Question

Calculate the heat of formation (∆Hf) of water (H2O) given the following reactions and their respective heats of reaction (∆Hr):2H2(g) + O2(g) → 2H2O(g); ∆Hr = -484 kJ/mol ½ O2(g) + H2(g) → H2O(l); ∆Hr = -285.8 kJ/mol Note: The heat of formation is the amount of heat released or absorbed when one mole of a compound is formed from its elements in their standard states.

Answer 1

To calculate the heat of formation of water, we need to find the heat of reaction for the formation of one mole of water from its elements in their standard states. We can use the given reactions to do this.First, let's look at the reaction for the formation of gaseous water:2H2 g  + O2 g   2H2O g ; Hr = -484 kJ/molSince we want the heat of formation for one mole of water, we need to divide the reaction by 2:H2 g  + ½ O2 g   H2O g ; Hr = -242 kJ/molNow, we need to convert gaseous water to liquid water. We can use the second reaction for this:½ O2 g  + H2 g   H2O l ; Hr = -285.8 kJ/molTo find the heat of formation of liquid water, we need to subtract the heat of reaction for gaseous water from the heat of reaction for liquid water:Hf  H2O, l  = Hr  H2O, l  - Hr  H2O, g Hf  H2O, l  = -285.8 kJ/mol -  -242 kJ/mol Hf  H2O, l  = -43.8 kJ/molSo, the heat of formation of water  H2O  is -43.8 kJ/mol.