Rates of reactions

Question 1

Describe the collision theory

Answer 1

For a successful reaction to occur, the reactants must undergo:

Collison

Have a sufficient energy, that which is greater than the activation energy

Have the correct molecular geometry/orientation during collision

Question 2

What is the definition for the ‘rate’ of a reaction?

Answer 2

The change in concentration of a reactant or a product in a given time.

Equation:
Change in concentration / Time taken (s)

Question 3

What factors affect the rate of a reaction?

Answer 3

Catalyst

Temperature

Concentration of reactants/ products

Pressure

Light

Surface area

Question 4

What is an order with respect to a reactant?

Answer 4

It is the power to which the concentration of the reactant is raised in the rate equation.

Question 5

What is the rate equation?

Answer 5

Rate = kA x [B] (to the power of m)

Question 6

What is the overall order?

Answer 6

The sum of the individual orders of reactants in the equation.

Question 7

Rate constant

Answer 7

The constant that links the rate of reaction with the concentrations of the reactants, raised to the power of their orders in the rate reaction.

Question 8

How are orders calculated?

Answer 8

Through information derived from experiments

Question 9

What is half life? (3)

Answer 9

The time taken for half of the reactant to be used up.

First orders have a constant half life as their concentration halves every half life.

This is an example of exponential decay.

Question 10

How is the rate of reaction and order calculated from a concentration time graph?

Answer 10

The gradient of the graph is equal to the rate.

The shape of the graph is an indication of the order with respect to a reactant.

0 order: Shape is a straight line with a negative gradient and a constant rate. The gradient is equal to the rate constant k.

1st Order: The shape shows a downwards curve with a decreasing gradient and reaction rate. The half life is constant.

2nd order: Steeper downwards curve that tails of at a slower rate.

Question 11

How is the rate constant k calculated from half life?

Answer 11

Uses the exponential relationship for a constant half-life.

k = ln2/ half life (t1/2)

Question 12

What is the rate- determining step?

Answer 12

Reactions happen in steps as specific particles have to collide for the reaction to progress. In some reactions more than two sets of particles have to collide.

The rate- determining step is the slowest step in the sequence. As the reaction can only be as quick as the slowest step.

Question 13

Describe reasons why a reaction might not be one step.

Answer 13

The stoichiometry in the rate equation might not match that of the overall equation.

It is very unlikely that more than 2 species will react at once.

Question 14

Describe the Arrhenius equation and how it can be used to graphically find the activation energy and the frequency factor.

Answer 14

The equation is the exponential relationship between the rate constant and the temperature.

k= A e(-Ea/RT)

A= frequency factor
Ea= activation energy
R= gas constant (8.314)
T= temperature

Can be re-expressed in the form: y=mx+c in order to find A and Ea graphically.

lnk= -Ea/R x 1/T + lnA

Ea= the gradient of the graph x -R
A= the exponential value of ln to the intercept of the graph (e(to the power of the intercept).

Question 15

Describe a 0 order rate-concentration graph.

Answer 15

A straight horizontal line with a gradient of zero.
The y intercept is equivalent to the rate constant.
Rate does not change despite change in concentration.

Question 16

Describe a 1st order rate-concentration graph.

Answer 16

A straight diagonal line that goes through the origin.
The rate is directly proportional to the concentration
K= gradient of the grap (rate/concentration)

Question 17

Describe a 2nd order rate-concentration graph.

Answer 17

An upward curve with an increasing gradient.
To obtain the rate constant:
Plot a rate against concentration squared graph.
The gradient of that graph is equal to the rate constant.

Question 18

Describe the experiment used to find the order of reactants: S2O8 2-, I-

Answer 18

1- Vary concentrations of I- whilst keeping other concentrations constant

2- Vary concentrations of S2O8 2- whilst keeping other concentrations constant.

Use Starch indicator. Time how long it takes for colour change: colourless to blue/black

Determine rate: 1/t

Plot a rate-concentration graph for each reactant. The shape determines the order.

Question 19

Describe the experiment that uses colorimetry to determine the order of a reactant: Iodine. (7)

Answer 19

1. Standard solutions of known concentrations of Iodine is used.
2. Select the appropriate filter in the colorimeter, complementary to the colour of iodine.
3. Calibrate the colorimeter using distilled water.
4. Take absorbance readings for each concentration of I2.
5. Plot a calibration curve: absorbance against concentration.
6. Now when Iodine reacts with a compound, absorbance readings are taken at time intervals. Calibration curve is used to work out concentration.
7. Plot a concentration-time graph from results. Shape determines order of reaction.