5 5 Standard Enthalpies of Formation Definition of

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 n. In a formation reaction, a substance is formed from elements in their standard states n. From what elements is water formed? n. H 2(g) + ½O 2 (g) H 2 O (l) ΔHfo = -285. 8 k. J n. 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