CHM 2045C Module Seven Sample Exam     

 

Module Seven Part C: Specific Heat/First Law Problems    10 points

 

If the temperature of a 50.0 gram block of aluminum increases by 10.9 K when heated by 500 joules, calculate the:  (See section 6.2 pages 241-246)

 

  1. heat capacity of the aluminum block.

q = heat transferred    C = specific heat capacity      

 m = mass of substance 

ΔT = change in K temperature

 

Heat capacity of Block =  q / Δ T

Heat Capacity of Block  = 500 J / 10.9 K  = 45.9 J/K

 

  1. molar heat capacity of aluminum.  Moles = Mass / Molar Mass

 Moles = 50.0 g / 27.0 g/mol = 1.85 mol

Molar heat capacity = q / Δ T mol

 

Molar Heat Capacity =  500 J / 10.9 K 1.85 mol = 24.8 J/K mol

 

  1. specific heat capacity of aluminum.

 

q = C x m x Δ T

 

500 J = C x 50.0 g x 10.9 K       C = 0.917 J/gK

 

If the internal energy of a thermodynamic system is decreased by 300 J when 75 J of work is done on the system, how much heat was transferred, and in which direction, to or from the system.  See section 6.4 p 253

Change                                                                  Sign Conversion      Effect of Esystem

Work done on the system by surroundings                        w > 0 (+)            E increases

Work done by the system on surroundings                        w < 0 (-)               E decreases

Heat transferred to system from surroundings                   q > 0 (+)              E increases

Heat transferred from system to surroundings                   q < 0 (-)               E decreases

 

Δ E = q  +  w

 

Given Δ E= - 300 J  w = + 75 J

 

- 300J = q + (+75J)

 

q = - 375 J of heat was transferred from the system to the surroundings