Chemistry C7 - Energy Changes

Define Exothermic and Endothermic reactions

[ 2 MARKS ]

Exothermic : Energy release to surroundings ( Temperature rises ) - Neutralisation or Combustion

Endothermic : Energy taken in from surroundings ( Temperature decreases ) - Thermal decomposition

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 electrode dipped in an electrolyte solution. Produces electricity from a chemical reaction.

What is a battery?

A battery consist of two or more cells connected in series

State the advantages and disadvantages of using cells and batteries

( + ) More or less cheap, some are rechargeable, convenient source of electrical energy

( - ) Harmful chemicals

Describe Rechargeable and non-rechargeable cells

Rechargeable - Chemical reactions are reverse when an external current is supplied

Non-Rechargeable - Reactants are used up ,

cannot be recharged

Describe the temperature change practical to investigate energy changes in a neutralisation reaction

[ 6 MARKS ]

1. Place a polystyrene cup in a beaker for support

2. Add a measured volume of acid

3. Record it's starting temperature

4. Add measured alkaline and stir with thermometer

5. Record highest temperature reached

6. Repeat and calculate mean temperature change

Explain in terms of bond energies why some reactions are exothermic

[ 3 MARKS ]

More energy is released when new bonds form than is absorbed to break bonds in reactants

NOTES :

A polystyrene cup is used instead of glass beaker because polystyrene is an insulator so reduces heat loss to surroundings leading to more accurate temperature change.

When ammonium chloride dissolves in water the temperature falls because it is an endothermic process so energy is absorbed from surroundings to break bonds in the solid and dissolve it