Chemistry: Topic 5: Energy changes

What happens to energy in chemical reactions?

It is conserved

What is the amount of energy in the universe like at the end of a chemical reaction compared to before it took place?

The same

What has to happen for a reaction to transfer energy?

The product molecules must have less energy than the reactants, by the amount transferred

What is an exothermic reaction?

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

Examples of exothermic reactions

• Combustion

• Many oxidation reactions

• Neutralisation

What are some everyday uses of exothermic reactions?

• Self-heating cans

• Hand warmers

What is an endothermic reaction?

A reaction that takes in energy from the surroundings, so the temperature of the surroundings decreases

Examples of endothermic reactions

• Thermal decomposition

• Reaction of citric acid and sodium hydrogencarbonate

What is an everyday uses of endothermic reactions?

Some sports injury packs

When can chemical reactions occur?

• When reacting particles collide with each other

• With the right orientation

• And the minimum activation energy

What is activation energy?

The minimum amount of energy that particles must have to react

What can reaction profiles be used to show?

• The relative energies of reactants and products

• The activation energy

• The overall energy change of a reaction

What does an exothermic reaction profile look like?

• Energy level of reactants is higher than products

• Negative energy change

What does an endothermic reaction profile look like?

• Energy level of reactants lower than products

• Positive energy change

What happens during a chemical reaction?

• Energy must be supplied to break bonds in the reactants

• Energy is released when bonds in the products are formed

What can the energy needed to break bonds and the energy released when bonds are formed be calculated from?

Bond energies

Overall energy change of a reaction equation / calculation

Sum of energy released when bonds in the products are formed - sum of the energy needed to break bonds in the reactants

What do cells contain?

Chemicals which react to produce electrcity

What is the voltage produced by a cell dependent upon?

A number of factors, including the:

• type of electrode and electrolyte

• temperature of electrolyte

• concentration of electrolyte

How can you calculate the voltage of a battery?

The difference in reactivity between the two metal electrodes

How can a simple cell be made?

By connecting two different metals in contact with an electrolyte

What do batteries consist of?

Two or more cells connected in series to provide a better voltage

In non-rechargeable cells and batteries, when do the chemical reactions stop?

When one of the reactants has been used up

Give an example of a non-rechargeable battery

alkaline batteries

Why can rechargeable cells and batteries be recharged?

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

What are fuel cells supplied by?

An external source of fuel (e.g. hydrogen) and oxygen/ air

How is a p.d. produced in fuel cells?

The fuel is oxidised electrochemically within the cell

What does the overall reaction in a hydrogen fuel cell involve?

The oxidation of hydrogen to produce water

What are hydrogen fuel cells?

A potential alternative to rechargeable cells and batteries

How can the water from hydrogen fuel cells be used?

• to generate energy

• can be used in electrolysis to get reactants again and continue reaction

Advantages of hydrogen fuel cells

• don’t go flat or need charging due to constant supply of fuel

• only waste product is water

• constant supply of electricity

Disadvantages of hydrogen fuel cells

• expensive

• use hydrogen which is flammable

• fossil fuels may be used to make hydrogen

Advantages of rechargeable cells and batteries

• can be used many times

• cheaper than fuel cells

Disadvantages of rechargeable cells and batteries

• takes time to recharge

• will eventually stop working

• non biodegradable - hard to dispose of

Describe the process in a hydrogen fuel cell using half equations

1. H₂ in

2. 2H₂ > 4H⁺ + 4e^- (reduction)

3. H⁺ ions travel through electrolyte, e^- through the wire (producing electricity)

4. O₂ in

5. O₂ + 4H⁺ + 4e^- > 2H₂O (oxidation)

6. H₂O out

What are the 2 half equations for hydrogen fuel cells?

• 2H₂ > 4H⁺ + 4e^-

• O₂ + 4H⁺ + 4e^- > 2H₂O

What type of reaction happens in a hydrogen fuel cell?

Redox