Question

Determine the standard enthalpy of formation for methane (CH4) using the combustion reactions of methane and hydrogen gas (H2). Given the standard enthalpy of combustion for CH4 is -890.3 kJ/mol and the standard enthalpy of formation for H2O is -285.8 kJ/mol.

Answer 1

To determine the standard enthalpy of formation for methane  CH4 , we will use the combustion reactions of methane and hydrogen gas  H2  and the given standard enthalpy values.The combustion reactions are as follows:1. CH4  g  + 2 O2  g   CO2  g  + 2 H2O  l    H1 = -890.3 kJ/mol2. H2  g  + 1/2 O2  g   H2O  l    H2 = -285.8 kJ/molWe need to manipulate these equations to isolate the formation of CH4. First, we will multiply equation 2 by 2 to match the amount of H2O in equation 1:2 *  H2  g  + 1/2 O2  g   H2O  l     2 * H2 = 2 *  -285.8 kJ/mol This gives us:3. 2 H2  g  + O2  g   2 H2O  l    H3 = -571.6 kJ/molNow, we will subtract equation 3 from equation 1 to isolate the formation of CH4: CH4  g  + 2 O2  g   CO2  g  + 2 H2O  l   -  2 H2  g  + O2  g   2 H2O  l  This results in:4. CH4  g  + O2  g  - 2 H2  g   CO2  g  - O2  g Simplifying the equation, we get:5. CH4  g  - 2 H2  g   CO2  g  - O2  g Now, we will subtract the enthalpy values of equations 1 and 3:H4 = H1 - H3 = -890.3 kJ/mol -  -571.6 kJ/mol  = -318.7 kJ/molSo, the standard enthalpy of formation for methane  CH4  is -318.7 kJ/mol.