Question

Calculate the standard enthalpy of formation of carbon dioxide (CO2) using the following data:C(graphite) + O2(g) → CO2(g)      ΔH° = -393.5 kJ/mol2H2(g) + O2(g) → 2H2O(l)        ΔH° = -572.0 kJ/molHint: Use Hess's law to calculate the standard enthalpy of formation of CO2.

Answer 1

To calculate the standard enthalpy of formation of CO2, we can use Hess's law. Hess's law states that the total enthalpy change for a reaction is the same, regardless of the pathway taken. In this case, we are given the following reactions:1  C graphite  + O2 g   CO2 g       H = -393.5 kJ/mol2  2H2 g  + O2 g   2H2O l         H = -572.0 kJ/molWe want to find the standard enthalpy of formation of CO2, which is the enthalpy change for the reaction:C graphite  + 1/2 O2 g   CO2 g We can manipulate the given reactions to obtain this reaction. The first reaction is already in the desired form:C graphite  + O2 g   CO2 g       H = -393.5 kJ/molThe second reaction is not needed in this case, as it does not involve the formation of CO2. Therefore, the standard enthalpy of formation of CO2 is simply the enthalpy change for the first reaction:Hf CO2  = -393.5 kJ/mol