Question

The heat required to convert 2 kg of water at 20 °C in a calorimeter to steam at 100 °C and at atmospheric pressure (1 atm) is _____ kJ. (Specific heat capacity of water is $4.2 kJ kg^{-1} K^{-1}$ and latent heat of steam is $2256 kJ kg^{-1}$)

Answer 1

Correct Answer: 5184

To determine the heat required, let's consider two stages: raising the temperature of water from 20°C to 100°C, and converting water at 100°C to steam. We'll calculate each part separately and sum them to find the total heat required.

Phase 1: Heating water from 20°C to 100°C 
The formula for heat required is given by Q = m × c × ΔT, where:

• m is the mass of the water (2 kg)
• c is the specific heat capacity of water (4.2 kJ kg^-1 K^-1)
• ΔT is the change in temperature, which is 100°C - 20°C = 80°C

Substituting the values, Q₁ = 2 × 4.2 × 80 = 672 kJ

Phase 2: Converting water at 100°C to steam
The heat required for the phase change is calculated using Q = m × L, where:

• L is the latent heat of steam (2256 kJ kg^-1)

Thus, Q₂ = 2 × 2256 = 4512 kJ

Total Heat Required
Adding the heat from both phases, we have Q_total = Q₁ + Q₂ = 672 + 4512 = 5184 kJ

The computed heat required is 5184 kJ.