure 4.5 Laboratory preparation of molar solutions. A Weigh the solid needed. Transfer the solid to a volumetric flask that contains about half the final volume of solvent. C Add solvent until the solution reaches its final volume. B Dissolve the solid thoroughly by swirling.

Converting a concentrated solution to a dilute solution. Figure 4.6

Sample Problem 4.6 Preparing a Dilute Solution from a Concentrated Solution PROBLEM: “Isotonic saline” is a 0.15 M aqueous solution of NaCl. How would you prepare 0.80 L of isotonic saline from a 6.0 M stock solution?

Sample Problem 4.7 Visualizing Changes in Concentration PROBLEM: The beaker and circle represents a unit volume of solution. Draw the solution after each of these changes: (a) For every 1 mL of solution, 1 mL of solvent is added. (b) One third of the volume of the solution is boiled off.

Figure 4.7: Writing Equations for Aqueous Ionic Reactions

The total ionic equation shows all soluble ionic substances dissociated into ions. This gives the most accurate information about species in solution. 2Ag+ (aq) + 2NO3– (aq) → Ag2CrO4 (s) + 2Na+ (aq) + CrO42– (aq) + 2Na+ (aq) + 2NO3– (aq) Spectator ions are ions that are not involved in the actual chemical change. Spectator ions appear unchanged on both sides of the total ionic equation. 2Ag+ (aq) + 2NO3– (aq) → Ag2CrO4 (s) + 2Na+ (aq) + CrO42– (aq) + 2Na+ (aq) + 2NO3– (aq)

The net ionic equation eliminates the spectator ions and shows only the actual chemical change. 2Ag+ (aq) + CrO42– (aq) → Ag2CrO4 (s)

Figure 4.7 An aqueous ionic reaction and the three types of equations.

Precipitation Reactions In a precipitation reaction two soluble ionic compounds react to give an insoluble product, called a precipitate. The precipitate forms through the net removal of ions from solution. It is possible for more than one precipitate to form in such a reaction.

The precipitation of calcium fluoride. Figure 4.8 The precipitation of calcium fluoride. 2NaF (aq) + CaCl2 (aq) → CaF2 (s) + 2NaCl (aq) 2Na+ (aq) + 2F– (aq) + Ca2+ (aq) + 2Cl– (aq) → CaF2 (s) + 2Na+ (aq) + 2Cl– (aq) Ca2+ (aq) + 2F– (aq) → CaF2 (s)

Predicting Whether a Precipitate Will Form Note the ions present in the reactants. Consider all possible cation-anion combinations. Use the solubility rules to decide whether any of the ion combinations is insoluble. Any insoluble combination identifies a precipitate that will form.

The precipitation of PbI2, a metathesis reaction. Figure 4.9 The precipitation of PbI2, a metathesis reaction. 2NaI (aq) + Pb(NO3)2 (aq) → PbI2 (s) + NaNO3 (aq) 2Na+ (aq) + 2I– (aq) + Pb2+ (aq) + 2NO3– (aq) → PbI2 (s) + 2Na+ (aq) + 2NO3- (aq) Pb2+ (aq) + 2I- (aq) → PbI2 (s) Precipitation reactions are also called double displacement reactions or metathesis reactions. 2NaI (aq) + Pb(NO3)2 (aq) → PbI2 (s) + 2NaNO3 (aq) Ions exchange partners and a precipitate forms, so there is an exchange of bonds between reacting species.

Sample Problem 4.8 Predicting Whether a Precipitation Reaction Occurs; Writing Ionic Equations PROBLEM: Predict whether or not a reaction occurs when each of the following pairs of solutions are mixed. If a reaction does occur, write balanced molecular, total ionic, and net ionic equations, and identify the spectator ions. potassium fluoride (aq) + strontium nitrate (aq) → ammonium perchlorate (aq) + sodium bromide (aq) →

Using Molecular Depictions in Precipitation Reactions Sample Problem 4.9 Using Molecular Depictions in Precipitation Reactions PROBLEM: The following molecular views show reactant solutions for a precipitation reaction (with H2O molecules omitted for clarity). Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Which compound is dissolved in beaker A: KCl, Na2SO4, MgBr2, or Ag2SO4? Which compound is dissolved in beaker B: NH4NO3, MgSO4, Ba(NO3)2, or CaF2?

Calculating Amounts of Reactants and Products in a Precipitation Reaction Sample Problem 4.10 PROBLEM: Magnesium is the second most abundant metal in sea water after sodium. The first step in its industrial extraction involves the reaction of Mg2+ with Ca(OH)2 to precipitate Mg(OH)2. What mass of Mg(OH)2 is formed when 0.180 L of 0.0155 M MgCl2 reacts with excess Ca(OH)2?

Figure 4.10 Summary of amount-mass-number relationships for a chemical reaction in solution.

Sample Problem 4.11 Solving Limiting-Reactant Problems for Precipitation Reactions PROBLEM: In a simulation mercury removal from industrial wastewater, 0.050 L of 0.010 M mercury(II) nitrate reacts with 0.020 L of 0.10 M sodium sulfide. How many grams of mercury(II) sulfide form? Write a reaction table for this process.