Reaction Rates

Reaction Kinetics,Factors that Influence Rates,

and Activation Energy

Standards• 8. Chemical reaction rates depend on factors that influence the

frequency of collision of reactant molecules. As a basis for understanding this concept:

• a. Students know the rate of reaction is the decrease in concentration of reactants or the increase in concentration of products with time.

• 8. b. Students know how reaction rates depend on such factors as concentration, temperature, and pressure.

• 8. c. Students know the role a catalyst plays in increasing the reaction rate.

• 8. d.* Students know the definition and role of activation energy in a chemical reaction.

Example Reaction #2

H2 (g) + I2 (g) 2 HI (g)

hydrogen iodine hydrogen gas gas iodide

+

Definitions

• r (reaction rate) – how fast we use up a reactanthow fast we make a product

2A AA

• chemical kinetics – the study of chemical reaction rates.

Definitions

• r (reaction rate) – how fast we use up a reactanthow fast we make a product

2A AA

• chemical kinetics – the study of chemical reaction rates.

Definitions

• r (reaction rate) – how fast we use up a reactanthow fast we make a product

2A AA

• chemical kinetics – the study of chemical reaction rates.

Factors that Affect Reaction Rates• Temperature– More reactant molecules are moving fast enough

• Warm glowsticks glow brighter• Chilled food spoils slower

• Concentration (pressure for gases)– Reactants are hitting each other more often

• Greater concentration = Faster reaction• Lower concentration = Slower reaction

• Surface Area– Reactants are hitting each other more often

• Start bonfires with crumpled paper or small twigs

• Catalysts– They hold reactant molecules in an optimal

orientation, which speeds up the reaction

Activation Energy

• Activation Energy (EA) – The amount of energy reactants need to form the transition-state (the in-between) and the reaction to proceed.

• aka “the hurdle” the reactants need to get over.

Molecular Speed vs. Temperature

Molecule Energy

Num

ber o

f Mol

ecul

es

EA

300K

400K

Looking at Reaction Energy

Reaction Proceeds

Ener

gy

Reactants

Products

EA

exothermic reaction

Looking at Reaction Energy

Reaction Proceeds

Ener

gy

Reactants

ProductsEA

endothermic reaction

Looking at Reaction Energy

Reaction Proceeds

Ener

gy

Reactants

Products

EA

With catalyst

Catalyst

• Catalyst – a substance not used up in a reaction that speeds up the reaction by lowering the activation energy, EA.

• A catalyst also holds onto one of the reactants and positions the reactant so that it is easier to hit in the critical spot.

2 H2O2 (l) 2 H2O (g) + O2 (g)I2

Transition State

I– + CH3Cl CH3I + Cl–

transition stateaka activated complex

Transition State

I– + CH3Cl Cl– + CH3I

Transition-State

I– + CH3Cl Cl– + CH3I

Transition State

I– + CH3Cl Cl– + CH3I

HCl(aq) + H2O(l) H3O+(aq) + Cl–

(aq)

hydro- water hydronium chloridechloric acid ion ion

++

N2 (g) + 3 H2 (g) 2 NH3 (g) + 92kJnitrogen hydrogen ammonia heat

gas gas gas

+ +

Reaction Steps

2 N2O (g) 2 N2 (g) + O2 (g)

N2O (g) N2 (g) + O (g) (slow)

N2O (g) + O (g) N2 (g) + O2 (g) (fast)+

Overall Reaction

rate-determining step

Reaction Steps

2 N2O (g) 2 N2 (g) + O2 (g)

N2O (g) N2 (g) + O (g) (slow)

N2O (g) + O (g) N2 (g) + O2 (g) (fast)+

1

2

1

2

transition-state

transition-state

intermediatereactants

products

1st Order Rate Law

• The greater the concentration, [A], the greater the reaction rate.

• This is a first-order rate law because it is concentration, [A], to the power of 1.

A B

rate = k·[A] mol/L

mol/L s

1

1st Order Rate Law Example

At 660 Kelvin, the rate constant, k, equals 4.5 × 10-2 s-1. What is the reaction rate when there is a concentration of 500,000 ppm?

SO2Cl2 (g) SO2 (g) + Cl2 (g)

rate = k·[SO2]1

rate = (4.5 × 10-2)·[500,000]rate = 22,500 ppm/s

Other Order Rate Laws

• The reaction rate may increase with increasing concentrations of both reactants, [A] and [B].

• The powers, x and y, may or may not relate to the coefficients on the balanced chemical equation.

A + B C

rate = k·[A]x·[B]y

2 N2O5 (g) 2 NO2 (g) + O2 (g)

NeFONCBBe

He

Li

H

Kr

ArCl

Br

XeI

SPSiMg Al

Ca

Na

K

NeFONCBBe

He

Li

H

Kr

ArCl

Br

XeI

SPSiMg Al

Ca

Na

K

4 e– in valence shell