Energy Changes

What is the conservation of energy principle?

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

What is an exothermic reaction? Give examples

• Exothermic reaction = A reaction where energy is transferred to the surroundings so that the surroundings temperature increases 

• Examples = combustion, oxidation reactions and neutralisation (acid + alkali) reactions. Negative sign of energy change.

What is an endothermic reaction? Give examples

• Endothermic reaction = A reaction where energy is taken in from the surroundings so the surroundings temperature decreases 

• Examples = thermal decomposition, reaction of citric acid and sodium hydrogencarbonate. Positive sign of energy change.

What is activation energy?

Minimum amount of energy that particles need to react

What is a reaction profile?

Reaction profile is a graph which shows the relative energies of reactants and product, as well as activation energy of the reaction

What is a cell?

A cell is composed of two electrodes dipped in an electrolyte solution. It produces

electricity from a chemical reaction.

What is a battery?

A battery consists of two or more cells connected in series.

What determines the voltage obtained from a cell?

Identities of metals used as electrodes and the identity and concentration of an

electrolyte.

State the advantages and disadvantages of using cells and batteries.

Advantage

• more or less cheap,

• some are rechargeable

• convenient source of electrical energy

Disadvantages 

• harmful chemicals

Describe rechargeable and non-rechargeable cells

Rechargeable – chemical reactions are reversed when an external current is supplied

Non-rechargeable – reactants are used up, cannot be recharged

What are the advantages and disadvantages of hydrogen fuel cells?

Advantages

• no pollutants

• no recharging

Disadvantages

• flammable

• H2 difficult to store

• fossil fuel production

• toxic chemicals 

• expensive production of H2 by electrolysis