Latent heat

 

1. Calculate the energy transferred to the surroundings when 50 g of aluminium changes phase from liquid to solid. E = mL_f  E = 0.050 × 398000 = 20 kJ
2. Calculate the energy required to change 2.5 kg of silver at its melting point from solid to liquid. E = mL_f E = 2.5 × 88000 = 220 kJ
3. When a kettle boils a space can be seen between the spout and the so called `cloud of steam'. If your finger were placed in the space a much more serious scald would result than if it were placed in the `cloud of steam'. Explain why, and criticize the expression `cloud of steam'. Steam is a colourless gas which fills the space, what we see is water vapour condensing in the air as it cools.
4. A 1 kW immersion heater takes 225 s to boil away 0.1 kg of water at 100 °C. Show that the specific latent heat of vaporization of water is about 2.1 KJkg^-1  

E =Pt = mL_v  1000 x 225 = 0.1L_v   Lv= 225000/0.1 = 2.25 kJkg^-1

5. Describe why the specific latent heat of vaporisation is normally greater than the specific latent heat of fusion for a particular substance. There is a greater change in internal energy changing phase from liquid to gas than from solid to liquid
6. A mixture of ice and water at 0 °C contains 0.060 kg of ice. A 750 W immersion heater takes 26.8 s to melt all the ice. Show that the value for the specific latent heat of fusion of ice is approximately 3.4 x 10⁵ Jkg^-1.

E =Pt = mL_v  750 x 26.8 = 0.060L_v   Lv= 20100/0.060 = 3.35 x 10⁵ Jkg^-1