Chemistry Reference
In-Depth Information
This equation is merely an expression of the law of conservation of energy. By
convention, energy, heat, and work added to a system are regarded as positive,
but any of these removed from a system is regarded as negative.
Be sure to get
the sign right.
Energy added to a system is
defined as positive; energy re-
moved is defined as negative.
EXAMPLE 14.14
(a) A certain acid-base reaction in solution produces 117 kJ of heat but does
no work. What is the sign of
q?
(b) All of the heat is used to warm the solution. What is the sign of
q
for the
warming process?
(c) What is the value of
q
for the overall process?
Solution
(a) Because heat is given off by the reaction,
q
is negative.
(b) Because the heat is absorbed by the solution, this
q
is positive.
(c) The overall
q
is zero because no heat was added to or taken from the
surroundings. All the heat from the reaction went into the warming
process.
Because we cannot measure the heat produced by a chemical reaction
directly, we measure it by determining its effect on something. For example, the
heat produced by burning 3.00 mol of methane (natural gas) might be measured
by determining the increase in temperature of a certain mass of water warmed
by the heat. In such an experiment, we must try to ensure that
all
of the heat
produced by the combustion reaction is used to warm the water and that none
escapes to the surroundings.
EXAMPLE 14.15
Burning 2.00 mol of
CH
4
warms 10.0 kg of water by
42.5°C.
Calculate the heat
produced in the combustion process:
CH
4
(g)
2 O
2
(g) £ CO
2
(g)
2 H
2
O(
/
)
Solution
The heat added to the water is
(10,000 g) (4.184 J/g
#
°C) (42.5°C)
10
6
J
Heat
mc
¢
t
1.78
1780 kJ
That is the magnitude of the heat produced by the reaction. Because the reac-
tion
produced
the heat,
q
1780 kJ
for the reaction.
Practice Problem 14.15
Burning 0.250 mol of
C
2
H
6
warms 5.65 kg
of water by
16.5°C.
Calculate the heat produced in the combustion process.
