GCSE AQA Chemistry C5

What is an exothermic reaction?

A reaction that transfers energy to the surroundings, causing the temperature to increase

What is an endothermic reaction?

A reaction that takes in energy from the surroundings, causing the temperature to decrease

Give three examples of exothermic reactions

Combustion, many oxidation reactions, and neutralisation

Give two examples of endothermic reactions

Thermal decomposition and the reaction of citric acid with sodium hydrogencarbonate

Give two everyday uses of exothermic reactions

Self-heating cans and hand warmers

Give an everyday use of endothermic reactions

Sports injury cold packs

Why is a polystyrene cup used in the temperature changes practical?

It is a good thermal insulator which reduces heat loss to the surroundings

Why is the polystyrene cup placed in a glass beaker in the temperature changes practical?

To prevent it from falling over and to provide extra insulation

Why is a lid used in the temperature changes practical?

To reduce heat loss to the surroundings by evaporation or convection

What is activation energy?

The minimum amount of energy that particles must have to react when they collide

In a reaction profile, where is the activation energy shown?

From the reactants line to the peak of the curve

What does the reaction profile for an exothermic reaction look like?

The products are at a lower energy level than the reactants

What does the reaction profile for an endothermic reaction look like?

The products are at a higher energy level than the reactants

How is the overall energy change shown on a reaction profile?

The difference in height between the energy of the reactants and the products

Is breaking chemical bonds an exothermic or endothermic process?

Endothermic, because energy must be supplied to break bonds

Is making chemical bonds an exothermic or endothermic process?

Exothermic, because energy is released when new bonds are formed

How do you calculate the overall energy change of a reaction using bond energies?

Sum of energy required to break bonds minus sum of energy released when making bonds

In terms of bond energies, why is a reaction exothermic?

The energy released forming new bonds is greater than the energy needed to break existing bonds

In terms of bond energies, why is a reaction endothermic?

The energy needed to break existing bonds is greater than the energy released forming new bonds

What does a simple chemical cell consist of?

Two different metals in contact with an electrolyte

What is an electrolyte in a chemical cell?

A liquid containing ions that react with the electrodes to conduct electricity

How is a battery formed?

By connecting two or more chemical cells together in series to provide a greater voltage

What happens to the voltage if the difference in reactivity between the two metals in a chemical cell increases?

The voltage produced by the cell increases

Why do non-rechargeable batteries eventually stop working?

The chemical reactions stop when one of the reactants has been completely used up

Give a common example of a non-rechargeable battery

Alkaline batteries

Why can rechargeable batteries be recharged?

The chemical reactions are reversed when an external electrical current is supplied

What is a fuel cell?

A cell that generates a continuous voltage by reacting a continuous supply of fuel and oxygen

What happens to the fuel inside a fuel cell to produce a potential difference?

The fuel is oxidised electrochemically

What is the overall word equation for a hydrogen fuel cell?

Hydrogen + oxygen -> water

What is the overall symbol equation for a hydrogen fuel cell?

2H2 + O2 -> 2H2O

What is the waste product of a hydrogen fuel cell?

Water

Write the half equation for the reaction at the negative electrode in a hydrogen fuel cell

H2 -> 2H+ + 2e-

Write the half equation for the reaction at the positive electrode in a hydrogen fuel cell

O2 + 4H+ + 4e- -> 2H2O

Give two advantages of hydrogen fuel cells over rechargeable batteries

They do not lose capacity over time and take less time to refuel than a battery takes to recharge

Give two disadvantages of hydrogen fuel cells compared to rechargeable batteries

Hydrogen is a highly flammable gas that is difficult to store safely and is often produced from non-renewable fossil fuels

What is the independent variable in the Required Practical 4 temperature changes experiment?

The volume of reactant added (e.g. sodium hydroxide added to hydrochloric acid)

What is the dependent variable in the Required Practical 4 temperature changes experiment?

The maximum temperature reached

Name two control variables in the Required Practical 4 temperature changes experiment?

The concentration of the acid and alkali, and the starting temperature of the solutions

Why does the temperature stop rising and start decreasing after the peak in the temperature changes practical?

All of the limiting reactant has reacted so the reaction stops, and heat is lost to the cooler surroundings

Why does a high activation energy mean a reaction takes place very slowly at room temperature?

Very few colliding particles have enough kinetic energy equal to or greater than the activation energy

Which electrode is hydrogen fed into in a hydrogen-oxygen fuel cell?

The negative electrode (anode)

Which electrode is oxygen fed into in a hydrogen-oxygen fuel cell?

The positive electrode (cathode)

What happens to hydrogen molecules at the negative electrode of a hydrogen fuel cell?

They lose electrons to become hydrogen ions (oxidation)

What happens to oxygen molecules at the positive electrode of a hydrogen fuel cell?

They gain electrons and react with H+ ions to form water (reduction)