Kinetics

Question 1

Under what circumstances do reactions occur?

Answer 1

A reaction will only occur between 2 particles if they collide with sufficient energy (to break bonds), and are of the correct orientation (part of the molecule that is to react).

Question 2

Define the term ‘activation energy’.

Answer 2

It is the minimum energy required for a reaction to occur/for a successful collision.
(the enthalpy difference between the reactants and the transition state; the minimum kinetic energy needed to break bonds).

Question 3

How often do reactions occur?

Answer 3

Most collision do not lead to a reaction; they have insufficient energy or are of the wrong orientation.

Question 4

What does a Maxwell-Boltzman distribution represent?

Answer 4

Particles in a gas all have different kinetic energies; they’re all moving at different speeds.
Plotting a graph of energy vs. fraction of particles w/that energy results in said distribution.
(shows the distribution of energy amongst the particles)

• No particles have zero energy.
• The peak shows the most probable energy.
• Most molecules are moving at a moderate speed (middley bit)
• The average energy is different to the most probable.
• Only molecules that have E_a ≤ E will react (have sufficient energy).

Question 5

What are the effects of temperature on a Maxwell-Boltzman distribution?

Answer 5

• At higher temperatures, the peak of the curve is lower, and moves to the right.
• More particles on average have more kinetic energy and will move faster.
• Total area remains the same.
• But more molecules have E_a or greater; higher percentage of collisions in a reaction.

Question 6

What effect does temperature have on the rate of reaction?

Answer 6

• Increases the speed of molecules; increasing their energy, thus the no. of successful collisions; rate increases. Molecules collide more often.
• Small increases in temperature lead to a large increase in reaction rate as a greater proportion of molecules have the required activation energy.

Question 7

How does concentration affect the rate of reaction?

Answer 7

• Increasing the concentration means collision are more likely to occur; more particles are present in a given volume, thus the rate increases.
• Particles on average are closer together; collide more often.

(As a reaction progresses, reactant is used up, concentration of it falls, thus the rate drops over time)

Question 8

What is a catalyst?

Answer 8

A substance that alters the rate of a chemical reaction without being chemically changed themselves; are not used up.

Question 9

How do catalysts work?

Answer 9

By providing an alternative reaction pathway of lower activation energy.

Question 10

State what is meant by the term ‘rate of reaction’.

Answer 10

(Measured) change in concentration (of a substance) in unit time/given time.