Reaction rates

Question 1

Definition of rate of reaction

Answer 1

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

Question 2

Describe 2 methods for following the progress of a reaction

Answer 2

1. Measure the volume of gas produced at regular time intervals using gas collection via a gas syringe or measuring cylinder.
2. Measuring loss in mass using a balance.

Question 3

5 Factors that affect the rate of reaction

Answer 3

1. Concentration
2. Temperature
3. Surface area
4. Catalysts
5. Pressure

Question 4

Definition of a successful collision

Answer 4

A collision that leads to a reaction.

Question 5

In order to have a successful collision…

Answer 5

1. Have enough energy.
2. Must collide in the correct orientation.

Question 6

Explain how increasing the concentration effects the rate of reaction.

Answer 6

Increasing the concentration of a reactant increased the rate of reaction as the number of particles in a given volume increases, so the number of collisions will increase. This means more frequent collisions and more successful collisions.

Question 7

Explain how increasing the pressure increases the rate of reaction

Answer 7

An increase in pressure will increase the rate.
The particles are pushed closer together, so more molecules per unit volume which means more frequent collisions, so more successful collisions.

Question 8

Explain how increasing the temperature increases the rate of reaction

Answer 8

Number of successful collisions increases as more molecules have energy greater than or equal to the activation energy.

Question 9

Draw a general Boltzman distribution diagram

Answer 9

*Refer to new reaction rates notes
Things the examiner is looking for:
1. Labelled axes.
2. Overall shape of curve is right.
3. Must start at the origin.
4. Doesn’t touch the x-axis (no such thing as maximum energy).

Question 10

Draw a Boltzman distribution to represent the effect of temperature on the rate of reaction

Answer 10

*Refer to new reaction rates notes

Question 11

Definition of catalysts

Answer 11

Increases the rate of reaction without being used up by the overall reaction- it’s regenerated.

Question 12

Describe how catalysts work

Answer 12

They provide an alternative route for the reaction with a lower activation energy. A lower activation energy means that more molecules will have an energy equal to or greater than the activation energy. Number of successful collisions increases as more particles with energy equal to or greater than the activation energy.

Question 13

Draw an energy profile diagram to represent the effect of a catalyst on an exothermic reaction

Answer 13

*Refer to new reaction rates notes

Question 14

Draw a Boltzman distribution diagram to show the effect of a catalyst on a reaction

Answer 14

*Refer to new reaction rates notes

Question 15

Definition of a homogenous catalyst

Answer 15

A catalyst that is in the same state as the reactants.

Question 16

2 examples of homogenous catalysts

Answer 16

1. Making esters, e.g: propyl ethanoate
   CH3COOH(l) + CH3CH2CH2OH(l) …. CH3COOCH2CH3(l) + H2O(l). The reaction uses H2SO4(l) as a catalyst.
2. Depletion of the ozone: O3(g) + O(g) … 2O2(g). The reaction uses chlorine radicals as a catalyst.

Question 17

Definition of heterogenous catalyst

Answer 17

A catalyst which is in a different state to the reactants, for example: reactants are gases, catalyst is a solid.

Question 18

Explain how heterogenous catalysts work

Answer 18

1. Absorption- gas molecules absorb onto (forms a weak bond with) the surface of the metal catalyst.
2. Weaking of bonds- the bonds within the reactant molecules are weakned by the interaction with the surface so the reaction happens faster.
3. Desorption- the product molecules are desorbed from the surface into the atmosphere.

Question 19

2 Examples of heterogenous catalyst reactions

Answer 19

1. Haber process- N2(g) + 3H2(g) …. 2NH3(g). Both reactants and products are gases- the catalst is a solid (iron).
2. Hydrogenation of unsaturated oils (alkenes) to make margarine. C2H4(g) + H2(g) …. C2H6(g). Both reactants and products are gases- the catalyst is Ni.

Question 20

How is the rate of reaction calculated?

Answer 20

Rate = (change in volume of gas produced) / (change in time).