Chemistry CFE Practice Test 2026 - Free Chemistry Exam Practice Questions and Study Guide

The key idea here is distinguishing two different heat changes: how a compound is formed versus how it is burned. Standard enthalpy of formation is the heat change when one mole of a compound is formed from its elements in their most stable forms at standard conditions. In other words, it’s the energy associated with making the compound from its elements—a synthesis from the elements. Standard enthalpy of combustion, by contrast, is the heat released when one mole of a substance is burned completely in excess oxygen to form oxides and water under standard conditions. So the difference lies in the process: formation from elements versus burning in oxygen. The statement that correctly defines the formation enthalpy is that it is the enthalpy change to form one mole of the compound from its elements. The other descriptions describe burning or the reverse of formation, not the formation process itself.