Topic 9 Kinetics I

Question 1

Rate of reaction =

Answer 1

Change in concentration
————————————-
Time for change to happen

Question 2

How can you calculate the rate of reaction from a graph

Answer 2

Draw a tangent to the curve at the point rate of reaction is to be measured
Find the gradient (change in y/change in x)

Question 3

What do you need to measure in order to determine the rate of a reaction?

Answer 3

How fast one of the products/reactants is being used up/formed

Question 4

What are the two requirements for a reaction to occur

Answer 4

The two molecules must collide with sufficient energy to cause a reaction (activation energy)
The two molecules must collide in the correct orientation

Question 5

What is steric hinderance

Answer 5

When the shapes of the molecules influence the reaction we say there is a ‘steric factor’ involved.
In some cases the atoms/groups of atoms in a molecule can hinder the course of a reaction (ie. If particularly large it can get in the way of the attacking species)
If this happens we say the reaction suffered from ‘steric hinderance’

Question 6

Describe the effect of concentration on the rate of reaction

Answer 6

For many reactions if the conc of a solution is increased then the frequency of collision between reacting solute particules also increases.
This is because they are closer together as there are more of them in a given volume of solution so the frequency of successful collisions increases

Question 7

Describe the effect of pressure on the rate of reaction

Answer 7

For a reaction in which molecules collide and react in gas phase an increase in pressure will cause an increase in rate of reaction
This is because there will be more reactant molecules in a given volume of mixture so the frequency of successful collisions will increase

Question 8

Describe the effect of surface area on the rate of reaction

Answer 8

For heterogeneous reactions involving a solid, a larger sa will result in an increased rate of reaction

Question 9

What are the Maxwell Boltzmann distribution curves?

Answer 9

Molecules in a sample have a wide range of energies
In order to estimate what fraction of collisions will have the required Ea, we need to know the energy distribution of the molecules

Question 10

If the temperature were increased what would be the effect on the maxwell Boltzmann distribution curve

Answer 10

Peak moves to right (higher energy) with a lower height
Area under curve is the same

Question 11

If a catalyst was added what would be the effect on the maxwell boltzmann distribution curve ?

Answer 11

The Ea line would shift to the left
Curve would remain the same exactly

Question 12

If the concentration was increased, what would be the effect on the maxwell boltzmann distribution curve ?

Answer 12

Peak remains at the same energy
Peak has a stretched shape |

Question 13

Why does a small increase in temperature result in a larger increase in the rate of a gaseous reaction

Answer 13

At a higher temp particles have more KE
Greater proportion can exceed the Ea & therefore collide with sufficient energy for a reaction to take place
More successful collisions = faster rate of reaction

Question 14

How does a catalyst work ?

Answer 14

Lowers Ea by providing an alternative pathway (may be an intermediate or a sa for reaction to occur on)
Rate of reaction is increased
They are regenerated at the end

Question 15

Describe the reaction profile diagram for an endothermic reaction

Answer 15

Enthalpy level of products is above that of the reactants

Question 16

What are the 2 major economic advantages of the uses of catalysts ?

Answer 16

• They increase the rate of a chemical reaction meaning that more of the desired product can be made in a given time period
• Reactions can take place at lower temperatures resulting in a decrease in the energy costs to the manufacturer

Question 17

What is a homogenous catalyst and how does it work ?

Answer 17

A catalyst that is in the same state as reactants
They work by reacting with the reactants to form an intermediate which breaks down to give products and regenerated catalyst

Question 18

Give 2 examples of use of homogeneous catalyst

Answer 18

Cl radical in ozone depletion
Sulphuric acid when making esters

Question 19

What is a heterogenous catalyst and how does it work?

Answer 19

Catalyst is in different state to reactants (usually solids)
Works by providing a surface on which the reaction can take place

Question 20

Give 2 examples of the use of a heterogenous catalyst

Answer 20

Use of iron in the Haber process
Use of Vanadium (V) oxide in the contact process

Question 21

What happens at the surface of a catalyst

Answer 21

One of the reactants approches the catalysts surface and settles onto an active site - ADSORPTION
Another reactant approches the catalyst
Re-arrangement of electrons takes place- REACTION
The products are released from the surface- DESORPTION