For a closed thermodynamic system of constant composition, specify the natural variables of the following state functions and write down an equation for the complete differential of the state function with respect to the natural variables. (i) internal energy u. enthalpy h. helmholtz free energy f. [1 mark] [1 mark] [1 mark] (iii) (b) using the first and second laws of thermodynamics, the equations from part (a), equations relating enthalpy and, separately, helmholtz free energy to internal energy and the maxwell relation for helmholtz free energy, derive an expression for the partial derivative of enthalpy with respect to volume at constant temperature in terms of temperature, volume and pressure. [5 marks] using the result from part (b), explain which physical properties of a pure substance should be measured to calculate the change in enthalpy with a change in volume at constant temperature.