5.5 Standard Enthalpies of Formation

Definition of ∆H˚f The standard molar enthalpy of formation is the quantity of energy that is absorbed or released when one mole of a compound is formed directly from its elements in their standard states.

Formation Reactions In a formation reaction, a substance is formed from elements in their standard states. From what elements is water formed? H2(g) + ½O2 (g)  H2O (l) ΔHfo = -285.8 kJ The enthalpy change of a formation reaction is called the standard molar enthalpy of formation, ∆H˚f.

∆H˚f of various compounds:

Note The standard enthalpies of formation of most compounds are negative. By definition, the enthalpy of formation of an element in its standard state is zero

Calculating Enthalpy Changes You can calculate the enthalpy change of a chemical reaction by adding the heats of formation of the products and subtracting the heats of formation of the reactants. ∆H˚ = Σ(n∆H˚f products) - Σ(n∆H˚f reactants)

Video on Enthalpies of Formation

Task Do practice problem 1 on page 323

Note It is important to realize that, in most reactions, the reactants do not actually break down into their elements and then react to form products. Since there is extensive data about enthalpies of formation, however, it is useful to calculate the overall enthalpy change this way.

Task Do the questions on the handout Do review questions 2, 3, 5 on page 324