Enthalpy change

In a reaction, bonds are broken and then made.

• Broken - energy taken in from surroundings (positive)

• Formed - energy given out from surroundings (negative)

Overall energy change of reaction depends on how much energy is transferred in these processes.

△H (Enthalpy change) = energy to break bonds (+) + energy to make bonds (-)

Measuring enthalpy change

Enthalpy measured under standard conditions: pressure - 100kPa, temperature - 298K

Enthalpy of Formation ( △_fH⁰): The enthalpy change when one mole of a substance is produced from its elements under standard conditions.

Enthalpy of combustion (△_cH⁰): 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 bonds enthalpy values calculated 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.