Chapter 19

Reaction Rates and Equilibrium

I. Rates of reaction A. Collision Theory

1. rates : measure the speed of any change during a time

interval

2. collision theory : particles react to form products when they

collide

3.

4. activation energy: minimum amount of energy that particles must

have in order to react

5. activated complex: arrangement of atoms at the peak of the activation – energy barrier

6. transition state: refers to the activated complex

B. Factors affecting reaction rates

1. Temperature a. Increase temperature

1. increase the number of particle collisions

2. enough energy to slip past the activation-energy barrier

3. allows more products to form faster

b. Decrease temperature: slows down a reaction

2. Concentration a. Increasing the number of

particles in a fixed volume causes more collisions

(increases the amount of products) b. decreasing the number of particles

slows the reaction

3. Particle size a. Smaller the particle size, the larger the

surface area b. Increase in surface area 1. increases the amount of reactant

exposed 2. increases the number of

collisions, increases the amount of products

c. How do you increase particle size?1. dissolve solid particles

2. crush solids into a powder

4. Catalysts a. Increases the rate of a reaction

without being used up

b. 2H2(g) + O2(g) 2H2O (l)

Pt

c.

d. Inhibitor 1. a substance that interferes with the

action of a catalyst 2. slows the reaction

II. Reversible Reactions and Equilibrium

A. Reversible reactions 1. happens simultaneously in both directions

2. 2SO2 (g) + O2 (g) 2SO3 (g)

B. Chemical equilibrium : when the rate of the forward reactions

equals the rate of the reverse reaction

C. Equilibrium position 1. changes with the concentration of the

substance

2. the arrow shifts towards the higher concentration

D. Le Chatelier’s Principle

1. If a stress is applied to a system in dynamic equilibrium,

the system changes to relieve stress.

2. Concentration a. Add more products, the shift is towards

the reactants

b. When a reactant is removed, the shift is towards the products

3. Temperature a. Increase in temperature causes a shift

that absorbs heat

b. Heating the reaction, favors the reactants

c. Cooling the reaction, favors the products

4. Pressure a. Affects only gaseous systems

b. Favors the least volume

E. Examples: N2 + 3H2 2NH3 + heat

stress results increase amount N2

increase pressure decrease pressure increase temp. decrease temp.

F. Equilibrium constant (Keq)

1. ratio of product concentration to

reactant concentration 2. a A + b B c C + dD a,b,c,d = # moles

3. Keq = [C]c x [D]d

[A]a x [B]b

4. [ ] indicate moles/L

5. Keq > 1, products are favored at equilibrium

6. Keq < 1, reactants are favored at

equilibrium

7. [ ] are for gases

8. Ex. A liter of gas mixture at 10 °C at equilibrium contains

0.0045 mole N2O4 and 0.030 mole NO2. Write the expression for the equilibrium constant and calculate the equilibrium constant (Keq) for the

reaction. N2O4 (g) 2NO2 (g)

9. Ex. At a certain temperature, Keq = 11.1, and the equilibrium mixture

contains 4.00 mole Cl2. How many moles of Br2 and BrCl are present in the equilibrium mixture?

BrCl (g) Cl2 (g) + Br2 (g)

III. Determining whether a reaction will occur

A. Free energy (G)

1. energy available to do work

2. only will happen if the reaction can take place

3. Spontaneous reactions a. Reactions that occur

naturally

b. Favor the formation of products

c. Release free energy

d. Ex. Colorful fireworks display

4. Nonspontaneous reactions

a. Does not favor the formation of products

B. Entropy (S)

1. measure of the randomness of a system

2. Law of disorder : the processes move in the direction of maximum disorder

or randomness a. Increasing entropy solid liquid gas

b. Increasing entropy crystalline solid solute particles

c. Increasing entropy the total number of product molecules

is greater than the total number of reactant molecules

2 H2O (l) 2 H2 (g) + O2 (g)

d. Increasing entropy happens when temperature increases,

molecules move faster and faster

• Heat, Entropy and Free Energy How changes in heat and entropy affect

reaction spontaneityHeat change Entropy Spontaneous reaction?Decreases increases yes (exo)Increases decreases only if unfavorable heat change is

offset by favorable entropy

change

decreases decreases only if favorable (exo) entropy change is offset by

favorable heat

change

increases decreases no (endo)

Calculating Entropy and Free Energy 1. Entropy calculations

a. S b. Units J/k c. So is the symbol for solids, liquid or

gases d. ΔSo = So(products) – So(reactants)

e. Ex.1 Calculate the standard entropy change (ΔSo) that occur when 1 mole H2O (g) at 25 oC and 101.3 kPa condenses to 1 mole H2O (l) at the same temperature.

2. NO (g) reacts with O2 to form NO2(g). What is the standard change in entropy for this reaction when reactants and products are in the specified physical states at 101.3 kPa and 25 oC?

NO (g) + O2 (g) NO2 (g)

2. Gibbs Free Energy (ΔG) a. The maximum amount of energy that

can be attached to another process to do work.

b. ΔG = ΔH – TΔS where ΔH : enthalpy T : temperature ΔS: entropy

c. ΔGo = ΔGfo(products) – ΔGf

o(reactants)

d. Summary: Reaction is ΔG ΔS

spontaneous - - nonspontaneous + -

e. Examples: Using the values for ΔH and S, determine

whether this reaction is spontaneous at 25 °C. C (s) + O2 (g) CO2 (g)

graphite

Substance ΔH(kJ/mol) S (J/Kmole) C (s) 0.0 5.69 O2 (g) 0.0 205

CO2 (g) -393.5 214

Ex. 2 Using the date from Table 19.4 and ΔG°f =ΔG°f (products) - ΔG°f (reactants)

determine whether the following equation is spontaneous

Cl2 (g) + H2O (g) 2HCl (g) + ½ O2 (g)