6) Reaction rates

Question 1

Define the rate of reaction

Answer 1

The change in concentration of a reactant or a product at a givin time

Question 2

What are the standard units for rate of reaction

Answer 2

mol dm-3 / s-1

Question 3

What factors affect the rate of reaction

Answer 3

• Temperature
• Pressure ( for gases)
• Concentration
• Surface area
• Catalyst

Question 4

How does concentration affect the rate of reaction

Answer 4

• The rate of reaction increases
• Number of particles in a given volume increases
• Number of collisions increases
• So more frequent collisions
• More successful collisions

Question 5

How does pressure affect the rate of reaction

Answer 5

• Increases the rate of reaction
• The particles are pushed closer together
• More frequent collisions
• Number of successful collisions increases

Question 6

How does temperature affect the rate of reaction

Answer 6

• Increases the rate of reaction
• Molecules are moving faster as they have more kinetic energy
• The number of collisions increases
• Number of successful collisions increase as more molecules have an energy equal to or greater then the activation energy

Question 7

Define catalyst

Answer 7

Speeds up the rate of reaction by lowering the activation energy .
Catalyst is regenerated at the end of the reaction

Question 8

How do catalysts work

Answer 8

Provide an alternative pathway for the reaction with a lower activation energy

Question 9

How does a catalyst affect the rate of reaction

Answer 9

-Increases the rate of reaction
- A lower activation energy means that more molecules will have an energy greater then or equal to the activation energy
- Number of successful collisions increases as more collisions with energy greater then activation energy

Question 10

Define homogeneous catalysts

Answer 10

Is a catalyst that is in the same state as the reactants

Question 11

Define heterogeneous catalysts

Answer 11

Is a catalyst which is in a different state to the reactants

Question 12

Name some examples of homogeneous catalysts

Answer 12

• Making esters ( H2SO4)
• Depletion of ozone ( Cl*)

Question 13

How do heterogeneous catalysts work

Answer 13

• Absorption = gas molecules absorb onto the surface of the metal catalyst (forms a weak bond with it)
• Weakening of the bonds = The bonds within the reactant molecules are weakened by the interaction with the surface so the reaction happens faster
• Desorption = The product molecules are desorbed from the surface into the atmosphere

Question 14

How do catalysts increase the sustainability of a process

Answer 14

1) They lower the activation energy which allows the reaction to run at a lower temperature. This saves energy costs
2) This also means less fossil fuels burnt so that they will last longer and less CO2 is emitted
3) As activation energy is lowered, an alternative reaction may be used with a better atom economy or less hazardous waste produced

Question 15

Describe the rate of reaction on a graph

Answer 15

1) At the start - The rate is fastest at the start since the reaction concentration is at its highest. Therefore, more frequent collisions so more successful collisions. So a faster rate
2) During the reaction - The rate of reaction decreases
3)When the one of the reactants has been used up the rate becomes zero