# Enthalpy Changes

#### Definitions

Enthalpy Change ( H)
A heat change at constant pressure measured in KJmol-1.

Exothermic
When energy is lost to the reactions surroundings due to the products having less energy than the reactants. H is negative.

Endothermic
When energy is gained from the reactions surroundings due to the products having more energy than the reactants. H is positive.

#### Enthalpy Profile Diagrams

This is where the examiners try to confuse you, they give you two different styles of diagrams to draw, both called enthalpy profile diagrams! This is the first set of diagrams. Example exothermic reactions
Combustion
C3H8 + 5O2 3CO2 + 4H2O
( H) = -890 KJmol-1

Respiration
C6H12O6 + 6O2 6CO2 + 6H2O
( H) = -2803 KJmol-1

Example endothermic reactions
Thermal decomposition
CaCO3 CO2 + CaO
( H) = +178 KJmol-1

Photosynthesis
6CO2 + 6H2O C6H12O6 + 6O2
( H) = +2803 KJmol-1

Standard Conditions
100 kPa (1 atm) and 298 K (25°C)

Standard enthalpy change of reaction H r
The enthalpy change when the amounts of reactants shown in the equation react together under standard conditions.

Standard enthalpy change of combustion H c
The enthalpy change when one mole of compound is completely burned in oxygen under standard conditions.

Standard enthalpy change of reaction H f
The enthalpy change when one mole of compound is formed from its elements under standard conditions.

Average bond enthalpy
The energy required to break one mole of covalent bonds.