206
Thermochemistry
PROBLEM:
The specific heat of Fe 2 O 3 is 0 151 cal/g °C How much heat is needed to raise the
temperature of 200 g of Fe 2 O 3 from 20 0°C to 30 0°C
7 What is the molar heat
capacity of Fe 2 O 3
9
SOLUTION.
Because 0 151 cal raises 1 g of Fe 2 O 3 1°C,
(
cal \
0 151 -— (100°C)
g C/
= 302 cal needed
The molar heat capacity is the product of specific heat and mole weight
Molar heat capacity = f 0 151 -^) fl59 6 ——}
y
\
g°C/\
mole/
= 24 1
g'
cal
mole °C
CALORIMETRY
The amount of heat that is absorbed or liberated in a physical or chemical
change can be measured in a well-insulated vessel called a calonmeter (Figure
14-1) Calonmetry is based on the principle that the observed temperature
change resulting from a chemical reaction can be simulated with an electrical
heater The electrical measurements of current (/), heater resistance (/?), and
duration (?) of heating make it possible to calculate how much heat is equivalent
to the amount of heat produced by the chemical change, using the formula
I
2 Rt/4 184 By using weighed quantities of reactants, one can calculate the heat
change per gram or per mole
The formula is derived as follows Electrical current, measured in amperes, is
the rate of flow of electrical charge (coulombs), by definition, it is
coulombs
amperes
-3—
seconds
The common relationship between volts (£), amperes (/), and resistance (R) is
known as Ohm's law
E = IR