Chapter 6: Chemical Kinetics

Question 1

Define rate of reaction

Answer 1

RATE OF REACTION: change in concentration of reactants or products per unit time

Question 2

Describe the experimental methods used to measure the rate of reaction

Answer 2

1) Measure the rate at which gas is produced
- Set-up: delivery tube allowing gas produced to displace the solution in an inverted measuring cylinder
- Rate = change in volume of solution/time (cm³ s⁻¹)
- Graph plotted (find rate at any point by drawing a tangent to the point
- Gradient decreases as time increases: rate is fastest at the start of the reaction and slowest at the end
- Inverse graph: concentration of solution (greater volume of gas produced = lower concentration of solution)
- Possible errors: gas escaping before rubber bung is put on (volume of gas lower than expected) + variations in size of solid (difference in surface area)

2) Measure the rate at which mass decreases
- Set-up: cotton wool placed over mouth of conical flask (allows gas to escape without mass being lost due to splashes), which is placed on measuring cylinder

3) Measure the change in colour
- Set-up: colorimeter (more light absorbed = darker colour)

Question 3

Explain collision theory

Answer 3

COLLISION THEORY: for a chemical reaction to occur, the reacting particles must:

1) Collide with more than a certain minimum amount of energy
- Activation energy (Eₐ): Minimum amount of energy that colliding particles must possess to result in a reaction (needed to overcome interelectronic repulsion and break bonds to allow for the rearrangement of atoms)
- Particles possessing energy lower than Eₐ: no reaction will occur
- Particles possessing energy greater than or equal Eₐ (assuming correct orientation): effective collision

2) Collide with the correct orientation
- Not every collision possessing energy ≥Eₐ will result in an effective collision

Question 4

State and explain the factors affecting rate of reaction

Answer 4

1) CONCENTRATION OF REACTANTS
- More reacting particles per unit volume –> greater frequency of collision –> greater frequency of effective collision

2) PRESSURE (for reactions involving gases)
- Increased pressure –> Increased concentration of gaseous particles per unit volume –> Higher frequency of collision –> Higher frequency of effective collision

3) SURFACE AREA (for reactions involving solid reactants)
- Smaller particle –> Increased surface area –> increased frequency of collision –> increased frequency of effective collision

4) TEMPERATURE
- Increased temperature –> Increased kinetic energy possessed by the reacting particles –> Increased number of particles possessing energy ≥ Eₐ –> Increased frequency of effective collision

5) CATALYST
- Catalyst: substance that increases rate of reaction by providing an alternate reaction pathway with a lower Eₐ, without itself being used up
- Catalyst present –> Lower Eₐ –> greater proportion of particles possessing energy ≥Eₐ –> Increased frequency of effective collision

Question 5

Sketch the Maxwell-Boltzmann distribution to explain the effect of increased temperature and a catalyst on rate of reaction

Answer 5

TEMPERATURE:
- 2 curves: 1 representing lower temperature, 1 representing higher temperature (flatter with lower peak so that area under curve remains the same to represent equal number of reacting particles)
- y-axis: No. of particles
- x-axis: Energy
- Dotted line to mark Eₐ
- Shade area under curve to show increased number of particles ≥Eₐ at higher temperature

CATALYST:
- 1 curve
- 2 different Eₐ marked out (Eₐ catalysed and Eₐ uncatalysed)
- Larger shaded area representing that more particles have energy ≥Eₐ