Chemical kinetics

Question 1

What must happen for a chemical reaction to be observed (3)

Answer 1

1. Energy is conserved (1st Law)
2. Disorder of the Universe increases (2nd Law)
3. The reaction rate is significant.

Question 2

What is metastability

Answer 2

A state of a system that can be observed for a short time but is not the system’s state of least energy

Question 3

What determines the rate of change

Answer 3

The rate of change is determined by kinetic factors, not just thermodynamics.

Question 4

Why is the rate of change from diamond to graphite negligible (3)

Answer 4

1. The rate of change is determined by kinetic factors, not just thermodynamics.
2. This kinetic barrier is the reason the rate is negligible, not the value of ΔG° itself.
3. ΔG° is negative because graphite is thermodynamically more stable than diamond under standard conditions.

Question 5

What is activation energy (4)

Answer 5

1. This is the minimum reactant energy required for the reaction.
2. High Ea = low rate
3. low Ea = high rate.
4. Catalysts lower Ea but don’t affect ΔG°(which determines the equilibrium position)

Question 6

What is the reaction rate (3)

Answer 6

1. The rate at which reactants (R) are consumed, or products (P) are made
2. Units = mol dm⁻³ s⁻¹
3. The less negative, the less reactants are available

Question 7

How is reaction rate calculated (3)

Answer 7

1. Rate (v) can be determined from a plot of concentration versus time.
2. v = d[P]/dt = -(d[R]/dt)
3. The less negative, the less reactants are available

Question 8

What is the rate constant (2)

Answer 8

1. The rate constant expresses rate as a function of concentration and k.
2. k is only constant at fixed temperature and varies with temperature according to the Arrhenius equation (SM).

Question 9

What is zero-order kinetics (4)

Answer 9

1. calculated by: v = k[A]⁰[B]⁰
2. v = k
3. The reaction rate stays constant during the reaction.
4. units = mol dm⁻³ s⁻¹

Question 10

What is first-order kinetics (5)

Answer 10

1. Calculated by: v = k[A]¹[B]⁰
2. v = k[A]
3. Called ‘’the first oder in A’’
4. The reaction rate is proportional to the amount of A left.
5. units = s⁻¹

Question 11

What is second-order kinetics (4)

Answer 11

1. Second order: Two general forms
2. v = k[B] → second order in B
3. v = k[A][B] → first order in A and B
4. Units = mol⁻¹ dm³ s⁻¹

Question 12

What is overall order kinetics (5)

Answer 12

1. general rate law is v = k[A]ᵃ[B]ᵇ
2. and overall order is given by a + b.
3. Units of k depend on the overall order
4. The overall order of some complex reactions can be fractional.
5. e.g. Cl2 + CH4 → CH3Cl + HCl (via UV) is three-halves order overall, one-half order in chlorine.

Question 13

What are complex reactions (SM) (7)

Answer 13

1. Most reactions are complex, a series of elementary reactions (or steps) called the ‘reaction mechanism’.
2. One step = a set of synchronous electron movements.
3. In a haloform reaction, there are nine steps.
4. Single-step bimolecular reaction → 2nd-order
5. But 2nd-order does NOT always mean a single-step bimolecular reaction.
6. Experiments indicate a 3-step reaction involving intermediates.
7. The rate law always arises from the slowest step: The ‘rate limiting’ step.

Question 14

What is molecularity (4)

Answer 14

1. Molecularity is the number of reactant particles involved in a step:
2. 1 → Unimolecular
3. 2 → Bimolecular
4. 3 → Termolecular