AQA GCSE Single Science Chemistry: Energy Changes

What is energy conservation?

• Energy is conserved in chemical reactions.

• The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place.

• If a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred.

What is an exothermic reaction?

• An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases.

• Exothermic reactions include combustion, many oxidation reactions and neutralisation.

• Everyday uses of exothermic reactions include self-heating cans and hand warmers.

What is an endothermic reaction?

• An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases.

• Endothermic reactions include thermal decompositions and the reaction of citric acid and sodium hydrogencarbonate.

• Some sports injury packs are based on endothermic reactions.

What is the method to reacting two solutions?

1. Place the polystyrene cup inside the glass beaker to make it more stable.

2. Measure an appropriate volume of each liquid, eg 25 cm³.

3. Place one of the liquids in a polystyrene cup.

4. Record the temperature of the solution.

5. Add the second solution and record the highest or lowest temperature obtained.

6. Change your independent variable and repeat the experiment. Your independent variable could be the concentration of one of the reactants, or the type of acid/alkali being used, or the type of metal/metal carbonate being used.

What is the method to reacting a solid with a solution?

1. Place the polystyrene cup inside the glass beaker to make it more stable.

2. Measure an appropriate volume of the solution, eg 25 cm³.

3. Measure an appropriate mass of the solid, or select a suitable sized piece of metal.

4. Place the solution in a polystyrene cup.

5. Record the temperature of the solution.

6. Add the solid and record the highest or lowest temperature obtained.

7. Change your independent variable and repeat the experiment. Your independent variable could be the surface area of the solid, or the type of acid being used, or the type of metal being used.

Define collision theory and activation energy

• Collision theory: the fact that chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.

• Activation energy: the minimum amount of energy that particles must have to react

Explain the formation and breaking of bonds in terms of energy

• To break bonds (in reactants), energy must be supplied

• To form bonds (in products), energy is released

What is the overall energy change of a reaction?

• The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed

Explain exothermic and endothermic reactions in terms of energy change

• In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds.

• In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.

What are cells?

• Cells contain chemicals which react to produce electricity.

• The voltage produced by a cell is dependent upon a number of factors including the type of electrode and electrolyte.

• A simple cell can be made by connecting two different metals in contact with an electrolyte.

What are batteries?

• Batteries consist of two or more cells connected together in series to provide a greater voltage.

• Alkaline batteries are non-rechargeable.

What’s the difference between non-rechargeable cells and batteries and rechargeable cells and batteries?

• In non-rechargeable cells and batteries, the chemical reactions stop when one of the reactants has been used up.

• Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied.

How do fuel cells work?

• Fuel cells are supplied by an external source of fuel (eg hydrogen) and oxygen or air.

• The fuel is oxidised electrochemically within the fuel cell to produce a potential difference.

• The overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water.

Evaluate the use of hydrogen fuel cells in comparison with rechargeable cells and batteries

• Alkaline cells are cheaper to manufacture, but may end up in landfill sites once fully discharged; recyclable though it is expensive

• Rechargeable cells can be recharged many times before being recycled, reducing the use of resources, but it costs more to manufacture

• Hydrogen fuel cells are easy to maintain but are very expensive to manufacture due to the constant supply of hydrogen fuel

What are the half equations for the electrode reactions in the hydrogen fuel cells?

• At the negative electrode: 2H₂ + 4OH^- → 4H₂O + 4e^-

• At the positive electrode: O₂ + 2H₂O + 4e^- → 4OH^-