Topic 6.1: Collision theory and rates of reaction

Question 1

Chemical kinetics

Answer 1

Study of the factors affecting the rate of a chemical reaction.

Question 2

Definition of reaction rate

Answer 2

Change in concentration of the products / reactants per time unit.

Question 3

Methods to monitor concentration changes (6)

Answer 3

a) Change in volume
b) Change in mass
c) Change in transmission of light
d) Change in concentration measuring titration.
e) Change in concentration measuring conductivity.
f) Clock reactions

Question 4

Change in volume of a gas (2)

Answer 4

a) The plunger of a gas syringe moves outwards as the gas produced is collected.
b) The water from an inverted burette

Question 5

Change in mass (2)

Answer 5

a) If the reaction giving off a gas, the decrease in mass can be measured by standing the reaction mixture directly on a balance.
b) It does not work if the evolved gas is too light such as hydrogen.

Question 6

Change in concentration measuring titration (2)

Answer 6

a) Samples are withdrawn from the reaction at regular intervals and analyzed by titration.
b) In quenching, a substance is introduced which stops the reaction in the sample while it is withdrawn

Question 7

Change in concentration measuring conductivity (2)

Answer 7

a) The total electrical conductivity of a solution depends on the total concentration of its ions and on their charges. If this changes, it indicates a change in the concertation of ions.
b) Conductivity can be measured directly using a conductivity meter in a solution

Question 8

Clock reactions

Answer 8

a) Measure the time it takes for a reaction to react a certain chosen fixed point.
b) Time taken to reach this point for the same reaction under different conditions can be compared.

Question 9

Why is the rate of reaction the steepest at the start of the curve?

Answer 9

a) There is a high concentration of reactant particles, so there is a thigh frequency of successful collisions.
b) The rate of reaction decreases over time as the concentration of reactant particles decreases, so the frequency of successful collisions also decreases.

Question 10

Formula of rate of reaction

a) Reactants
b) Products

Answer 10

a) - [∆R] / ∆T

b) [∆R] / ∆T

Question 11

Units of rate of reaction

Answer 11

mol dm-3 s-1

Question 12

What are the conditions for a reaction between two particles to occur?

Answer 12

a) Collision
b) Appropriate geometry / orientation
c) Sufficient energy

Question 13

Definition of activation energy

Answer 13

Minimum amount of KE that colliding particles require to have a successful collision.

Question 14

Why is the activation energy needed? (2)

Answer 14

a) Overcoming repulsion between molecules

b) Breaking some bonds in the reactants before they can react.

Question 15

Relationship between rate of reaction / Ea

Answer 15

a) Rate of reaction depends on the proportion of particles that have values greater than Ea.
b) Reactions with high Ea proceed more slowly than those with low Ea.

Question 16

Factors that affect the rates of reaction

Answer 16

a) Particle size (⇓)
b) Temperature (⇑)
c) Concentration (⇑)
d) Pressure (⇑)
e) Catalyst (⇑)

Question 17

Explanation of the effect of particle size in rate of reaction

Answer 17

a) The greater the particle size, the smaller the exposed SA.
b) Reactions require collisions for reactions to occur.
c) If the SA is smaller, fewer collisions will occur per unit time.

Question 18

Explanation of the effect of temperature in rate of reaction

Answer 18

a) As temperature increases, the particles will move faster so there will be more collisions per second.
b) As temperature increases, more colling particles will possess the necessary Ea, resuling in more successful collisions.

Question 19

Explanation of the effect of reactant concentration in rate of reaction

Answer 19

a) Increasing the concentration increases the frequency of collisions.
b) This increases the frequency of successful collisions, thus, increasing the rate.
c) As reactants are used up, their concentration falls and the reaction rate decreases.

Question 20

Explanation of the effect of pressure in rate of reaction

Answer 20

a) Higher pressure compresses the gas, increasing the concentration.
b) This increases the frequency of collisions and thus the rate too.

Question 21

Explanation of the effect of a catalyst in rate of reaction.

Answer 21

Catalysts speed up chemical reactions by providing an alternative pathway that a lower Ea than its original one.

Question 22

What does the Maxwell-Boltzmann curve show?

Answer 22

Distribution of kinetic energy among the particles of a system.
x-axis) KE
y-axis) Number of particles with KE

Question 23

Proportion of particles at a given temperature with sufficient KE to overcome Ea.

Answer 23

Small

Question 24

Effect of increasing temperature in Maxwell-Boltzmann curve

Answer 24

a) More particles have a very high velocity resulting in an increase in the average KE.
b) This is represented as a broadening of the curve, but with the same Ea.

Question 25

Statement of Kinetic Molecular Theory

Answer 25

Particles in a substance move randomly as a res