Upon completion of this topic, learners will be able to:

• define and use the term standard conditions.
• define and use the term enthalpy change with particular reference to enthalpy changes of reaction, formation, combustion and neutralization.

Standard conditions

To make any fair comparison of enthalpy changes, we must use the same conditions, called standard conditions. These are:

• Pressure of 101 kPa (approximately normal atmospheric pressure)
• Temperature of 298 K (25°C)
• Each substance involved in the reaction is in its normal physical state (solid, liquid or gas) at 101 kPa and 298 K.

The symbol (⁰) is used to indicate that the enthalpy change refers to a reaction carried out under standard conditions.

We can describe enthalpy changes according to the type of chemical reaction taking place. These includes

• Enthalpy change of combustion
• Enthalpy change of formation
• Enthalpy change of neutralisation.

In more general cases we can use the term:

• Enthalpy change of reaction.

A. Enthalpy of neutralization, ∆H⁰_neut

The standard molar enthalpy of neutralization, 0neutHD is the change in enthalpy when one mole of an acid or a base is completely neutralized. For example, the heat of neutralization for:

For any acid-base reaction the ionic equation is:

The other ions in solution (Cl^- and Na⁺) are spectator ions and take no part in the reaction.