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Unit #6
Thermochemistry
Thermochemistry The study of the heat changes in chemical
reactions and physical processes.
Examples of heat changes:
Fire
Chemical
Exo. Chemical potential to
heat/light via
combustion.
Friction
Physical
Exo. Kinetic to heat
Hair Drier
Physical
Exo. Electrical to heat
Air
Conditioner
Physical
?
Electrical to heat (high
pressure) to absorb heat
(low pressure).
Examples of heat changes con’t:
Ice
Melting
H2O
Evaporating
Physical
Endo.
Physical
Endo.
Low kinetic to
high kinetic
Low kinetic to high
kinetic.
All of the above result in a change in heat (energy).
Energy - The capacity to do work or supply heat.
Chemical potential energy Energy stored in chemicals because of their
composition.
Where is “this energy” stored ?
In the chemical
bonds.
The type of atoms and their arrangements
determine the amount energy stored in the bonds.
Heat A form of energy that ALWAYS flows from a
warmer object to a cooler object.
In physical sciences it is represented as a “q”
It is measured in “Joules”
What are the observable effects of adding heat ?
1. Increase of temperature.
2. Change of state (melting, boiling…)
3. Starting of a chemical reaction (combustion,
decomposition).
Temperature A measure of the average kinetic energy of the
molecules of an object.
Heat Capacity The amount of heat (energy) required to
change an object’s temperature by exactly 1C.
Measured in J/C or calories.
calorie The amount of heat required to increase the
temperature of 1 gram of water from 4C to 5C.
Notice this is a lower case word.
Calorie with a capital “C” is used when refering
to food. It equals 1000 calories (lower case “c”), or 1
Kcal (Kilocalorie).
Trivia:
A runner in a marathon (26.2 miles) could burn
1800 Calories during the run.
1800 Calories is about 4.5 servings of spaghetti.
1 Kcal = 4186 joules
Joule SI unit of heat and energy.
1 joule = 1 Kg x m2
s2
= 0.239 calories
Specific Heat Capacity The amount of heat required to raise 1 gram
of a substance 1C.
Measured in
J
g x C
Heat capacity (not specific heat) is dependant on
two things:
1. The mass of the substance.
The greater the mass, the greater the heat
capacity.
2. The chemical composition of the substance.
Example #1:
A single microwavable burrito lists its Calories
as 310. How many joules is this ?
310 Cal x
1000 calories
1 Cal
=
1,297,071.13 joules
Example #2:
Consider the following substances and their
specific heats:
H2O = 4.186 J/g xC
Al
= 0.90 J/g xC
Fe
= 0.44 J/g xC
Cu
= 0.38 J/g xC
Which heats
quicker??
cools slower
HCu
2O
Example #3:
A piece of silver has a heat capacity of 42.8
J/C. If the silver has a mass of 181 grams, what is
the specific heat ?
Specific heat =
Specific heat =
J
g x C
=
0.236 J/g xC
42.8 J
181 g x C
Example #4:
Calculate the heat (q) absorbed by 15 grams
of water to raise its temperature from 20C to 50C.
(assume constant pressure).
Specific heat =
4.186 J
g x C
Heat absorbed = 1881 J
=
XJ
15 g x 30C
Example #5:
You wish to heat water for coffee. How much
heat in Joules must be used to raise the temperature of
0.180 Kg of tap water (enough for 1 cup) from 15 C
to 96 C (the ideal brewing temp).
Specific heat =
Heat added =
4.186 J
g x C
61031.88 J
=
XJ
180 g x 81C
Example #6:
An iron skillet weighing 1.51 Kg is heated to
178C on the stove. It then cooled to room the temp of
21C . How much heat energy (in Joules) was
removed ?
Specific heat of Fe =
0.450 J
g x C
Heat removed = -106681.5 J
=
XJ
1510 g x -157C