Question

Calculate the enthalpy change for the complete combustion of 5 grams of methane gas (CH4) given that the enthalpy of formation for CH4 is -74.8 kJ/mol and the enthalpy of formation for H2O is -285.8 kJ/mol.

Answer 1

To calculate the enthalpy change for the complete combustion of 5 grams of methane gas  CH4 , we first need to write the balanced chemical equation for the combustion reaction:CH4  g  + 2 O2  g   CO2  g  + 2 H2O  g Next, we need to find the enthalpy of formation for CO2. The enthalpy of formation for CO2 is -393.5 kJ/mol.Now, we can use Hess's Law to calculate the enthalpy change for the reaction:H = [Hf  CO2  + 2 * Hf  H2O ] - Hf  CH4 H = [ -393.5 kJ/mol  + 2 *  -285.8 kJ/mol ] -  -74.8 kJ/mol H =  -393.5 - 571.6 + 74.8  kJ/molH = -890.3 kJ/molNow we need to find the number of moles of methane in 5 grams:Moles of CH4 = mass / molar massMoles of CH4 = 5 g /  12.01 g/mol  C  + 4.03 g/mol  H  Moles of CH4 = 5 g / 16.04 g/molMoles of CH4 = 0.3118 molFinally, we can calculate the enthalpy change for the combustion of 5 grams of methane:H = -890.3 kJ/mol * 0.3118 molH = -277.4 kJThe enthalpy change for the complete combustion of 5 grams of methane gas is approximately -277.4 kJ.