What is the standard enthalpy change of formation?

The standard enthalpy change of formation is the energy change when one mole of a substance is formed from its elements in their standard states.

The standard enthalpy change of formation, often denoted as ΔHf°, is a specific type of enthalpy change. It is defined as the energy change that occurs when one mole of a substance is formed from its constituent elements in their most stable states under standard conditions (1 atm pressure and 298K temperature). This process is always carried out at constant pressure.

For example, the standard enthalpy change of formation for water would involve the reaction of hydrogen and oxygen gases to form one mole of water. It's important to note that the elements must be in their standard states. For instance, oxygen is in the form of O2 molecules and hydrogen is in the form of H2 molecules in their standard states.

The standard enthalpy change of formation is a crucial concept in thermodynamics and is extensively used in Hess's Law calculations. It allows us to calculate the enthalpy change for any chemical reaction, given the standard enthalpies of formation of all the reactants and products.

The values of standard enthalpy changes of formation for many substances have been experimentally determined and are usually given in kilojoules per mole (kJ/mol). They are typically found in the back of chemistry textbooks or reliable online databases. It's worth noting that the standard enthalpy change of formation for an element in its standard state is zero, as there is no formation process needed for an element in its most stable form.

Understanding the standard enthalpy change of formation is fundamental to grasping energy changes in chemical reactions, which is a key aspect of IGCSE Chemistry.