Untitled - Kelly Walsh High School

100 CHEMISTRY FOR THE UTTERLY CONFUSED
The coffee-cup calorimeter can be used to measure the heat changes in reactions
that are open to the atmosphere, q p, constant pressure reactions. We use
this type of calorimeter to measure the specific heats of solids. We heat a known
mass of a substance to a certain temperature and then add it to the calorimeter
containing a known mass of water at a known temperature. The final temperature
is then measured. We know that the heat lost by the added substance (the
system) is equal to the heat gained by the surroundings (the water and
calorimeter, although for simple coffee-cup calorimetry the heat gained by the
calorimeter is small and often ignored):
q solid q water
Substituting the relationships for q gives:
(c solid mass solid T solid) (c water mass water T water)
We can then solve this equation for the specific heat capacity of the solid.
We use the constant-volume bomb calorimeter to measure the energy changes
that occur during combustion reactions. We add a weighed sample of the substance
under investigation to the calorimeter and then add excess compressed
oxygen gas. We ignite the sample electrically and measure the temperature
change of the calorimeter and the known mass of water. Generally, we know the
heat capacity of the calorimeter and can determine heat absorbed by the water.
Suppose we have a problem such as the one below:
We ignited a 1.5886 g sample of glucose (C 6H 12O 6) in a bomb calorimeter. The
temperature increased by 3.682C. The heat capacity of the calorimeter was
3.562 kJ/C, and the calorimeter contained 1.000 kg of water. Find the molar heat
of reaction (J/mol of glucose) for the reaction:
C 6H 12O 6(s) 6 O 2(g) l 6 CO 2(g) 6 H 2O(l)
In solving problems of this type, you must realize that the oxidation of the glucose
released energy in the form of heat and that some of the heat was absorbed
by the water and the remainder by the calorimeter. You can use both the heat
capacity of the calorimeter and the mass and specific heat of the water with the
temperature change to calculate the heat absorbed by the calorimeter and
water:
(Heat capacity of the calorimeter) (change in temperature)
Heat absorbed by the calorimeter
(Grams of water) (specific heat of water) (temperature change)
Heat absorbed by the water