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

• Br2 (l) → Br2 (g) ΔH = 7.4 kcal/mol

– Is this reaction endothermic or exothermic– Is this reaction spontaneous with respect to

enthalpy?

• 2C8H18 + 25O2 → 16CO2 + 18H2O + 239.5 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

– I2 (g) + 3F2 (g) → 2 IF3 (g)

– A precipitate forming when two solutions are mixed– Demolition of a building

– CS2(g) + 4H2(g) →  CH4(g) + 2H2S(g)

– 2HBr(g)  → H2(g) + Br2(g)

Why do Chemical Reactions Occur? Free Energy

• Free Energy

Why do Chemical Reactions Occur? Free Energy

H

S G

(-) favorable (+) favorable (-) spontaneous always

(+) unfavorable (-) unfavorable (+) nonspontaneous always

(-) favorable (-) unfavorable (-) spontaneous @ Low T

(+) nonspontaneous @ High T

(+) unfavorable (+) favorable (+) nonspontaneous @ Low T

(-) spontaneous @ High T

Problem

• H2 (g) + Br2 (l) → 2 HBr (g)

– Is this reaction spontaneous with respect to entropy?

– If the ΔH = -17.4 kcal/mol is the reaction spontaneous with respect to enthalpy?

– If the ΔH = -17.4 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 Fe3O4(s) --> 4 Al2O3(s) + 9 Fe(s) + 3350 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 25oC 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

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

– N2(g) + 3H2(g) → 2NH3(g)

– FeCl3(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) + O2(g) → 2CO2(g) K = 1.4 x 102

– H2O (l) + HNO2(aq) → H3O+(aq) + NO2-(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) + 2H2

(g)   →  CH3

OH(g) ΔH°rxn = -90.7 kJ

• What happens when – The temperature is raised by 50oC?– 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