Question

How much energy (in kilojoules) is released when 29.0 g of ethanol vapor at 88.0 ∘C is cooled to -10.0 ∘C? Express your answer to two significant figures. q = ______________ kJ Ethanol has mp = -114.5 ∘C, bp = 78.4 ∘C, ΔHvap = 38.56 kJ/mol and ΔHfusion = 4.60 kJ/mol. The molar heat capacity is 113 J/(K⋅mol) for the liquid and 65.7 J/(K⋅mol) for the vapor.

Answer 1

Dear friend your answer is

The amount 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_g) multiplied by the change in temperature (final temperature - initial temperature)

q = m x C_g x (T_f - T_i)

Specific Heat Capacity (C_g) of a substance is the amount of heat required to raise the temperature of 1g of the substance by 1^oC (or by 1 K).

The units of specific heat capacity are J ^oC^-1 g^-1 or J K^-1 g^-1                 for ethanol C_g = 2.44

Energy = (29.0g)(2.44 J/g)(-10-88)      
Energy = -6934.48 J*(1 KJ/ 1000 J)
Energy = -6.93 KJ

negative sign means energy released.