Kinetics

Question 1

Rate of reaction

Answer 1

Change in the concentration of a particular reactant or product per unit time

Rate of reaction = Change in concentration of a product or reactant / time taken

Units: mol dm^-3 s^-1

Question 2

Instantaneous rate

Answer 2

The rate at a particular time; the rate at a particular instant during the reaction

Measured by tangent of curve

Question 3

Average rate

Answer 3

The change in concentration of a reactant or a product over a set time interval

Measured by drawing triangle

Question 4

Initial Rate

Answer 4

Instantaneous rate at the start of the reaction, when an infinitely small amount of the the reactant has been used up

Measure by the tangent of curve at time t= 0

Question 5

Rate equation

Answer 5

For hypothetical reaction xA + yB → zC

rate = k[A]^p[B]^q

where:
k = rate constant
p, q = orders of reaction wrtx to A and B

Question 6

Rate constant (k)

Answer 6

The proportionality constant, k, in the rate equation of a reaction at a given temperature

k is constant for a given reaction at a particular temperature

affected only by change in temperature or activation energy (eg catalyst)

Question 7

Order of reaction

Answer 7

The degree to which reaction rate is affected by concentration of reactants

1. Zero-order → Reaction rate is independent of the [reactant]
2. First-order → Reaction rate is directly proportional to the [reactant]
3. Second-order → Reaction rate is proportional to the [reactant]²

Question 8

Initial rate method of determining order of reaction

Answer 8

Comparing experiments I and II

[A] double and initial rate doubled
Therefore rate is directly proportional to [A]

Hence reaction is first order wrt A

Question 9

Half-life (t_1/2)

Answer 9

The half-life of a reaction is the time taken for the concentration of a reactant to fall to half its initial value

For a first-order reaction:
t_1/2 = ln2/k

Question 10

Pseudo-order reactions

Answer 10

When one reactant is in large excess or is also the solvent/catalyst, a change in its concentration has little impact on the rate of the reaction

Thus reaction appears to be zero-order wrt to it

Thus the reactions is pseudo zero-order wrt to this reactant

Question 11

Experimental methods of measuring reaction rates

Answer 11

1. Identify change in physical property (colour intensity, volume , pressure, electrical; conductivity) which is directly proportional to [reactant] or [product] to be measured
2. Start the reaction and measure the physical property at regular time intervals
3. Plot graph of physical property against time.
4. Deduce order of reaction, rate and rate constant may be found

Question 12

The collision theory

Answer 12

• Reaction occurs due to the collision of reactant particles
• A collision only results in a reaction of reactant particles collide with a certain minimum energy known as the activation energy
• A collision will not result in a reaction unless the colliding particles are correctly orientated to one another

Question 13

Activation energy (E_a)

Answer 13

The activation energy of a reaction is the minimum energy that the reactant particles must possess before they can collide to result in a reaction

Question 14

Transition state theory

Answer 14

• As the reactants configurations change toward product configuration, there is an increase in potential energy until a maximum is reached
• At this energy molecules have a critical configuration known as the transition state or activated complex

Question 15

Factors affecting rate of reaction

Answer 15

1. Concentration
2. Temperature
3. Surface area of reactants
4. Catalyst
5. Light (for photochemical reactions)

Question 16

Catalyst

Answer 16

A substance that increases the rate of chemical reaction without itself undergoing any permanent chemical change

• Catalyst does not affect ΔH of a reaction
• Catalyst does not affect POE
• Catalysts are often highly specific
• Catalysts is chemically involved in a reaction. It is consumed in one step then regenerated in a subsequent step