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Sodium reacts spontaneously and vigorously with oxygen to form Na₂O. The heat given off by this reaction is the enthalpy of formation ΔH_ƒ of Na₂O. Suppose that you react 23 g of Na metal with oxygen in a calorimeter that has the effective heat capacity of 5 kg of water. The heat capacity of water is 75.3 J/mol K. If the calorimeter has a temperature of 20°C before the reaction and a temperature of 29.9°C after the reaction, what is ΔH_ƒ of Na₂O? Assume that the Na₂O contributes negligibly to the heat capacity of the system.

Answer: The heat capacity of the calorimeter is

The heat required to raise its temperature by 9.9 K is then

which is the enthalpy of this reaction. Our experiment created 0.5 moles of Na₂O, so ΔH is −414.16 kJ/mol.