Problem 67 - Chemistry: The Central Science

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?

Step-by-Step Solution

Answer

(a) Standard conditions refer to the specific conditions, typically 298 K (25°C) and 1 atm (101.3 kPa), under which enthalpy changes are measured, ensuring consistency and comparability across experiments. (b) Enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its constituent elements in their most stable forms at constant pressure. (c) Standard enthalpy of formation, represented as ΔHf°, is the change in enthalpy when one mole of a compound is formed from its constituent elements in their most stable forms under standard conditions (298 K and 1 atm of pressure).

Step 1: (a) Definition of Standard Conditions

Standard conditions, with reference to enthalpy changes, refer to the specific conditions under which the measurements (like enthalpy changes) are taken. The standard conditions are set to ensure consistency and comparability in results across various experiments. Some common standard conditions are temperature at 298 K (25°C) and pressure at 1 atm (101.3 kPa).

Step 2: (b) Definition of Enthalpy of Formation

Enthalpy of formation is the change in enthalpy (heat content) when one mole of a compound is formed from its constituent elements in their most stable forms (reference states) at constant pressure.

Step 3: (c) Definition of Standard Enthalpy of Formation

Standard enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its constituent elements in their most stable forms under standard conditions (298 K and 1 atm of pressure). It is represented as ΔHf°, where the superscript "°" denotes the standard state. Standard enthalpy of formation can be either positive or negative depending on whether the formation process is endothermic (absorbing heat) or exothermic (releasing heat), respectively.