Reaction Rates and Equilibrium

Ch. 19

Rates of Reaction

19-1

Collision Theory• The rate or speed of a reaction varies

tremendously depending on the reaction

• Rate = measures the speed of any change over time.

• Collision Theory = atoms/ions/molecules react to form products when they collide, if they have enough kinetic energy.

– If they lack enough kinetic energy, will just bounce off each other when they collide

Activation Energy

• Activation Energy (Ea) = minimum amount of energy that particles must have in order to react.

Energy Diagrams – Exothermic Reaction

Ex: 2H2 + O2 2H2O + 286kJ

Energy Diagrams – Endothermic Reaction

Ex: 2H2O + 286kJ 2H2 + O2

DRAW!

R

P

P

R

+ΔH-ΔH

ENDOTHERMIC EXOTHERMIC

REACTION REACTION

EN

ER

GY

EN

ER

GY

Factors Affecting Reaction Rates

1. Temperature Reaction Rate-Inc. temp., inc. kinetic E, inc. # of collisions, inc. rate

2. Concentration Reaction Rate-Inc. # of particles, more collisions, inc. rate

1. Surface Area Reaction Rate-More surface area, inc. reactant exposure to collisions, inc. rate

4. Catalyst = substance that increases rate of reaction without being used up in rxn.

– Catalyst’s lower the activation energy of a rxn.

– Catalyst written above the “yield arrow”

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

5. Inhibitor = substance that interferes with action of catalyst.

Pt

Reaction without a catalyst

Reaction with a catalyst

Effect of a Catalyst

-H

DRAW!!!!

(Or has an Inhibitor)

Reversible Reactions and Equilibrium

19-2

Reversible Reactions

• Reversible Reactions = reactions which occur simultaneously in both directions.

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

• Chemical Equilibrium = forward and reverse reactions take place at the same rate.

Factors affecting Equilibrium:Le Chatelier’s Principle

• Le Chatelier’s Principle = if a stress is applied to equilibrium, the system will change to relieve the stress.

1. Concentration: changing the amount of any reactant or product at equilibrium disturbs EQ.

H2CO3(aq) CO2(aq) + H2O(l)-If add CO2 (product), rxn direction shifts LEFT-If remove CO2 (product), rxn direction shifts RIGHT-If add H2CO3 (reactant), rxn direction shifts RIGHT-If remove H2CO3 (reactant), rxn direction shifts LEFT

2. Temperature = increasing temperature causes equilibrium to shift in direction that absorbs heat.

Exothermic: 2SO2(g) + O2(g) 2SO3(g) + heat

-If add heat (product), rxn shifts LEFT

-If remove heat (product), rxn shifts RIGHT

Endothermic: N2(g) + O2(g) + heat 2NO(g)

-If add heat (reactant), rxn shifts RIGHT

-If remove heat (reactant), rxn shifts LEFT

3. Pressure = change in pressure affects only equilibrium rxn’s with an unequal # of moles of gaseous reactants AND products.

N2(g) + 3H2(g) 2NH3(g)

4 moles reactant to 2 moles product

-If increase pressure, shift RIGHT (move to less moles side)

-If decrease pressure, shift LEFT (move to more moles side)

Another pressure example:

2Ag2O(g) 4Ag(g) + O2(g)

2 moles reactant to 5 moles product

-If increase pressure, shift LEFT (move to less moles side)

-If decrease pressure, shift RIGHT (move to more moles side)

Equilibrium Constants• Equilibrium Constant (Keq) = ratio of product

concentration to reactant concentrations at equilibrium.– aA + bB cC + dD– Keq = [C]c x [D]d

[A]a x [B]b

– Put product concentrations over reactant concentrations.

– Coefficients become exponents of concentrations.– Concentrations are in mol/L or M– Keq will change if temperature changes– If Keq < 1, then reactants are favored (more reactants

than products)– If Keq > 1, then products are favored (more products

than reactants)

• Equilibrium example:– Using the following equation; 4.00 moles/L of Cl2,

1.2 mol/L BrCl, and 4.00 moles/L of Br2. What is the Keq?

• 2BrCl(g) Cl2(g) + Br2(g)

Keq = [Cl2] x [Br2]

[BrCl]2

Keq = 4.00mol/L x 4.00mol/L

(1.2mol/L)2

Keq = 11.1

Keq > 1 Products are favored!!