Some chemical reactions give out heat, these are called exothermic reactions and have a negative enthalpy change (DH= -ne).
Other chemical reactions are endothermic, meaning that they take in heat (DH= +ne).
When a reaction gives out heat, the chemical reactants are losing energy. This energy is used to heat up the surroundings. The products end up with less energy than what the reactants had, but the surroundings end up with more energy.
In an endothermic reaction, the reactants take in energy from the surroundings, resulting in the products having more energy than the reactants.
We call the heat changes associated with chemical reactions enthalpy changes (DH).
The equation to work out the enthalpy change is
The enthalpy change is defined as the heat exchange with the surroundings at constant pressure, but generally it does not matter about the pressure.
Enthalpy changes are measured in Kilojoules per mol (KJmol-1).
This means that for every one mole of methane that has reacted, 890 KJ are released as heat to the surroundings.
When calcium carbonate decomposes, it takes in heat.
CaCO3(s) à + CaO(s) + CO2(g)
DH= +572 KJ mol-1
572 KJ of energy is needed to decompose 1 mole of calcium carbonate.
The enthalpy change is affected by the temperature and pressure/concentration of solutions. When referring to enthalpy changes, the standard used is 1 atmosphere of pressure (1.01x105 Pascal's), a temperature of 25oC (298K), and a concentration of 1 mol dm-3.
If DH refers to the standard conditions, it is written as DHq.
Standard enthalpy of combustion
This refers to the enthalpy change that occurs when 1 mole of fuel is burned fully. In theory the fuel needs to burn in standard conditions, but in practice this is near impossible.