Hess's Law - West Branch High School

Hess's Law - West Branch High School

Hesss Law 17.4 Hesss Law If a series of reactions are added together, the enthalpy change for the net reaction will be the sum of the

enthalpy changes for the individual steps Example: Smog is a problem in large cities. It is composed of NO2 formed from N2 and O2

N2 + 2O2 2NO2 the formation of NO2 takes 2 steps N2(g) + O2(g) 2NO(g) H = +181 kJ 2NO(g) + O2(g) 2NO2(g) H = -113 kJ N2(g) + 2O2(g) 2NO2(g)

Hnet = H1 + H2 Hnet = (+181kJ) + (-113kJ) = +68kJ The reaction is ? Rules for manipulating reactions: If you multiply coefficients in the reaction by some number, then the enthalpy change must also be

multiplied by that number. If an equation is reversed, then the sign of enthalpy change is also reversed. Example: If C(s) + H2O(g) H2(g) + CO(g)

H = +113kJ Then the reverse reaction is H2(g) + CO(g) C(s) + H2O(g) H = -113kJ The production of nitrogen dioxide occurs in two steps. Use the thermochemical equations

to determine the heat of reaction for N2O4(g) 2NO2(g)

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