Rates and energy

Question 1

Two ways of measuring rates of reactions and examples.

Answer 1

Time taken for a certain mass of reactant to be used
Time taken for a certain amount of product to be formed
E.g. Time taken for a certain amount of solid to appear in a solution.
Time taken to collect a certain amount of gas.

Question 2

Equation for rate of reaction

Answer 2

rate of reaction = amount of reactant used or amount of product formed ÷ time

Question 3

What four factors affect the rate of reaction?

Answer 3

1) Temperature
2) Surface area
3) Concentration/pressure
4) The presence of a catalysts

Question 4

Define activation energy.

Answer 4

The smallest amount of energy particles need in collisions before they can react.

Question 5

Why do catalysts speed up the rate of reactions?

What are the benefits of using catalysts in chemical processes? (2)

Answer 5

Catalysts that speed up the rate of reaction, lower the activation energy of the reaction, thus increasing the frequency of successful collisions.

1) They reduce energy costs and they are not used up in reactions.
2) If fossil fuels are burned to provide energy for industrial reactions, using catalysts will help to conserve resources and reduce pollution.

Question 6

What type of substance are catalysts often made?

What are the disadvantages of these substances?

Answer 6

Transition metals and their compounds.

1) They can be toxic and harm the environment if they escape.
2) They can also be rare and expensive.
3) Catalysts often only work with one type of reaction, so different reactions require different catalysts.

Question 7

What is an exothermic reaction?

What is an endothermic reaction?

Answer 7

Exothermic reactions transfer, often heat energy to the surroundings, and so causes the temperature to increase.

Endothermic reactions take in energy from the surroundings, some cause a decrease in temperature and others require a constant supply of energy.

Question 8

Give two examples of exothermic reactions and two everyday uses.
What is special about some of these products?
Give an example of this?

Answer 8

1) Combustion - burning fuels
2) Neutralisation reactions - involving acids and bases.

1)Self-heating cans
2) Hand warmers
The reactions are reversible so they can be reused again and again.
The crystallisation of a salt, once used, the pack can be heated in boiling water to re-dissolve the salt.

Question 9

Give one example of an endothermic reaction and one everyday uses.
Can these products be reused and why?

Answer 9

1) Thermal decomposition - needs to be heated continually to keep the reaction going.
1) Sports injury packs
No, although the reaction is reversible it is not reversible in the pack so this type of pack can be used only once.

Question 10

A reversible reaction that is exothermic in one direction must be what in the opposite direction?
Describe the energy transfers in a reversible reaction.

Answer 10

Endothermic.

The amount of energy released by the exothermic reaction is equal to the energy taken in by the endothermic reaction.

Question 11

If hydrated copper sulphate produces anhydrous copper sulphate and water(g) with heat and is an endothermic reaction, what happens when water is added to anhydrous copper sulphate?

Answer 11

It produces hydrated copper sulphate and is exothermic (releases heat).

Question 12

For a graph of amount of reactant or product over time, what does the gradient represent?
What does a steep gradient represent?

Answer 12

The rate of reaction

Fast rate of reaction

Question 13

At ordinary temperatures what affect will increasing the temperature by 10ºC have on the rate of reaction?

Answer 13

It will roughly double the rate of reaction.

Question 14

Why do we refrigerate or freeze food?

Answer 14

So it stays fresh for longer.

Question 15

How are catalysts used as effectively as possible?

Answer 15

They are often used in forms that have large surface areas.

Question 16

What would be the benefits of replacing traditional catalysts with enzymes; biological catalysts?

Answer 16

They will reduce energy costs even further.

Question 17

How does increasing the temperature increase the rate of reaction?

Answer 17

Increasing the temperature increases the speed of the particles in a reaction mixture. This means the frequency of collisions increases, which increases the rate of reaction. As well as colliding more frequently, they collide with more energy which also increases the rate of reaction.

Question 18

Why does increasing the surface area increases the rate of reaction?

Answer 18

Increasing the surface area means more particles are exposed so there are more collisions in a given time.

Question 19

Why does increasing the pressure of a gas or increasing the concentration of a liquid increase the rate of reaction?

Answer 19

If the concentration of a liquid or the pressure of gas is increased there are more particles dissolved in the same volume, this means the dissolved particles are closer together and so collide more often. The frequency of successful collisions therefore increases and so does the rate of reaction.