Topic 6 & 16 - Kinetics

Question 1

Define rate of reaction

Answer 1

The increase in concentration of products per unit time

(the decrease in concentration of reactants per unit time)

Question 2

List suitable experimental procedures for measuring rates of reactions

Answer 2

1. Change in volume of gas produced
2. Change in mass
3. Change in transmission of light (colorimetry/spectrophotometry)
4. Change in concentration (titration)
5. Change in concentration (conductivity)
6. Clock reactions

Question 3

Describe how the volume of gas produced can be used to determine the rate of reaction

Answer 3

• Collecting the gas and measuring the change in volume at regular time intervals gives a volume-time graph
• A gas syringe or inverted burette is used
• Using warm water will reduce error

Mg(s) + 2HCL(aq) → MgCl₂(aq) + H₂(g)

Question 4

Describe how the change in mass can be used to determine the rate of reaction

Answer 4

• Can be used if the reaction is giving off gas
• Corresponding decrease in mass can be directly measured on a balance
• Allows for continuous readings
• Does not work if hydrogen is released (too light to show change)
• Mass-time graph

CaCo₃(s) + 2 HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)

Question 5

Describe how the change in transmission of light can be used to determine the rate of reaction

Answer 5

• Can be used if one of the reactants is coloured
• A light of selected wavelength is passed through the solution
• Intensity of light transmitted is measured
• As the concentration increases, it absorbs more light → less light is transmitted
• A photocell detects light and creates a current
• Allows for continuous readings and absorbance-time graph

2 HI(g) → H₂(g) + I₂(g)

Question 6

Describe how the change in concentration (titration) can be used to determine the rate of reaction

Answer 6

• Measuring the concentration by titrating it against a known standard
• Cannot done continuously because it chemically changes the reaction
• Samples must be withdrawn at regular time intervals and analysed by titration
• Quenching is used to stop the reaction when withdrawn
• It’s necessary to repeat the process at several time intervals

H₂O₂(aq) + 2 H⁺(aq) + 2 I^-(aq) → I₂(aq) + 2 H₂O(l)

Question 7

Describe how the change in concentration (conductivity) can be used to determine the rate of reaction

Answer 7

• Electrical conductivity depends on the concentration of ions and their charges
• If this changes during a reaction, the change can be followed
• Conductivity meter is used

BrO₃^-(aq) + 5 Br^-(aq) + 6 H⁺(aq) → 3 Br₂(aq) + 3 H₂O(l)

Question 8

Describe how clock reactions can be used to determine the rate of reaction

Answer 8

• Used when it is difficult to record the continuous change
• Measuring the time it takes to reach a fixed point (something observable)
• Gives an average rate
  e. g. time taken to react completely (no longer visible)

Mg(s) + 2 HCl(aq) → MgCl₂(aq) + H₂(g)

Question 9

Define the kinetic theory

Answer 9

Temperature in kelvin is proportional to the average kinetic energy of the particles in a substance

Question 10

Define activation energy Ea

Answer 10

The minimum value of kinetic energy which particles must posess before they are able to react

Question 11

Describe the collision theory

Answer 11

The rate of a reaction depends on:

• collision frequency
• no. of particles with kinetic energy greater than E_a
• appropriate collision geometry

Question 12

List five factors affecting the rate of reaction

Answer 12

1. Particle size
2. Temperature
3. Concentration
4. Pressure
5. Catalyst

Question 13

Predict and explain the effect of particle size on the rate of a reaction

Answer 13

Decreasing the particle size increases the rate of reaction. Subdividing a large particle into smaller parts increases the total surface area and allows more contact between reactants

Question 14

Predict and explain the effect of temperature on the rate of a reaction

Answer 14

Increasing temperature causes an increase in the rate of reaction. This is due to the increase in kinetic energy of the particles, that exceed the required activation energy. There is also an increase in collision frequency which further increases the rate.

Question 15

Predict and explain the effect of concentration on the rate of a reaction

Answer 15

Increasing the concentration of reactants increases the rate of reaction. This is due to the increase in the frequency of collisions.

Question 16

Predict and explain the effect of pressure on the rate of a reaction

Answer 16

Increasing pressure (in gases) increases the rate of reaction. Higher pressure compresses the gas, effectively increasing its concentration.

Question 17

Describe the effect of a catalyst on the rate of a reaction

Answer 17

A catalyst increases the rate of a chemical reaction without itself undergoing chemical change (can be reused). Provides an alternate route with a lower E_a. Doesn’t increase the temperature. Also causes an equal reduction for both forward and reverse reaction, thus does not change the position of the equilibrium or the yield.

Question 18

Sketch and explain the Maxwell-Boltzmann energy distribution curve for a gas at different temperatures and its consequences for changes in reaction rate

Answer 18

• The area represents the no. of particles
• The area under the curve does not change as the total no. of particles remains constant
• With increased temperature, more particles have a higher velocity, which shifts the curve to the right

Question 19

Distinguish between *rate constant, overall order of reaction *and order of reaction

Answer 19

**Rate constant, k **= a constant for a particular reaction at a specified temperature

Order of reaction = the powers (m,n) to which the concentrations of reactants are raised

rate = k[A]^m[B]ⁿ

Overall order = the sum of the individual orders, m + n

Question 20

Distinguish between zero, first, second and third order reactions

Answer 20

zero

rate = k

first

rate = k[A]

second

rate = k[A]²

third

rate = k[A]³

Question 21

How is the order of a reaction determined?

Answer 21

In the end the orders are added up

Question 22

What can you deduce if a reactant has a constant half-life?

Answer 22

It is first order with respect to that reactant

Question 23

Can reactions occur by more than one step?

Answer 23

Yes

Question 24

Which step of a reaction determines the rate of the whole reaction?

Answer 24

The slowest step

Question 25

What does the term molecularity refer to in reaction mechanisms?

Answer 25

Refers to the number of reactant species involved Unimolecular = reaction involves a single reactant particle Bimolecular = two reactant particles

Question 26

What is the relationship between rate constant, k, and temperature?

Answer 26

The rate constant is dependent on temperature, when temperature increases, the constant increases