Question

Calculate the energy needed to heat 4 kg of water from 25°C to 45°C.

Answer 1

(1 cal/g °C) x (4000 g) x (45 - 25)°C = 80000 cal = 80 kcal. So the answer is 80 kcal .

Answer 2

Answer:

80 Kcal

Explanation:

Specific heat is the energy required to raise the temperature of a unit mass of a substance by one degree. Its units are kJ/kg·K or kJ/kg·°C in the SI system.

There are two types of specific heat: 1) when the process is at constant volume. 2) When it's at constant pressure. In the case of liquids, the value is the same for the two cases. The specific heat of water is equal to 4.18kJ/kg·°C (in that range of temperatures)

The formula which relates energy needed, specific heat, change in temperature, and mass of the substance is the following:

ΔU=m·c·(T2-T1)

T2: final temperature of the substance

T1: initial temperature of the substance

c: specific heat of the substance

m: the mass of the substance

ΔU: energy required to raise the temperature the substance from T1 to T2

Plugging the given values values on the equation:

ΔU=4kg·(4.18kJ/kg·°C)·(45°C-25°C)

ΔU=334.4kJ

Now, it only rests to convert from kJ to cal:

1kJ=0.2390kcal

So:

334.4kJ=334.4*0.2390kcal=79.9kcal≈80kcal

So the energy needed to heat 4 kg of water from 25°C to 45°C is 80kcal