Kinetics

Question 1

What is the rate of reaction?

Answer 1

It is the change in the amount of a reactant or product per unit time (how fast a reaction takes place)

Question 2

How does a reaction occur?

Answer 2

The reactant molecules must collide successfully - collide with sufficient energy and at the correct orientation

Question 3

What is the rate of reaction proportional to?

Answer 3

The frequency of successful collisions between the reactant molecules

Question 4

What makes a collision successful?

Answer 4

The correct orientation to allow contact between atoms that will become bonded together in the product. The collision must occur with adequate energy (activation energy) to allow the reacting species’ valence shells to come close enough so that the electrons can rearrange and form new bonds.

Question 5

How can you show the activation energy?

Answer 5

Draw an energy profile diagram.

Question 6

What should you include in an energy profile diagram?

Answer 6

X axis - extent of reaction
Y axis - energy
Reactants
Products
Arrow from reactants to peak (activation energy)
Arrow from reactants to products (delta H)

Question 7

How can you show what proportion of reactive molecules present have activation energy?

Answer 7

You draw the Maxwell-Boltzmann distribution graph

Question 8

What should you include on a Maxwell-Boltzmann distribution graph?

Answer 8

X axis - energy
Y axis - no of molecules
Line to show the activation energy
Line to show most probable energy

Question 9

What does the area under the curve of a Maxwell-Boltzmann distribution graph equal?

Answer 9

The total number of molecules in the sample

Question 10

What factors affect the rate of reaction?

Answer 10

Surface area
Concentration
Pressure
Temperature
Presence of a catalyst

Question 11

How does the surface area affect the rate of reaction?

Answer 11

When the surface area increases the frequency of collisions increase so the rate of reaction increases.

Question 12

How does the concentration affect the rate of reaction?

Answer 12

It increases the frequency of successful collisions and so the reaction rate increases.

Question 13

How does a change in pressure affect the reaction rate?

Answer 13

Higher pressure = the closer the molecules are so the frequency of successful collisions increases and so the reaction rate increases

Question 14

How does temperature affect the rate of reaction?

Answer 14

1. Average kinetic energy increases = so molecules move faster and collide more frequently
2. A greater proportion of the molecules have energy greater or equal to the activation energy, so more of the collisions are successful.

Question 15

What is a catalyst

Answer 15

A catalyst is a substance that provides an alternative reaction pathway for a reaction which has a lower activation energy. A catalyst does not get used up because it regenerates

Question 16

What is the effect on the rate of reaction when a catalyst is present?

Answer 16

It increases the rate because a greater proportion of the reaction molecules will have energy equal to the reaction energy, therefore, more collisions will be successful

Question 17

Why are rates of reaction studied?

Answer 17

1. To discover the best conditions to make the reaction go as quickly as possible.
2. To gather information about the mechanism of a reaction (how the reaction actually takes place.)

Question 18

Give 2 methods of physically measuring the rate of reaction.

Answer 18

1. Volume against time e.g. the reaction between HCl and CaCO3.
   The reaction gives off a gas whose volume can be measured in a gas syringe. Time it using stop watch from start of reaction to end (when gas syringe stops moving.)
2. Decrease in mass against time e.g. the same reaction but using an open flask on a top pan balance with cotton in the mouth. Record how much mass decreases by end of reaction. Time from start of reaction to when mass stops decreasing.

Question 19

What is the chemical method for testing rate of reaction?