GCSE AQA Chemistry - Energy Changes

Exothermic reactions

Transfer energy to the surroundings.

Endothermic reactions

Take in energy from the surroundings.

Describe the set up of the temperature changes practical

Pour 30cm3 HCl to a polystyrene cup in a beaker, a thermometer and lid are also used.

Temperature changes practical

• Initial temperature of the acid is measured

• Add 5cm3 of sodium hydroxide to the acid and place the lid over the cup

• Measure the highest temperature reached

Activation energy

The minimum amount of energy particles need to collide with each other and react.

How should I represent activation energy and overall energy change?

Using arrows

What type of process is bond breaking?

Endothermic (energy must be supplied)

What type of process is bond forming?

Exothermic (energy is released)

Electrolyte

A solution that can conduct electricity

Describe the setup of an electrical cell

Two metals (the electrodes) connected in series are submerged in an electrolyte.

How does an electrical cell produce electricity?

The chemical reactions on the surfaces of the two electrodes allows a charge to flow and electricity is produced.

What are the two factors affecting potential difference produced in an electrical cell?

• The difference in reactivity between the metals

• The electrolyte used

Battery

Two or more cells connected in series to produce a greater potential difference.

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

Eventually the chemicals in the cell will run out and the reactions will stop.

Why is it that some cells are non-rechargeable?

Because the reactions that happen at the electrodes are irreversible.

How can rechargeable reactions be recharged?

By applying a current to reverse the chemical reactions.

What is a fuel cell?

An electrical cell that is supplied with a fuel and oxygen, and the chemical reaction inside it produces electricity

What happens in hydrogen fuel cells?

Hydrogen is oxidised, producing water and electrical current

What is the reaction at the cathode in a hydrogen fuel cell?

2H₂ → 4H⁺ + 4e^-

What is the reaction at the anode in a hydrogen fuel cell?

O₂ + 4H⁺ + 4e^- → 2H₂O

What is the overall reaction in a hydrogen fuel cell?

2H₂ + O₂ → 2H₂O

What are the advantages of hydrogen fuel cells? (3)

• Can produce electricity for as long as hydrogen is provided

• Don’t get less efficient

• Can provide a source of drinking water

What are the disadvantages of hydrogen fuel cells? (2)

• Hydrogen is an explosive gas, so is difficult to store

• They produce low potential differences, so multiple are needed together

What are the disadvantages of rechargeable batteries? (2)

• They need to be recharged when they run out

• The more charging cycles they go through, the less electricity they can store

What are the advantages of rechargeable batteries? (2)

• No dangerous fuels required

• Produce a greater potential difference than hydrogen fuel cells