To calculate the standard enthalpy of formation for hydrogen peroxide H2O2 , we need to know the standard enthalpy change of the reaction that forms it. The formation reaction for hydrogen peroxide can be written as:H2 g + O2 g H2O2 l However, we don't have the direct enthalpy change for this reaction. We can use Hess's Law to find it indirectly by using the enthalpy changes of formation for water and oxygen gas. We can write two reactions that involve these compounds:1 H2 g + 1/2 O2 g H2O l H1 = -285.8 kJ/mol2 H2O l + 1/2 O2 g H2O2 l H2 = ?We want to find the enthalpy change for reaction 2. To do this, we can manipulate reaction 1 and add it to reaction 2 to get the desired reaction. We can reverse reaction 1 and multiply it by 1:1' H2O l H2 g + 1/2 O2 g H1' = 285.8 kJ/molNow, we can add reaction 1' and reaction 2:1' H2O l H2 g + 1/2 O2 g H1' = 285.8 kJ/mol2 H2O l + 1/2 O2 g H2O2 l H2 = ?----------------------------------------------H2 g + O2 g H2O2 l H = H1' + H2Now we can solve for H2:H2 = H - H1'H2 = 0 kJ/mol - 285.8 kJ/molH2 = -285.8 kJ/molSo, the standard enthalpy of formation for hydrogen peroxide H2O2 at 298 K is -285.8 kJ/mol.