Question

(a) What is meant by the term standard conditions, with reference to enthalpy changes? (b) What is meant by the term enthalpy of formation? (c) What is meant by the term standard enthalpy of formation?

Short answer

(a) Standard conditions refer to a set of agreed-upon conditions, specifically 298 Kelvin (25 degrees Celsius) temperature and 1 atmosphere (atm) pressure, for conducting experimental observations and measurements related to enthalpy changes in chemical reactions. (b) Enthalpy of formation (ΔH_f) represents the change in enthalpy during the formation of one mole of a substance from its constituent elements in their most stable forms at constant pressure. (c) Standard enthalpy of formation (ΔH_f°) describes the change in enthalpy when one mole of a substance is formed from its constituent elements in their most stable forms under standard conditions (298 K temperature and 1 atm pressure).

Step-by-step solution

a) Standard Conditions

Standard conditions are defined as a set of conditions agreed upon for conducting experimental observations and measurements in the field of chemistry and physics. For enthalpy changes, standard conditions refer to a temperature of 298 Kelvin (25 degrees Celsius) and a pressure of 1 atmosphere (atm). These conditions are used to ensure consistency and comparability when dealing with enthalpy changes in chemical reactions.

b) Enthalpy of Formation

Enthalpy of formation, denoted as ΔH_f, is the change in enthalpy during the formation of one mole of a substance from its constituent elements in their most stable forms at a constant pressure. For example, the enthalpy of formation of water would be the change in enthalpy when one mole of water is formed from hydrogen and oxygen molecules. It helps determine whether a particular reaction is exothermic (releases heat) or endothermic (absorbs heat).

c) Standard Enthalpy of Formation

The standard enthalpy of formation, denoted as ΔH_f°, is the change in enthalpy when one mole of a substance is formed from its constituent elements in their most stable forms under standard conditions (298 K temperature and 1 atm pressure). By convention, the standard enthalpy of formation for an element in its most stable and standard state is zero. Knowing the standard enthalpies of formation for various compounds, chemists can easily compare the energy changes that occur during different chemical reactions under standard conditions.