Chapter 7 Chemical Reactions: Energy, Rates, and Equilibrium

Energy and Chemical Bonds Chapter 6 –Kept a careful accounting of atoms as they rearranged themselves Reactions also involve a transfer of energy

Energy and Chemical Bonds Two fundamental kinds of energy. –Potential energy is stored energy. –Kinetic energy is the energy of motion. Law of Conservation of Energy –Energy can be converted from one kind to another but never destroyed

Energy and Chemical Bonds A chemical –Potential - attractive forces in an ionic compound or sharing of electrons covalent compound –Kinetic – (often in form of heat) occurs when bonds are broken and particles allowed to move

Heat Changes during Chemical Reactions Bond dissociation energy: The amount of energy that must be supplied to break a bond and separate the atoms in an isolated gaseous molecule.

Heat Changes during Chemical Reactions Bond breakage requires energy to be added to the system (+ energy) Bond formation gives off energy as the bonds form (- energy)

Heat Changes during Chemical Reactions Heat of reaction: (Enthalpy) –Represented by  H –is the difference between the energy absorbed in breaking bonds and that released in forming bonds Endothermic: –More energy is required than released. –  H is positive Exothermic: –More energy is released than required –  H is negative

Exothermic and Endothermic Reactions

Problem Br 2 (l) → Br 2 (g) ΔH = 7.4 kcal/mol –Is this reaction endothermic or exothermic –Is this reaction spontaneous with respect to enthalpy? 2C 8 H O 2 → 16CO H 2 O kcal –Is this reaction endothermic or exothermic? –What is the sign of ΔH?

Why do Chemical Reactions Occur? Free Energy Events that lead to the system having less energy are said to be spontaneous with respect to enthalpy –Exothermic reactions are spontaneous –Why would endothermic reactions occur? Free Energy (ΔG) –Enthalpy – ΔH – heat of reaction –Entropy (S)

Entropy Entropy – measures the spreading out of energy – universe moves toward disorder –Entropy favored system is one that goes from a concentrated area of energy to the energy being more spread out ΔS is positive –Unfavorable process involves concentrating the energy into less area ΔS is negative

Why do Chemical Reactions Occur? Free Energy

Problem Identify each of the following as entropy favored or disfavored. For each state the sign of the ΔS. –Assembling a jig-saw puzzle –I 2 (g) + 3F 2 (g) → 2 IF 3 (g) –A precipitate forming when two solutions are mixed –Demolition of a building –CS 2 (g) + 4H 2 (g) → CH 4 (g) + 2H 2 S(g) –2HBr(g) → H 2 (g) + Br 2 (g)

Why do Chemical Reactions Occur? Free Energy Free Energy

Why do Chemical Reactions Occur? Free Energy HH SS GG (-) favorable(+) favorable(-) spontaneous always (+) unfavorable(-) unfavorable(+) nonspontaneous always (-) favorable(-) unfavorable(-) Low T (+) High T (+) unfavorable(+) favorable(+) Low T (-) High T

Problem H 2 (g) + Br 2 (l) → 2 HBr (g) –Is this reaction spontaneous with respect to entropy? –If the ΔH = kcal/mol is the reaction spontaneous with respect to enthalpy? –If the ΔH = kcal/mol and ΔS = 27.2 cal/mol K, is the reaction spontaneous with respect to free energy? –What is the value of ΔG at 300 K?

Problem Given the reaction: 8 Al(s) + 3 Fe 3 O 4 (s) --> 4 Al 2 O 3 (s) + 9 Fe(s) kJ –Is the reaction endothermic or exothermic? –The sign of ΔH should be positive or negative? –According to enthalpy, is the reaction favored or not favored? –According to entropy, is the reaction favored or not favored? –The sign of ΔS should be positive or negative? –Calculate Gibb’s free energy for this reaction at 25 o C if ΔS=215.1 J/K and has the sign you determined in part e. –Is the reaction favored according to free energy?

How do Chemical Reactions Occur? Reaction Rates  G indicates whether a reaction will occur –But how fast will it occur? –To what extent does the reaction occur?

Rates of Reaction Rate of Reaction –How fast does a reaction go? Properly oriented collisions Sufficient energy to break the bonds of the reactants –Factors affecting collisions and energy Concentration of reactants Temperature of system

How do Chemical Reactions Occur? Reaction Rates Orientation

How do Chemical Reactions Occur? Reaction Rates Sufficient energy –Energy of activation

Effects of Temperature, Concentration, and Catalysts on Reaction Rates

A third way to speed up a reaction is to add a catalyst— a substance that accelerates a chemical reaction but is itself unchanged in the process. A catalyzed reaction has a lower activation energy.

Problem

Chemical Equilibrium Equilibrium –To what extent a reaction occurs

Reversible Reactions and Chemical Equilibrium Many reactions result in complete conversion from reactant to product. Many however do not

Chemical Equilibrium

Equilibrium Equations and Equilibrium Constants Consider the following general equilibrium reaction: aA + bB + …  mM + nN + … –Where A, B, … are the reactants; –M, N, …. are the products; –a, b, ….m, n, …. are coefficients in the balanced equation. –At equilibrium, the composition of the reaction mixture obeys an equilibrium equation.

Equilibrium Equations and Equilibrium Constants The value of K varies with temperature.

Problem Write an equilibrium constant equation for –N 2 (g) + 3H 2 (g) → 2NH 3 (g) –FeCl 3 (aq) + 3NaOH(aq) → Fe(OH) 3 (s) + 3NaCl(aq)

Equilibrium Equations and Equilibrium Constants K larger than 1000: Reaction goes essentially to completion. K between 1 and 1000: More products than reactants are present at equilibrium. K between 1 and 0.001: More reactants than products are present at equilibrium. K smaller than 0.001: Essentially no reaction occurs.

Problem Indicate the primary substance or substances in the reaction vessel given the K values of the reactions –2CO(g) + O 2 (g) → 2CO 2 (g) K = 1.4 x 10 2 –H 2 O (l) + HNO 2 (aq) → H 3 O + (aq) + NO 2 - (aq) K = 4.50 x 10 -4

LeChatelier’s Principle: The effect of Changing Conditions on Equilibia Le Châtelier's Principle: When a stress is applied to a system at equilibrium, the equilibrium shifts to relieve the stress. The stress can be any –change in concentration –pressure –Volume –temperature that disturbs original equilibrium.

LeChatelier’s Principle

Le Chatelier’s Principle: The Effect on Changing Conditions on Equilibria

Problem Methanol can be synthesized by combining carbon monoxide and hydrogen. CO(g) + 2H 2 (g) → CH 3 OH(g) ΔH°rxn = kJ What happens when –The temperature is raised by 50 o C? –The pressure is raised? –Methanol is added? –Hydrogen is removed?

Optional Homework Text – 7.17, 7.18, 7.19, 7.20, 7.22, 7.23, 7.30, 7.38, 7.40, 7.46, 7.48, 7.54, 7.56, 7.58, 7.62, 7.64, 7.66, 7.68, 7.80 Chapter 7 Homework online

Required Homework Assignment 7