Energy Changes

Question 1

What is an exothermic reaction?

Answer 1

They transfer energy from the reacting molecules to the surroundings. So the temperature of the surroundings increases.
E.g.
Combustion
In an exothermic energy profile, the products have less energy than the reactants.
Uses:
Handwarmers
Self-heating cans

Question 1

What is an exothermic reaction?

Answer 1

They transfer energy from the reacting molecules to the surroundings. So the temperature of the surroundings increases.
E.g.
Combustion
In an exothermic energy profile, the products have less energy than the reactants.
Uses:
Handwarmers
Self-heating cans

Question 2

What is an endothermic reaction?

Answer 2

Take in energy from their surroundings, so the temperature decreases.
E.g. thermal decomposition
In an endothermic energy profile, the products have more energy than the reactants.

Question 3

What is meant by activation energy?

Answer 3

The minimum amount of energy that particles need to collide and react.

Question 4

Required Practical 4: Temperature changes

Answer 4

Independant variable: volume of sodium hydroxide solution
Dependant variable: maximum temperature reached
Control variables: volume of hydrochloric acid and the concentrations of both hydrochloric acid and sodium hydroxide solution

1. Use a measuring cylinder to measure 30cm^3 of dilute hydrochloric acid.
2. Then transfer the acid into a polystyrene cup.
3. Stand the cup inside a beaker -this stops the cup from falling over.
4. Use a thermometer to measure the temperature of the acid and record this in a table.
5. Use a measuring cylinder to measure 5cm^3 of sodium hydroxide solution and transfer this to the polystyrene cup.
6. Fit a plastic lid to the cup and place the thermometer through the hole in the lid. The bulb of the thermometer must be in the solution.
7. Use the thermometer to gently stir the solution.
8. This reaction is exothermic so it releases energy. So the temperature of the solution will increase.
9. Look carefully at the temperature rise on the thermometer.
10. When the reading on the thermometer stops changing, we record the highest temperature reached.
11. Rinse out and dry the polystyrene cup.
12. Repeat the whole experiment using 10cm^3 of sodium hydroxide.
13. Carry it out several times, just increasing by 5cm^3 until we reach a maximum of 40cm^3 of sodium hydroxide solution.
14. And then repeat the whole thing again so there are two sets of results.
15. These can be used to calculate a mean value for the maximum temperature reached for each volume of sodium hydroxide solution.

Question 4

Required Practical 4: Temperature changes

Answer 4

Independant variable: volume of sodium hydroxide solution
Dependant variable: maximum temperature reached
Control variables: volume of hydrochloric acid and the concentrations of both hydrochloric acid and sodium hydroxide solution

1. Use a measuring cylinder to measure 30cm^3 of dilute hydrochloric acid.
2. Then transfer the acid into a polystyrene cup.
3. Stand the cup inside a beaker -this stops the cup from falling over.
4. Use a thermometer to measure the temperature of the acid and record this in a table.
5. Use a measuring cylinder to measure 5cm^3 of sodium hydroxide solution and transfer this to the polystyrene cup.
6. Fit a plastic lid to the cup and place the thermometer through the hole in the lid. The bulb of the thermometer must be in the solution.
7. Use the thermometer to gently stir the solution.
8. This reaction is exothermic so it releases energy. So the temperature of the solution will increase.
9. Look carefully at the temperature rise on the thermometer.
10. When the reading on the thermometer stops changing, we record the highest temperature reached.
11. Rinse out and dry the polystyrene cup.
12. Repeat the whole experiment using 10cm^3 of sodium hydroxide.
13. Carry it out several times, just increasing by 5cm^3 until we reach a maximum of 40cm^3 of sodium hydroxide solution.
14. And then repeat the whole thing again so there are two sets of results.
15. These can be used to calculate a mean value for the maximum temperature reached for each volume of sodium hydroxide solution.

Question 5

(RP4)

What happens as we increase the volume of sodium hydroxide solution?

Answer 5

As we add more particles of sodium hydroxide they react with hydrochloric acid. More energy is released and the maximum temperature reached increases.
At a certain volume of sodium hydroxide solution, the maximum temperature reached starts to decrease. This means we are adding so much sodium hydroxide, that there isn’t enough hydrochloric acid to react with.

Question 6

(RP4) Why is a polystyrene cup with a lid used?

Answer 6

We are measuring temperature, so any heat losses should be reduced.
Polystyrene is a good thermal insulator so this reduces heat loss through the sides and the bottom.
The lid reduces heat loss to the air.

Question 7

How is electricity produced?

Answer 7

Take two different metals and place them into an electrolyte.

Question 8

What is an electrolyte?

Answer 8

A solution which can conduct electricity, for example a solution of an ionic compound.

Question 8

What is an electrolyte?

Answer 8

A solution which can conduct electricity, for example a solution of an ionic compound.

Question 9

Why can a cell only produce electricity for a certain period of time?

Answer 9

Eventually, the chemicals in the cell run out and the reaction stops.

Question 9

Why can a cell only produce electricity for a certain period of time?

Answer 9

Eventually, the chemicals in the cell run out and the reaction stops.

Question 10

Under what conditions can cells produce electricity?

Answer 10

If we use metals with different reactivities.

Question 11

What does the size of the potential difference in a cell depend on?

Answer 11

The greater the difference between the reactivity of the metals, the greater the potential difference produced by the cell.
The electrolyte also affects the potential difference.

Question 12

What is a battery?

Answer 12

Contains two or more cells connected in series to produce a greater voltage.

Question 12

What is a battery?

Answer 12

Contains two or more cells connected in series to produce a greater voltage.

Question 13

Describe alkaline batteries.

Answer 13

The reactants in the battery run out and no more electricity is produced.
There is no way to reverse these reactions so they are non-rechargeable.

Question 14

Describe rechargeable batteries:

Answer 14

They can be recharged because we can reverse the chemical reactions when we apply an electrical current.

Question 15

At the negative electrode in a fuel cell, what is the half equation?

Answer 15

2H2 —-> 4H+ + 4e-

These electrons pass through the wire and that is the electrical current.

Question 16

At the positive electrode in a fuel cell, what is the half equation?

Answer 16

O2 + 4H+ + 4e- —-> 2H20

Oxygen molecules combine with the hydrogen ions and electrons produced at the negative electrode.

Question 17

What is the overall equation in the fuel cell?

Answer 17

2H2 + O2 —> 2H2O

The hydrogen is being oxidised.

Question 18

Hydrogen fuel cells vs rechargeable batteries

Answer 18

Hydrogen fuel cell advantages:

• will produce electricity for as long as you provide hydrogen
• do not get less efficient the longer they run
• can be a source of drinkable water eg. spacecraft

Rechargeable batteries disadvantages:

• run out and need to be recharged
• can store less electricity the more charging cycles they go through and eventually need to be replaced
• some types can catch fire if not manufactured correctly

Hydrogen fuel cell disadvantages:

• run on hydrogen which is an explosive gas and is very difficult to store safely
• produce a relatively low potential difference so several are needed together

Rechargeable battery advantages:

• no dangerous fuels are required
• rechargeable batteries can produce a larger potential difference than a hydrogen fuel cell.