The equation we need to use is:
change in thermal energy = mass * specific heat capacity * temperature change
We know the mass is 1kg, the specific heat capacity of water is 4200 J/kg°C, and the temperature change is from 20°C to 50°C, which is a change of 30°C.
Plugging these values into the equation, we get:
change in thermal energy = 1kg * 4200 J/kg°C * 30°C
change in thermal energy = 126000 J
Therefore, it would require 126000 J of energy to increase the temperature of 1kg of water from 20°C to 50°C.