Suppose you want to know the enthalpy change for the formation of methane. CH-4.

Question

Suppose you want to know the enthalpy change for the formation of methane. CH-4. from solid carbon (as graphite) and hydrogen gas: C(s)+2 H_2(g) rightarrow CH_4(g) DeltaH_rdegree = ? The enthalpy change for this reaction cannot be measured in the laboratory because the reaction is very slow We can. however, measure enthalpy changes for the combustion of carbon, hydrogen, and methane. Eq 1: C(s) + O_2(g) rightarrow CO_2(g) DeltaH_r^degree 1 = -393.5 kJ/mol-1xn Eq 2: H_2(g) + 1/2 O_2(g) rightarrow H_2O(l) DeltaH_r^degree2 = -285.8 kJ/mol-1xn Eq 3: CH_4(g) + 2 O_2(g) rightarrow CO_2(g) + 2 H_2O(l) DeltaH_r^degree3 = -890.3 jJ/mol-1xn Use this information to calculate DeltaH_rdegree for the formation of methane from its elements. jJ/mol-1xn

Explanation / Answer

C (s) + O2 (g) -----> CO2 (g) ; dHf = -393.5 kJ
2H2 (g) + O2 (g) -----> H2O ; dHf = 2* -285.8 kJ
CO2 (g) + 2H2O (l) ----> CH4 (g) + 2 O2 (g) ; dHf = 890.3 kJ
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C (s) + 2H2 (g) ------> CH4 (g) ; dHf = -393.5 +(2* -285.8) + 890.3
               dHf = -74.8 kJ

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