3.2 Reaction Mechanism and the Arrhenius Equation

Question 1

Thermodynamics and Kinetics

Reaction coordinate diagrams

Answer 1

Question 2

Collision Theory

Answer 2

Used to explain the varios factors that influence reaction rates

Question 3

Collision Theory

Activation Energy

Answer 3

• In order for a chemical reaction to occur, the reactants must first overcome an activation barrier (hilltop)
• the energy required to overcome this barrier is called the activation energy Ea

Question 4

Collision Theory

Activation Energey:

The heat-temperature relationship

Question 5

Collision Theory

Activation Energy:

What has to happen for a chemical reaction to occur?

Answer 5

• Reactants must collide with sufficient energy to overcome the activation barrier
• they must collide in a proper orientation
• at the moment of transition, the affected species can proceed to products or return to the reactants

Question 6

Collision Theory

The Arrhenius Equation

Answer 6

Ea = activation energy for a specific reaction

R = gas constant 8.314 Jmol^-1K^-1

T = temperature (Kelvin)

Note: when the value of Ea increases, the value of k decreases

Question 7

Collision Theory

Effect of Temperature on Rate

Answer 7

• if the reaction is run at two diff tem the rate of the reaction will change
• we can use the two rates at the two temperatures to determine the Ea

Question 8

Collision Theory

Determining Activation Energy Graphically

Answer 8

• Ea can be determined experimentally by using a reaction rate at different temperatures
• then plotting the graph of ln k (calculated form the rate) versus 1/T

Question 9

Collision Theory

Effect of a Catalyst on Activation Energy

Answer 9

• an increase in temperature will result in an increase in the rate of reaction, as more molecules can overcome Ea (Ea doesn’t change)
• acatalyst does change the Ea

Question 10

Reation Mechanisms

Answer 10

• a reaction mechanism describes the sequences of steps that occur.
• each step in a reaction mechanism is called an elementary step
• in an elementary step the coefficients do become exponents
• Reaction intermediate something that is formed and then consumed

Question 11

Reaction Mechanisms

Molecularity:

unimolecular process, bimolecular process, RDS

Answer 11

• It refers to how many species react together in an elementary step
• if a process incolces only one reactant species, it is termed a unimolecular process
• Bimolecular process involves two species, either identical or different, and is second-order
• The overall rate of a reaction is determined by the rate of the slowest or rate determining step (RDS)

Question 12

Reaction Mechanisms

Determining Reaction Mechanisms:

Guidelines

Answer 12

1. Look for the slow, or RDS
2. Write a rate law in terms of concentrations of reactants in the RDS. A maximum of two species should appear in the rate law at this stage.
3. If there are intermediates in the rate law, express their concetrations in terms of stable reactants appearing in the overall reaction equation. This may be done by writing equilibrium constant expressions for steps preceding the RDS
4. Substitute concentrations of stable reactants for concentrations of intermediates in the rate law.
5. Fast steps following the RDS in the mechanistic sequence may be ignored

Question 13

Reaction Mechanisms

determining Reaction Mechanisms:

The following reaction occurs in three steps:

Cl2 + CO → COCl2

rate = k[Cl2]^x[CO]^y

Step 1: Cl2 → 2Cl

Step 2: CO + Cl → COCl

Step 3: COCl + Cl2 → COCl2 + Cl

a) What would be the overall rate law expected if step 1 is slow and steps 2 and 3 are fast?
b) What if steps 2 is slow, steps 3 is fast, but step 1 is a fast equilibrium?
c) What if step 3 is slow, but steps 1 and 2 are fast equilibria?

Answer 13