Question

Calculate the enthalpy change for the reaction below using Hess's Law and the given thermochemical equations:2H2(g) + O2(g) -> 2H2O(g)    deltaH1 = -484 kJ/molH2(g) + 1/2O2(g) -> H2O(g)    deltaH2 = -242 kJ/mol 2H2O(g) -> 2H2(g) + O2(g)    deltaH3 = +484 kJ/mol

Answer 1

To use Hess's Law, we need to manipulate the given thermochemical equations so that they add up to the desired reaction. The desired reaction is:2H2 g  + O2 g  -> 2H2O g The given thermochemical equations are:1  2H2 g  + O2 g  -> 2H2O g     deltaH1 = -484 kJ/mol2  H2 g  + 1/2O2 g  -> H2O g     deltaH2 = -242 kJ/mol3  2H2O g  -> 2H2 g  + O2 g     deltaH3 = +484 kJ/molWe can see that equation 1 is already the desired reaction, so we don't need to manipulate equations 2 and 3. Therefore, the enthalpy change for the desired reaction is simply the enthalpy change of equation 1:deltaH = deltaH1 = -484 kJ/mol