Define standard enthalpy of formation.

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

The standard enthalpy of formation, often denoted as ΔHf°, is a specific type of enthalpy change. It is defined as the heat change that occurs when one mole of a compound is formed from its constituent elements in their most stable physical states, under standard conditions (1 atm pressure and 298.15 K temperature). This process is always carried out at constant pressure.

For example, consider the formation of water from hydrogen and oxygen. The standard enthalpy of formation for water would be the heat change that occurs when one mole of water is formed from its elements, i.e., hydrogen and oxygen, in their standard states. Hydrogen is a gas at standard conditions, and oxygen is also a gas. Therefore, the standard enthalpy of formation of water is the enthalpy change for the reaction: H2(g) + 1/2 O2(g) → H2O(l).

It's important to note that the standard enthalpy of formation for an element in its standard state is zero. This is because there is no formation process needed for an element in its most stable form. For instance, the standard enthalpy of formation for oxygen gas, O2, or for solid carbon, C(s), is zero.

The standard enthalpies of formation are very useful in calculating the enthalpy changes in chemical reactions. This is done using Hess's Law, which states that the total enthalpy change for a chemical reaction is the sum of the enthalpy changes for each step in the reaction. Therefore, by knowing the standard enthalpies of formation for the reactants and products in a reaction, we can calculate the overall enthalpy change for the reaction.

In summary, the standard enthalpy of formation is a fundamental concept in thermodynamics and plays a crucial role in understanding and predicting the energy changes that occur during chemical reactions.