Unifying heat of reaction

Question

Calculate the amount of heat needed to melt 135.g

 of solid ethanol and bring it to a temperature of -47.6° C

. Be sure your answer has a unit symbol and the correct number of significant digits.

Answer 1

(1) find the heat (q) needed to melt the solid. This is a PHASE CHANGE so there is NO change in temperature.

You only need what is called the enthalpy of fusion, ∆Hf.

(2) find the heat (q) needed to raise the temperature of the now melted solid (a liquid) from the current temperature to the desired temperature.

(3) sum up all the values of q to get the total heat

Step (1) q = m∆Hf = (135.0 g)(106.5 J/g) = 14,378 J Note, since there is no change in temperature during a phase change, you don't need a ∆T term. Just use the ∆H and the mass.

Step (2) q = mC∆T = (135.0 g)(2.46 J/g/deg)(-47.6 - (-114)) = (135.0 g)(2.46 J/g/deg)(66.4 deg) = 22,051 J

Step (3) Add them up: 14,378 J + 22,051 J = 36,429 J = 36,400 J (to 3 sig. figs.)