QUANTITY OF HEAT

The quantity of heat energy (Q) gained or lost by a substance is equal to the mass of the substance (m) multiplied by its specific heat capacity (C) multiplied by the change in temperature (final temperature - initial temperature)

Q = m x C x (T_f - T_i)

Memorize this formula well.

• Specific Heat Capacity (C) of a substance is the amount of heat required to raise the temperature of 1 gram of the substance by 1^o C (or by 1 K).
• Heat capacity (H) is defined as the mass multiplied by the specific heat capacity.        H = m x C
• The quantity of heat can therefore be defined as heat capacity multiplied by the change in temperature. Q = H x C

Example:

Calculate the quantity of heat needed to raise the temperature of 250 grams of water from 20^o C to 56^o C. (Heat capacity of water is C_g = 4.18 J ^oC^-1 g^-1)

Solution: Q = m x C x (T_f - T_i) 

m = 250 g; T_f = 56^oC; T_i = 20^oC

q = 250 x 4.18 x (56 - 20) 

q = 250 x 4.18 x 36 

q = 37620 J = 38 kJ