4. Polyethylene has a heat capacity of 2.3027 Jlg°C. You need to decide if 1.0 o

Question

4. Polyethylene has a heat capacity of 2.3027 Jlg°C. You need to decide if 1.0 ounce of polyethylene can be used to package a material that will be releasing heat when in use. Consumer safety specifications indicate that the maximum allowable temperature for the polyethylene is 45°C, it can be assumed that the plastic is initially at room temperature. (a) What temperature will the polyethylene reach if the product generates 1500 J of heat and all of this energy is absorbed by the plastic package? (b) Is this a realistic estimate of the temperature that the polyethylene packaging would reach? Explain your answer. (c) What is the enthalpy change of the polyethylene? (d) Estimate the entropy change of the polyethylene. You will need to assume that the temperature of the plastic is constant.

Explanation / Answer

4 Polyethylene

(a) heat = q = mCpdT

heat = 1500 J

m = 1 ounce = 28.35 g

Cp = 2.3027 J/g.oC

dT = Tf - 23 oC

let 23 oC be the room temperature

so,

final temperature Tf = (1500/28.35 x 2.3027) + 23 = 46 oC

(b)This is not a realistic temperaure for polyethylene to reach, as the maximum allowable temperature for polyethylene reported is 45 oC.

(c) Enthalpy chane for polyethylene dH = -q = -1500 J = -1.5 kJ

(d) Entropy change dS = dH/T = -1500/(45+273) = -4.72 J/K

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