Worksheet 09 – Energy and Chemistry (Part 2)

WORKSHEET > Worksheet 09 – Energy and Chemistry (Part 2)

University of British Columbia CHEM 154 Useful values: R = 8.3145 J mol-1 K-1 or 0.08206 L atm mol-1 K-1 0 1 2 3 4 5 6 7 8 9 Energy unit conversion: 1 L atm= 101.325 J 1) Calculate the work done (in J mol-1) for each of the following physical changes: a) The conversion of 1 mol of ice to 1 mol of water at 273 K and 1 atm. The molar volumes of ice an ...[Show More]

University of British Columbia
CHEM 154
Useful values: R = 8.3145 J mol-1 K-1 or 0.08206 L atm mol-1 K-1 0 1 2 3 4 5 6 7 8 9

Energy unit conversion: 1 L atm= 101.325 J

1) Calculate the work done (in J mol-1) for each of the following physical changes:

a) The conversion of 1 mol of ice to 1 mol of water at 273 K and 1 atm. The molar volumes of ice and

water at 273 K are 0.0196 L mol-1 and 0.0180 L mol-1, respectively.

? = -?∆? = - 1 atm 0.0180 L molQR - 0.0196 L molQR = - 1 atm -0.016 L molQR

= 0.016 L atm molQR 101.325 J

L atm

= 0.162 J molQR

b) The conversion of 1 mol water to 1 mol of steam at 373 K and 1 atm. The molar volumes of water

and steam at 373 K are 0.0188 L mol-1 and 30.61 L mol-1.

? = -?∆? = - 1 atm 30.61 L molQR - 0.0188 L molQR = - 1 atm 30.59 L molQR

= -30.59 L atm molQR 101.325 J

L atm

= -3099 J molQR

2) A CHEM 154 student is interested in commercializing a hot pack that takes advantage of the exothermic

dissolution of CaCl2 in water. To this end, the student collects the data in the table below.

a) How much CaCl2 must the student weigh to heat 30mL of water at 5° to 30°C? (C = 4.18 J g-1 K-1)

Mass of CaCl2 (g): Ti (°C):

Volume of water (g): Tf (°C):

? = ??∆? = ?×?. ??

?

? ?

×∆?

?

???? ????? =

?

???? ????? ×??????? ???? ????? = ???×?. ??

?

? ?

×???

b) Would switching the solvent from water to methanol (C = 1.7 J g-1 K-1) make the hot pack work

better than with water? What other factors must the CHEM 154 student consider?

Based solely on heat capacity yes. Because less CaCl2 will be required to heat methanol by the same

amount. However, there may be solubility issues (CaCl2 may not dissolve readily in MeOH). Also,

the dissolution may not be as exothermic. As a positive, the freezing temperature of methanol is -

90 °C so it could be used at temperatures below freezing. The point is, there are many factors to

be considered.