Question

How much energy is needed to melt 25.4g of iodine? ( $ \Delta {H_{fusion}} = 61.7J/g $ ).

Answer 1

Hint: In this question we’ll have to find the energy required for fusion of iodine. The mass of the substance, along with the latent heat of fusion, needs to be provided to find the heat change (q) during a phase change process, which in this case is fusion.

Complete answer:
Let us first know what latent heat is. Consider the process of boiling water. Once it reaches its boiling point, the phase change from liquid to vapour doesn’t instantly occur. This is where latent heat comes into play. Once the liquid has reached its boiling point, it gains energy equal to its latent heat to overcome the forces of liquid and turn into vapour. Similar case happens during fusion. When ice melts, at the melting point it requires some heat to break the solid bonds and turn into liquid. This is the latent heat of fusion.
Therefore, the latent heat of fusion can be defined as the heat required to melt 1 gram of solid into liquid, when it reaches its melting point. The formula for finding the heat/energy required is given by the formula: $ q = m \times \Delta {H_{Fusion}} $
Where q is the heat, m is the mass of the substance and $ \Delta H $ is the enthalpy of fusion. The information given to us is:
 $ m = 25.4g,\Delta {H_{fusion}} = 61.7J/g $
Therefore, the energy required to melt $ = 25.4 \times 61.7 \approx 1.57 \times {10^3}J $
Therefore, the answer required to us is $ 1.57 \times {10^3}J = 1.57kJ $.

Note:
Latent heat cannot be seen but it is observed very easily. The latent heat of the substances is the reason why vapour can cause more harsh skin burns than hot liquid at its boiling point. For this reason, steam engines work more powerfully.