18 Rates of Reactions

Question 1

What is rate of reaction? (1 mark)

Answer 1

Quantity reacted or produced/ change in time

Question 2

What is the shorthand for concentration of A? (1 mark)

Answer 2

[A]
Must be square brackets

Question 3

What is rate of reaction proportional to? (1 mark)

Answer 3

[A]^n
n= order of reaction

‘Concentration of a reactant to the power of n’

Question 4

What is the order of the reaction if a reactant has no effect on the rate? (1 mark)

Answer 4

0

Question 5

What does a reaction order of 1 mean, give an example? (2 marks)

Answer 5

The rate depends on it’s concentration raised to the power of 1.
So if the concentration of A is doubled, the rate of reaction increases by a factor of 2^1

Question 6

What does a reaction order of 2 mean, give an example? (2 marks)

Answer 6

The rate depends on it’s concentration raised to the power of 2.
So if the concentration of A is doubled, the rate of reaction increases by a factor of 2^2

Question 7

What is the overall order? (1 mark)

Answer 7

The overall effect of the concentration of all the reactants on the rate of reaction.

Question 8

How do you work out overall order? (1 mark)

Answer 8

The sum of all orders with respect to each reactant.

Question 9

What is an equation for rate which contains the rate constant and order of reaction? (2 marks)

Answer 9

rate= k x [A]^m x [B]^n

k- rate constant
m- order with respect to A
n- order with respect to B

Question 10

How is the data for a concentration-time graph collected? (3 marks)

Answer 10

Continuous monitoring over the course of a reaction.
By collecting gas formed, mass loss, or colour change (using a colorimeter).

Question 11

Describe how to analyse using colorimetry. (6 marks)

Answer 11

Prepare standard solutions of coloured chemical.
Use a filter with complementary colour.
Zero the colorimeter with water.
Measure the absorbance of the standard solutions and plot a calibration curve.
Conduct reaction and take absorbance readings at intervals.
Use the calibration curve to calculate the concentration of the chemical at the absorbance reading.
Plot a concentration-time graph.

Question 12

What does the shape of a concentration-time graph tell you? (1 mark)

Answer 12

the order of the reaction (for 1st and 0th)

Question 13

What does the concentration-time graph of a zero-order reaction look like? (2 marks)

Answer 13

Straight line
Negative gradient

Question 14

In a zero-order reaction, what does the gradient of the concentration-time graph tell you? (1 mark)

Answer 14

The rate constant (k).

Question 15

What does the concentration-time graph of a first-order reaction look like? (3 marks)

Answer 15

Downward curve
Decreasing concentration
Half life is constant (the time the concentration takes to halve)

Question 16

What does the concentration-time graph of a second-order reaction look like? (2 marks)

Answer 16

Downward curve
Steeper at the start and tails off more slowly.

Question 17

How do you calculate k from rate of reaction (for first order reaction)? (3 marks)

Answer 17

Choose a random concentration, on a concentration-time graph, and draw a tangent to find rate at that concentration.
Substitute this value into the rate equation.

Question 18

How do you calculate k from half life (for a first order reaction)?

Answer 18

k= ln2/ half life

Question 19

What does the rate-concentration graph look like for a zero order reaction? (1 mark)

Answer 19

Horizontal straight line

Question 20

What does the rate-concentration graph look like for a first order reaction? (1 mark)

Answer 20

Straight line through origin.

Question 21

What does the rate-concentration graph look like for a second order reaction? (2 mark)

Answer 21

Upward curve with increasing gradient.

Question 22

Where is k on a rate-concentration graph for a zero order reaction, why? (2 marks)

Answer 22

The y intercept
Because the rate equation is:
Rate= k[A]^0
So Rate= k

Question 23

Where is k on a rate-concentration graph for a first order reaction, why? (2 marks)

Answer 23

The gradient of the line
Because rate equation is:
Rate= k[A]^1
So Rate= k[A]

Question 24

How do you find k from a rate-concentration graph for a second order reaction? (3 marks)

Answer 24

Plot a second graph (rate against concentration^2)
K is the gradient of the straight line.

Question 25

Describe the clock reaction. (3 marks)

Answer 25

If there is no significant change in rate, then record the time until visual change.
In this case, the initial rate is inversely proportional to the time.

Question 26

What do iodine clocks rely on? (2 marks)

Answer 26

The formation of iodine (the colour would change from orange to black).

Question 27

How accurate are clock reactions, over time?

Answer 27

It measures the average rate of change in reactant over time, so the less of a reaction taken place, the more accurate it is.

Question 28

What is the slowest step in a reaction mechanism called? (1 mark)

Answer 28

The rate-determining step

Question 29

What does the rate equation contain? (1 mark)

Answer 29

Only the reactants from the rate determining step.
E.g. EF + F —> G
rate= k[E][F]^2

Question 30

What happens to the rate constant as temperature increases? (1 mark)

Answer 30

The rate constant increases.

Question 31

Why does the rate constant increase when temperature increases? (5 marks)

Answer 31

It shifts the Boltzmann distribution to the right, so more particles exceed the activation energy.
It also causes particles to move faster and collide more frequently at the right orientation.
However, the number of particles exceeding activation energy has much more of an effect than frequency collisions.

Question 32

What does the exponential factor, in the Arrhenius equation, represent? (1 mark)

Answer 32

The proportion of molecules/particles that exceed Ea.

Question 33

What does the frequency factor, in the Arrhenius equation, take into account? (2 marks)

Answer 33

The frequency of collisions
with the correct orientation.

Question 34

The Arrhenius equation is shown below:
k= Ae^(-Ea/RT)
What is the logarithmic form of this equation, pair each term up with a term form the equation y= mx + c?

Answer 34

lnk = -Ea/RT • 1/T + lnA
y = m • x + c

Question 35

If you plot a graph of lnk against 1/T, what is the gradient (according to the Arrhenius equation)?

Answer 35

-Ea/R

Question 36

If you plot a graph of lnk against 1/T, what is the y-intercept (according to the Arrhenius equation)?

Answer 36

lnA