OCR AS Chemistry Unit 2 (Module 3)



Chemical energy is a special form of potential energy that lies within chemical bonds.

When chemicals react to form new substances, bonds break in the reactants and new bonds are formed as products are made.


KEY DEFINITION: Enthalpy, H, is the specific heat content that is stored in a chemical system.

We mesaure the energy absorbed or released to the surroundings during a chemical change. It most often occurs as heat.

Conservation of energy

If heat is released, the amount of energy that leaves a chemical system is exactly the same as the amount that goes into the surroundings. No heat energy is lost. This called the law of conservation of energy.

  • heat loss in a chemical system = heat gain to surroundings (accompanied by a temperature increase)
  • heat gain in a chemial system = heat loss from surroundings (accompanied by a temperature decrease)

Enthalpy change

In some reactions, the products of the reactions have more chemical energy than the reactants. In others, the products have less chemcial energy than the reactants.

An enthalpy change deltaH is:

  • the heat exchange with the surroundings furing a chemical reaction, at constant pressure;
  • the different between the enthalpy of the products and the enthalpy of the reactants.

deltaH = H products - H reactants

  • In general, all reactions are either exothermic or endothermic.

In an exothermic reaction: 

  • the enthalpy of the products is smaller than the enthalpy of the reactants. 
  • there is a heat loss from the chemical system to the surroundings
  • deltaH has a sign because heat has been LOST by the chemical system

(see exothermic/endothermic enthalpy change diagrams on page 184-185)

If two chemicals are mixed together in an exothermic reaction the container will warm to touch.

In an endothermic reaction:

  • the enthalpy change of the products is greater than the enthalpy change of the reactants.
  • there is heat gain to the chemical system from the surroundings
  • deltaH has a positive sign because heat has been GAINED to the chemical system

Self cooling cans (for beer) and self heating cans (for soup) have been developed. Chemicals are separated by a barrier, when you want to cool/heat the beer/soup, you break the barrier and the chemicals mix. 


Exothermic reactions:

Oxidation of fuels:

  • A common example is the oxidation of methane in natural gas to form carbon dioxide and water.
  • the products have less enthaply than the reactants, so the excess energy is released as heat

The enthalpy change (deltaH) is included in the equation, as the enthalpy change of reaction.

deltaH is measured in kJmol-1


The most important exothermic reaction for life:

  • Sugars such as glucose are oxidised to carbon dioxide and water.

Endothermic reactions:

Thermal decomposition of limestone:

Heat provides the energy that drives many reactions, these reactions are usually endothermic, the supplied energy being taken in by the chemical system.

CaCO3 ----> CaO + CO2

The calcium oxide product is commonly known as lime and has many uses: in cement and for treatment…




thanks for sharing :)



Thank you!