Sucrose (table sugar, C12H22O11) can be oxidized to CO2 and H2O, and the enthalpy change for the reaction can be measured.
C12H22O11(s) + 12 O2(g) ? 12 CO2(g) + 11 H2O(?)
?Hrxn° = -5645 kJ/mol-rxn
What is the enthalpy change when 7.00 g of sugar is burned under conditions of constant pressure?
Suppose you want to know the enthalpy change for the formation of methane, CH4, from solid carbon (as graphite) and hydrogen gas:
C(s) + 2 H2(g) ? CH4(g) ?Hr° = ?
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) + O2(g) ? CO2(g)?Hr°1 = -393.5 kJ/mol-rxnEq 2:H2(g) + ½ O2(g) ? H2O(?)?Hr°2 = -285.8 kJ/mol-rxnEq 3:CH4(g) + 2 O2(g) ? CO2(g) + 2 H2O(?)?Hr°3 = -890.3 kJ/mol-rxn
Use this information to calculate ?H