Physics 9702 · AS & A Level · The first law of thermodynamics

The first law of thermodynamics — practice question

(a)[2]

What does specific latent heat of vaporisation mean?

(b(i))[1]

Calculate the thermal energy $q$ supplied to the water.

(b(ii))[3]

The mass of $1.0\,\text{mol}$ of water is $18\,\text{g}$. Treat water vapour as behaving like an ideal gas. Show that the volume of water vapour produced is $0.64\,\text{m}^3$.

(b(iii))[2]

Assume that the starting volume of the liquid water is negligible compared with the volume of water vapour produced. Determine the magnitude of the work done by the water as it expands against the atmosphere as it vaporises.

(b(iv))[2]

Use your answers in (b)(i) and (b)(iii) to determine the increase in internal energy of the water when it vaporises at $100\degree\text{C}$. Give a reason for your answer.

(c)[2]

Use the first law of thermodynamics to suggest, with a reason, how the specific latent heat of vaporisation of water at a pressure greater than atmospheric pressure differs from its value at atmospheric pressure.

Worked solution & mark scheme

This 12-mark question has a full step-by-step worked solution and mark scheme. One marking point: thermal energy transferred for each unit mass

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