#27Q2) Heat

Determination of specific latent heat of fusion of ice by the method of mixtures

DifficultyMedium
Est. Time45 mins

Required Apparatus

A calorimeter, a stirrer, a thermometer, water, a sufficient quantity of ice, filter papers, a four beam balance, chemical balance, and a box of weights.

Scientific Theory

In the experiment above, let:

  • m1m_{1}: The mass of an empty calorimeter with a stirrer
  • m2m_{2}: Mass of the calorimeter with a quantity of water
  • θ1\theta_{1}: The initial temperature of the water
  • θ2\theta_{2}: Minimum temperature of the mixture when mixed with ice
  • m3m_{3}: Mass of the calorimeter with final contents
  • cwc_{w}: Specific heat capacity of water
  • cc: Specific heat capacity of calorimeter metal
  • LL: Specific latent heat of fusion of ice

Then since 0C<θ2<θ10^{\circ}C<\theta_{2}<\theta_{1}, assuming that no heat was absorbed from the surroundings during mixing:

Heat gained by ice = Heat lost by water and calorimeter (with stirrer)

(m3m2)L+(m3m2)cwθ2=[m1c+(m2m1)cw](θ1θ2)(m_{3}-m_{2})L+(m_{3}-m_{2})c_{w}\theta_{2}=[m_{1}c+(m_{2}-m_{1})c_{w}](\theta_{1}-\theta_{2})

Experimental Method

  1. Measure the mass (m1)(m_{1}) of the calorimeter with the stirrer.
  2. Fill about two thirds of the calorimeter with water at room temperature and measure its mass (m2)(m_{2}) again. Measure also the temperature (θ1)(\theta_{1}) of the water.
  3. Wipe out water from small pieces of ice using filter paper and put those one by one into the water in the calorimeter while stirring, taking care to insert one piece after the previous one has dissolved. Use a square-net stirrer to prevent the ice from floating in water.
  4. When the temperature of the water has fallen sufficiently (by about 5C)5^{\circ}C) stop adding ice, stir the mixture well and record the lowest temperature of the mixture (θ2)(\theta_{2}).
  5. Finally measure the mass of the calorimeter with its contents again (m3)(m_{3}).
  6. Substitute values of m1m_{1}, m2m_{2}, θ1\theta_{1}, θ2\theta_{2} and standard values for cc and cwc_{w} in the expression given in the theory and calculate the value of LL.

Important Points

1
  • Conclude the value obtained from the calculation as the specific latent heat of fusion of ice.
2
  • Compare the value of specific latent heat of fusion of ice you obtained from the experiment with its standard value you would obtain from a data book and calculate its percentage error.
3
  • It is advisable to determine the dew point approximately before mixing ice. Then by preventing the final temperature of the mixture going below the dew point, the error caused by the dew depositing on the calorimeter surface can be minimized.
4
  • When the temperature of the calorimeter begins to fall below room temperature with the mixing of ice, it begins to gain heat from the surroundings. This can be minimized by lagging the calorimeter with heat absorbing materials.
5
  • Or else the compensation method used in the method of mixtures can be used. Heat the calorimeter with water by about 5C5^{\circ}C above room temperature. Considering this temperature as the initial temperature θ1\theta_{1} mix pieces of ice until the temperature of the water falls below the room temperature by the same number of degrees (5C)(5^{\circ}C). Assuming that the heat lost by the system during the 5C5^{\circ}C above room temperature compensated with the heat gained by the system during the 5C5^{\circ}C below room temperature, the error due to heat gained from surroundings can be considered to have minimized.