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Hess's law

More Thermochemistry

HESS"S LAW: says that when there is more that one reaction, the total enthalphy change is the sum of the enthalpy changes of each reaction.

CS2(l) + 3 O2(g) → CO2(g) + 2 SO2(g)

Using the equations below calculate the enthalpy of the reaction.

C(s) + O2(g) → CO2(g)   ΔH= -393.5 kJ/mol
S(s) + O2(g) → SO2(g)    ΔH = -296.8 kJ/mol

C(s) + 2 S(s) → CS2(l)    ΔH = 87.9 kJ/mol

The solution

 Leave the first reaction alone C(s) + O2(g) → CO2(g)   ΔH= -393.5 kJ/mol Double the second reaction 2S(s) + 2O2(g) → 2SO2(g)    ΔH = -296.8 kJ/mol Reverse the third reaction CS2(l)→C(s) + 2 S(s)     ΔH = -87.9 kJ/mol Combine the reactions and cancel species that appear on both sides. Add up the ΔH. CS2(l) + 3 O2(g) → CO2(g) + 2 SO2(g) ΔH = -1075 kJ/mol

Example 2

HCl(g) + NaNO2(s)  HNO2(l) + NaCl(s)

Using the equations below calculate the enthalpy of the reaction.

2NaCl(s) + H2O(l) →  2HCl(g) + Na2O(s)                     ΔH = 507 kJ
NO(g) + NO2(g) + Na2O(s)  2NaNO2(s)                    ΔH = -427 kJ
NO(g) + NO2(g)  N2O(g) + O2(g)                               ΔH = -43 kJ
2HNO2(l)  N2O(g) + O2(g) + H 2O(l)                          ΔH = 34 kJ

 Reverse and 1/2 the first reaction HCl(g) + 1/2 Na2O(s)→NaCl(s) + 1/2H2O(l) ΔH = -1/2(507 kJ) Reverse and 1/2 the second reaction NaNO2(s)→1/2 NO(g) +1/2 NO2(g) +1/2 Na2O(s) ΔH = -1/2(-427 kJ) 1/2 the third reaction 1/2NO(g) + 1/2NO2(g) → 1/2N2O(g) + 1/2O2(g)                               ΔH = 1/2(-43 kJ) Reverse and 1/2 the second reaction 1/2N2O(g) + 1/2O2(g) + 1/2H 2O(l)→HNO2(l)                           ΔH = -1/2(34 kJ) Combine the reactions and cancel species that appear on both sides. Add up the ΔH. HCl(g) + NaNO2(s) → HNO2(l) + NaCl(s) ΔH =-78.5kJ