Question

How much heat must be removed to freeze a tray of ice cubes at ${{0}^{0}}C$? The mass of the water is 225 g?

Answer 1

Hint: Use the formula $Heat=Mass\times Latent\,heat$ and substitute the values given in the question, to find the heat evolved. The heat will come out to be negative, which means that heat is taken out of the system, which is necessary to freeze the solution.

Complete step-by-step answer: In order to answer our question, we need to learn about molar heat of fusion and thermodynamics. We know that energy change occurs during the reaction, keeping in fact the temperature and volume constant given by internal energy change. It is also meant that the change in the internal energy is equal to the heat absorbed at constant volume i.e., $\Delta U={{q}_{V}}$. However most of the reactions in the laboratory are carried out in test tubes or open beakers., etc. In such cases, the reacting system is open to the atmosphere. Since atmospheric pressure is almost constant, therefore, such reactions may involve change in volume. The energy change occurring during such reactions may not be equal to the internal energy change. As the sign of the molar heat of fusion is positive, we can say that heat gets absorbed, for the change of state.
Latent heat is that type of heat which is needed for the change of state from one phase to another. Now, for water, the latent heat for changing the state from liquid to solid, which is ice is $80cal\,{{g}^{-1}}$. This is the heat which must be given out by the system. Now, the total heat which must be removed is given by the formula:
     \[Heat=Mass\times Latent\,heat\]
So, let us substitute the values given in the question, in the equation. So, we get the heat as:
     \[Heat=(80cal\,{{g}^{-1}})\times (225g)=18kcal\]
So, we get the heat evolved out as 18kcal, which is the required answer for our question.

Note: The best example to see the difference in latent heat is by observing water and steam at ${{100}^{0}}C$. Steam contains the latent heat, boiling water does not, so steam causes more burns when the hand is kept.