Chemistry 222 – WS#4

PCC/Rock Creek/Backes

(5 pts - due February 2, 2005)                                             Name

1. Cold packs, whose temperatures are lowered when ammonium nitrate dissolves in water, are carried by athletic trainers when transporting ice is not possible.  Is this reaction exothermic or endothermic?

2.  When Karl Kaveman adds chilled grog to his new granite mug, he removes 10.9 kJ of energy from the mug. If it has a mass of 625 g and was at 25°C, what is its new temperature?  Specific heat capacity of granite = 0.79 J/(g·K)

q = m c ΔT

q = - 10.9 kJ  or  -10.9 x 103 J

m = 625 g

c= 0.79 J/(g °C)

ΔT = Tf – Ti  (Ti = 25 °C)

-10.9 x 103 J = 625 g x 0.79 J/(g °C) x (Tf – 25 °C)

Tf = 3 °C

3.  A 20.0-g piece of metal at 203 °C is dropped into 100.0 g water at 25.0 °C.  The water temperature rises to 29.0 °C.  Calculate the specific heat of the metal (J/g°C).  Assume no heat loss to the surroundings.

Energy is conserved.  The energy released by the piece of metal is absorbed by the water; so qH2O = -qmetal, and q = m c ΔT

100.0 g x 4.184 J/(g °C) x 4.0 °C   =   -{20.0 g x cmetal x (-174 °C)}

cmetal  = 0.48 J/(g °C)

3.  How many kilojoules of energy are released by the reaction of 3.41 g Al with excess oxygen, according to                   2Al(s)  +  1½O2(g)  ®  Al2O3(s)     DHrxn = -1675.7 kJ

3.41 g Al   x   1mol   x    -1675.7 kJ    =  -1.06 x 102 kJ

26.98 g          2 mol Al

4.  Calculate DH°rxn for the combustion reaction of octane and DH°rxn for the combustion reaction of methanol.  Methanol is sometimes used as a fuel in high performance engines.

Using the data in Appendix B, also compare the standard enthalpy of combustion per gram of methanol, with that of octane (gasoline is a mixture of substances, but mainly octane; DH°f = -269 kJ/mol for octane).

1. Balance reactions first.  Both are combustion reactions and produce CO2 and H2O.

2. Calculate DHrxn  using ΔHreaction = SΔHproducts - SΔHreactants

3. Compare prices.

2 C8H18(l)  +  25O2(g)  →  16 CO2(g)  +  18H2O(l)                DHrxn = -1.09 x 104 kJ

-1.09 x 104 kJ   x    1 mol     =  -47.8 kJ/g

2 mol C8H18          114 g

2 CH3OH(l)  +  3O2(g)  →  2 CO2(g)  +  4H2O(l)                  DHrxn = -1.45 x 103 kJ

-1.45 x 103 kJ   x    1 mol     =  -22.7 kJ/g

2 mol CH3OH       32 g

Octane is the better fuel.