CI 1 - Kinetics

Question 1

What is meant by rate of reaction?

Answer 1

The change in concentration of reactants or products over time

Question 2

What is a rate constant?

Answer 2

The constant of proportionality linking the rate of reaction and the concentrations of the reactants raised to their orders in the rate equation

Question 3

How do you calculate the units of a rate constant?

Answer 3

1. Rearrange the rate equation to make k the subject
2. Substitute units into the equation
3. Cancel the common units to find the units for k

Question 4

What is the order of reaction?

Answer 4

The order with respect to a reactant is the power to which the concentration of the reactant is raised in the rate equation
Overall order = sum of all the individual orders of the reactants

Question 5

What do the different orders of reaction mean?

Answer 5

Zero order: if rate proportional to [A] is 0, then the rate of reaction is unaffected by changing [A]
First order: if rate proportional to [A] is 1, then the rate of reaction increases at the same rate as [A] changes
Second order: if rate proportional to [A] is 2, then rate will increase by the square that [A] increases by

Question 6

What is a rate equation?

Answer 6

For a reaction between A and B:
Rate = k[A]^m[B]^n
m and n represent the orders with respect to each reactant
k is the rate constant

Question 7

How does the rate constant, k vary with increasing temperature?

Answer 7

As temperature increases there are more particles with energy > activation energy
As increase in temperature therefore causes an increase in k

Question 8

What does a rate-concentration graph look like for a zero order reaction?

Answer 8

Straight line across with gradient zero

Question 9

What does a rate-concentration graph look like for a first order reactant?

Answer 9

Straight line going up with constant positive gradient

Question 10

What does a rate-concentration graph look like for a second order reactant?

Answer 10

Curved line going up with and increasing positive gradient

Question 11

What is meant by the half-life of a sample?

Answer 11

The average time taken for the concentration of a reactant to decrease by 1/2

Question 12

How do you calculate the rate constant, k from half life? (only applies to first order)

Answer 12

k = (ln2) / (half life)
(this equation ONLY applies to first order reactants)

Question 13

What does a concentration-time graph look like for a zero order reactant?

Answer 13

Straight line going slightly down with a constant negative gradient

Question 14

What does a concentration-time graph look like for a first order reactant?

Answer 14

Curved line going down with and increasing negative gradient (it is getting less steep)

Question 15

How do you calculate half life from a first order concentration-time graph?

Answer 15

Using the graph, find the time taken for the concentration to halve. Then find the time taken for it to halve again (to increase accuracy). Calculate the average of these values and this will be the half-life

Question 16

How do you calculate the rate from a first order concentration-time graph?

Answer 16

1. Draw a tangent at the time you want to calculate the rate of reaction for
2. The gradient of this tangent will equal the rate of reaction

Question 17

How do you calculate k from a first order rate-concentration graph?

Answer 17

1. Calculate the gradient of the line of best fit
2. The gradient will equal the rate constant, k

Question 18

How do you calculate the gradient of a line/tangent?

Answer 18

Gradient = (change in y-coordinate) / (change in x-coordinate)

Question 19

What is the Arrhenius equation?

Answer 19

k = Ae^(Ea / RT)
k = rate constant
A = pre-exponential factor
Ea = activation energy
R = gas constant
e = base of the natural logarithm, 2.72 to 3s.f.
T = temperature in kelvin

Question 20

How do you determine Ea and A for a reaction, using the Arrhenius equation graphically?

Answer 20

1. Take natural logs of the Arrhenius equation:
   ln k = ln A - (Ea / R x 1 / T)
2. Plot a graph of ln k against 1 / T
3. The gradient = Ea / R so Ea = gradient x R
4. Y-intercept = ln A so A = e^(y-intercept)

Question 21

What is the rate determining step?

Answer 21

The slowest step of the reaction
Only species that take part in the rate determining step (or steps that take place before it) affect the rate

Question 22

What is the relation between the rate-determining step and the orders with respect to the reactants?

Answer 22

The species present in the rate equation are those that take part in the rate determining step
For any reactant in the rate equation, the order attached to it tells you how many molecules of it are involved in the rate determining step