OZ - More on reaction rates

Question 1

What factors speed up the rate of reaction?

Answer 1

Increasing concentration.
Increasing pressure.
The use of catalysts.
Increasing temperature.

Question 2

How does increasing the concentration of reactant in solution affect the rate of a reaction?

Answer 2

Speeds up the reaction because the particles are closer together on average so the collision frequency would increase.

Question 3

How does increasing the pressure of a gas affect the rate of a reaction?

Answer 3

Speeds up the reaction because the particles are closer together on average so the collision frequency would increase.

Question 4

How does the use of catalysts affect the rate of a reaction?

Answer 4

They speed up reactions by lowering the activation enthalpy by providing an alternate pathway for the bonds to be broken and remade. This means more particles will have enough energy to react.

Question 5

How does increasing the temperature affect the rate of a reaction?

Answer 5

Speeds up the reaction because the particles have more energy so they are more likely to react when they collide. Also, there is an increased collision frequency.

Question 6

What methods can be used to measure the rate of a reaction?

Answer 6

• Measuring the volume of gas produced.
• Measuring the loss of mass as a gas is produced.
• Measuring the change in pH during a reaction.
• Measuring a temperature change.
• Taking samples at regular intervals and analysing them by titration.

Question 7

How do you plot a graph to show rate of reaction?

Answer 7

Time is plotted on the x-axis and the change you measured goes on the y-axis.

Question 8

How do you find the rate of reaction from a graph?

Answer 8

Draw a tangent to the curve at that time. The gradient of the tangent is the rate of reaction.

Question 9

What units does rate have?

Answer 9

Change/unit of time.

Question 10

What is the equation for finding the gradient of a curve of a graph?

Answer 10

Gradient = Change in y/Change in x

Question 11

At what point during a reaction is the rate always the quickest and why?

Answer 11

At the beginning because as the reaction continues, the concentration of reactants will decrease (as they form products), so there will be less frequent collisions between reactant particles and the rate will decrease.

Question 12

What are homogeneous catalysts?

Answer 12

Catalysts that are in the same state as the reactants.

Question 13

How do catalysts work?

Answer 13

They increase the rate of a reaction by providing an alternative reaction pathway with a lower activation enthalpy. The catalyst is chemically unchanged at the end of the reaction.

Question 14

How many types of catalyst are there?

Answer 14

2

Question 15

What are the 2 types of catalysts?

Answer 15

Homogeneous and heterogeneous.

Question 16

What are heterogeneous catalysts?

Answer 16

Catalysts that are in a different state to the reactants.

Question 17

Give an example of homogeneous catalysis in the body

Answer 17

When enzymes (biological catalysts) catalyse reactions in body cells, everything is aqueous.

Question 18

How do homogeneous catalysts work?

Answer 18

They speed up reaction by forming one or more intermediate compounds with the reactants. The products are then formed from the intermediate compounds.

The activation enthalpy needed to form the intermediates (and to from the products from the intermediates) is lower than that needed to make the products directly from the reactants.

Question 19

What will the enthalpy profile look like of a reaction is sped up by a homogeneous catalyst?

Answer 19

It will have 2 humps in it.

Question 20

What happens to the catalyst during a reaction?

Answer 20

It reforms and carries on catalysing the reaction remaining chemically unchanged.

Question 21

What differences are there in the Boltzmann distribution when a catalyst is used compared to when one isn’t?

Answer 21

There is no difference, only more particles have enough energy to react.