5.1 - 5.3 + 18.1 - 18.5 Kinetics

Question 1

Explain collision theory

Answer 1

For a reaction to take place particles must collide with enough energy and in the correct orientation. This is called a successful collision.

Question 2

Define activation energy, Ea

Answer 2

The minimum amount of energy required to start a reaction

Question 3

What are 3 reasons might there be for a collision between reactant particles not leading to a reaction?

Answer 3

• They do not collide with enough energy
• They are not in the correct orientation
• The collision was not between the correct particles

Question 4

What does a Maxwell-Boltzmann distribution graph show?

Answer 4

The different amounts of energy that particles in a given gas or liquid have

Question 5

Define rate of reaction

Answer 5

The change in concentration of a substance per unit time

Question 6

List 4 things that can have an effect on the rate of reaction

Answer 6

1. The concentration or pressure
2. The surface area
3. The temperature
4. Adding a catalyst

Question 7

Why does an increase in concentration or pressure increase the rate of reaction?

Answer 7

More particles in a given volume so more frequent successful collisions will happen

Question 8

Why does an increase in surface area increase the rate of reaction?

Answer 8

More particles of solid exposed so more chance of a successful collision with other reactant particles

Question 9

Why does an increase in temperature increase the rate of reaction?

Answer 9

• Particles move faster so collide more often
• More particles with more energy than the activation energy

Question 10

Why does adding a catalyst increase the rate of reaction?

Answer 10

Catalyst provides an alternative reaction pathway with a lower activation energy. Lowering the activation energy means more particles with have energy exceeding the activation energy so more successful collisions will happen.

Question 11

List three ways of measuring the rate of reaction

Answer 11

1. If a gas is produced, measure volume produced or mass lost over time
2. Precipitation/colour change
3. Changes in electrical conductivity or pH

Question 12

What is the effect of increase in rate on increase in concentration if the reactant is first order?

Answer 12

Increase in rate = Increase in concentration

Question 13

What is the effect of increase in rate on increase in concentration if the reactant is second order?

Answer 13

Increase in rate = (Increase in concentration)²

Question 14

What is the effect of increase in rate on increase in concentration if the reactant is zero order?

Answer 14

Increase in concentration has no effect on rate

Question 15

What is the rate equation?

Answer 15

rate = k [A]x^ [B]y^

k = rate constant
[ ] = concentration
x = order with respect to A
y = order with respect to B
(x+y) = overall order of reaction

Question 16

Describe what a graph showing zero order would look like

Answer 16

Concentration vs time:
- A straight line of constant decline

Rate vs concentration:
- A straight horizontal line

Question 17

Describe what a graph showing first order would look like

Answer 17

Concentration vs time:
- A slightly sloping declining curve with a constant half-life

Rate vs concentration:
- A straight line diagonally up

Question 18

Describe what a graph showing second order would look like

Answer 18

Concentration vs time:
- A steeply declining curve that then levels out

Rate vs concentration:
- A line curving up

Rate vs concentration²:
- A straight line diagonally up

Question 19

What is the rate constant, k?

Answer 19

The value of the rate at any particular temperature when the concentration of all species in the rate equation are 1 mol dm⁻³
Bigger k = Faster reaction

Question 20

Describe the link between the rate constant and activation energy

Answer 20

As Ea↑, rate ↓ so k↓

Question 21

Describe the link between the rate constant and temperature

Answer 21

As temp↑, rate ↑ so k↑

Question 22

What do the different parts of the Arrhenius equation stand for?

Answer 22

k = rate constant
A = Arrhenius constant
e = mathematical quantity, e
Ea = Activation energy (J mol⁻¹) 
R = Gas constant (8.31 J k⁻¹ mol)
T = temp (k)

Question 23

What is the conversion for °C to K?

Answer 23

°C + 273 = K

Question 24

On a graph of lnk against 1/T, what does the gradient represent?

Answer 24

Gradient = -Ea/R
Y-intercept = lnA

Question 25

Explain why using a large excess of all other reactants means the rate of reaction depends only on the one not in excess

Answer 25

• The concentration of the reactants in excess is effectively constant
• This means these reactants become zero order

Question 26

What must be done to a solution of reactants before the concentration of one of them is measured?

Answer 26

• Stop the reaction
• By cooling

Question 27

rate = k[E]
Why would doubling the temp have a much greater effect on rate of reaction than doubling [E]?

Answer 27

• Reaction occurs when molecules have E>Ea
• Doubling T may cause many more molecules to have E>Ea
• Doubling [E] will only double molecules with E>Ea

Question 28

What effect does introducing a catalyst have on a Maxwell-Boltzmann distribution curve?

Answer 28

No effect

Question 29

Give the logarithmic arrhenius equation

Answer 29

lnK = (-Ea/R) x (1/T) + lnA