1. Hess’s Law
17.4

2. 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

3. 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…

4. 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 ?

5. 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.

6. 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

7. 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)