A 42.30 g sample of liquid lead at 505.00°C is poured into a mold and allowed to cool to 28.00°C. How many kJ of energy are released in this process?

Question

The following information is given for lead at 1atm:

= 1740.00°C	(1740.00°C) =  J/g
= 328.00°C	(328.00°C) =  J/g
Specific heat solid = 0.1300 J/g °C
Specific heat liquid = 0.1380 J/g °C

A 42.30 g sample of liquid lead at 505.00°C is poured into a mold and allowed to cool to 28.00°C. How many kJ of energy are released in this process?

(Report the answer as a positive number.)

Energy =  kJ

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Answer 1

melting point of lead = 328.00ºC

To go from 505.00º to the melting point of 328.00º, we have...

q = mC∆T

q = heat = ?

m = mass = 42.30 g

C = specific heat = 0.1380 J/gº

∆T = change in temperature = 505 - 328 = 177º

q = (42.30 g)(0.1380 J/gº)(177)º = 1033 J

To go from liquid lead to solid lead at 320º (phase change, no change in temp), we have...

q = m∆Hfusion (looking up ∆Hfusion, I found 23.2 J/g

q = (42.30 g)(23.2 J/g) = 981.4 J

To cool the solid lead from 320º to 28.00º, we have...

q = mC∆T = (42.30 g)(0.1300 J/gº)(292º)

q = 1606 J

Adding up all of the joules, we have ...

1033 J + 981 J + 1606 J

q = 3620 J = 3.620 kJ