Using standard heats of formation, calculate the standard enthalpy change for the following reaction. 2HBr(g)H2(g) + Br2(l)

Answer 1

The reaction is 2HBr ====> H2 + Br2.

The general equation for determining heats of reaction from standard heats of formation is

Sum (heats of formation of products) - Sum (heats of formation of reactants).

The standard heat of formation for elements in their standard states is zero, and the heat of formation of HBr in nshs-science.net is given as -36.2 Kj/mole. As 2 moles of HBr are in the balanced equation, the standard heat of formation of HBr should be multiplied by 2.

0 + 0 - (2 x -36.2) = + 72.4 Kj for the reaction as written.