16.1 The First Law of Thermodynamics

Define the work done by a gas.

A mass of water with negligible volume is vapourised into water vapour of volume 0.82 m³ at a temperature of 100 °C and atmospheric pressure 1.0 × 10⁵ Pa. The thermal energy supplied to the water is 9.5 × 10⁵ J. 

Determine the increase in internal energy of the water when it vapourises at 100 °C. Explain your reasoning.

Suggest, with a reason, how the specific latent heat of vapourisation of water at a pressure greater than atmospheric pressure compares with its value at atmospheric pressure.

State the first law of thermodynamics and include any relevant units.

A fixed mass of water is in a beaker at atmospheric pressure. The initial temperature of the water is 0 °C. The water is supplied with thermal energy q, so that its temperature increases to 10 °C. There is no net change in the volume of the water. 

Determine the work done and the increase in internal energy of the water by completing Table 1.1, considering the first law of thermodynamics.

Table 1.1

The water from part (b) is now heated so it's temperature increases by a further 10 °C to a final temperature of 20 °C. This process causes the volume of the water to increase so that work W is done.

Assume that the change in internal energy is the same as in part (b). 

Determine the work done, thermal energy supplied and the increase in internal energy of the water by completing Table 1.2, considering the first law of thermodynamics.

Table 1.2

Explain how the average specific heat capacity of the water, in parts (b) and (c), between 10 °C and 20 °C compares with its average value between 0 °C and 10 °C.

Define internal energy.

State and explain, in molecular terms, whether the internal energy of the following increases, decreases or does not change.
 

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