Question

Calculate the heat (in joules) absorbed by the system for each of the following examples (the system is given in italics). Specify the sign of the heat.

a. 100g of liquid water is heated from 0⁰C to 100⁰C at 1 bar

b. 100g of liquid water is frozen to ice at 0⁰C at 0.01 bar

c. 100g of liquid water is evaporated to steam at 100⁰C at 1 bar

Answer 1

a) As, it requires 4.184 J to raise the temperature of one gram of water by one degree celsius. multiply this by the no. of grams of water and the change in the temperature, we get the heat absorbed by the water..

q = m c delta T

   = 100g x 4.184 J⁰C^-1g^-1 x 100 ^oC

   = 41.8 KJ

b) This variation does not involve a temperature change in the water. As due to, whole ice-water system stays at zero degree celsius.

q = 100g x 334.16 J g^-1

     = 33.416 J

c) q = m H_v

As, for water at its normal boiling point of 100 C , the heat of vaporisation is 2260 J/g. This means that to convert 1 g of water at 100 C to 1 g of steam at 100 C , 2260 J g^-1 heat must be absorbed by the water. so here for 100 g of water it should be ..

q = 100g x 2260 J g^-1

     = 226 KJ