Question

Explain what happens to the molecular motion and energy of $1\mathrm{kg}$ of water at $273k$ when it changes to the ice at the same temperature. How is the latent heat of fusion related to the energy exchange that takes place during this change of state?

Answer 1

The molecular motion and energy when water changes to the ice:

• When water changes to the ice, the average energy of the molecules decreases.
• The amount of heat removed is great enough that the attractive forces between the molecules draw the molecules close together, and arrange them into fixed positions.

Latent heat of fusion:

• Latent heat of fusion is the amount of heat (energy) that is taken from the surroundings or added to the system in order to change it from solid to liquid at a constant temperature.
• The latent heat of freezing water is $-80\mathrm{cal}{\mathrm{g}}^{-1}$(the negative sign is indicating the energy release).
• Upon conversion of $1\mathrm{kg}$ $\left(1000g\right)$ water to the ice at the same temperature, there are changes involved in internal potential energy.
• The molecular motion decreases as heat are released into the surroundings as the water gets converted into ice.
• The energy involved in the phase change is,$∆\mathrm{Q}=\mathrm{m}\times \mathrm{L}$ where $\mathrm{L}$ is latent heat and $m$ is the mass

Therefore,

$$\begin{gathered} ∆\mathrm{Q}=\mathrm{m}\times \mathrm{L} \\ =-80\times 1000 \\ =-80000\mathrm{cal}{\mathrm{g}}^{-1} \end{gathered}$$

Therefore, when water changes to ice the energy released is$-80000cal{g}^{-1}$and the molecules get a fixed position.