Topic 4: Energetics
Enthalpy change
In a reaction, bonds are broken and then made. For bonds to be broken, energy is taken in
from the surroundings and when bonds are formed, energy is given out. The overall energy
change of the reaction depends on how much energy is transferred in these processes.
When energy is taken in from the surroundings, the enthalpy change is positive. When
energy is released, the enthalpy change is negative.
Overall enthalpy change (∆H) can be calculated:
∆H = (+) energy to break bonds + energy to make bonds (-)
Measuring Enthalpy Change:
Enthalpy is measured under standard conditions of 100 kPa pressure and a temperature of
298 K.
Enthalpy of Formation (∆fH o ):
The enthalpy change when one mole of a substance is produced from its elements under
standard conditions.
Enthalpy of Combustion (∆cH o ):
The enthalpy change when one mole of a substance is burned completely in oxygen under
standard conditions.
Mean Bond Enthalpies:
Different covalent bonds require different amounts of energy to be broken. Values can be
found experimentally using calorimetry methods. The bond enthalpy values calculated in
this way often differ from the data book values as they are not exact and vary in each
situation. This means the data book values are averaged values.
Enthalpy change
In a reaction, bonds are broken and then made. For bonds to be broken, energy is taken in
from the surroundings and when bonds are formed, energy is given out. The overall energy
change of the reaction depends on how much energy is transferred in these processes.
When energy is taken in from the surroundings, the enthalpy change is positive. When
energy is released, the enthalpy change is negative.
Overall enthalpy change (∆H) can be calculated:
∆H = (+) energy to break bonds + energy to make bonds (-)
Measuring Enthalpy Change:
Enthalpy is measured under standard conditions of 100 kPa pressure and a temperature of
298 K.
Enthalpy of Formation (∆fH o ):
The enthalpy change when one mole of a substance is produced from its elements under
standard conditions.
Enthalpy of Combustion (∆cH o ):
The enthalpy change when one mole of a substance is burned completely in oxygen under
standard conditions.
Mean Bond Enthalpies:
Different covalent bonds require different amounts of energy to be broken. Values can be
found experimentally using calorimetry methods. The bond enthalpy values calculated in
this way often differ from the data book values as they are not exact and vary in each
situation. This means the data book values are averaged values.
