Hess’s Law 17.4.

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Presentation transcript:

Hess’s Law 17.4

Hess’s 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…

DHnet = (+181kJ) + (-113kJ) = +68kJ The reaction is ? N2(g) + O2(g)  2NO(g) DH = +181 kJ 2NO(g) + O2(g)  2NO2(g) DH = -113 kJ N2(g) + 2O2(g)  2NO2(g) DHnet = DH1 + DH2 DHnet = (+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) DH = +113kJ Then the reverse reaction is H2(g) + CO(g)  C(s) + H2O(g) DH = -113kJ

The production of nitrogen dioxide occurs in two steps The production of nitrogen dioxide occurs in two steps. Use the thermochemical equations to determine the heat of reaction for N2O4(g)  2NO2(g)