Char MacDonald
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Thermochemistry
Ch 16 Read p 624- 638
1. Thermochemistry
Thermochemistry is the study of heat changes that occur during chemical reactions.
Fuels such as coal, oil and gasoline have high chemical potential energy. Their
combustion products, carbon dioxide and water, have very low chemical potential energy.
Energy is the capacity for doing work or supplying heat. We can also state that the
energy is the capacity to produce changes. It is found in two forms:
Energy
Form
Potential
Symbol Definition
Chemistry Example
Ep
Stored energy
Kinetic
Ek
Energy of motion
Energy in chemical bonds (Chemical
Energy)
Movement of particles (Thermal Energy)
Kinetic Molecular Theory: The particles in a substance are constantly moving and
colliding. Adding heat energy to an object increases its energy content (Ek + Ep).
Increases Ek → increases particle motion
Adding Heat Energy
Increases Ep → increases bond energies
Furthermore, kinetic energy is continually converted to potential energy, and vice versa.
Consider a head-on collision between two gas molecules with the same initial velocity –
energy must be conserved.
Event
Diagram
Energy Description
Approach
Ek is high
Ep is low
Collision
Ek is zero
Ep is high
Rebound
Ek is high again
Ep is low again
Heat is a form of energy. Heat and temperature are not the same. Heat involves a transfer
of energy between two objects because of a difference in temperature. Temperature is the
measure of the average kinetic energy of molecules. Any temperature change (∆T)
depends on the amount of heat (q) flowing into or out of the substance.
The SI unit for measuring energy, including heat, is the Joule (J).
Only temperature can be measured. Heat is not measurable but can be calculated from
measure temperatures changes.
The temperature change produced by an amount of heat varies directly with three factors:
q ∞ mass (m) of the object
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q ∞ specific heat (c) of the object
q ∞ temperature change (ΔT) in the object
Therefore, q = mcΔT
(i) Specific Heat (c)
o
C.
= heat requires to raise the temperature of 1 g of a substance by 1
= has the unit J / g · oC
(ii) Some specific heat values have been tabled for you. Be careful. They depend on the
physical state of the substance.
(iii) ∆T = Tfinal - Tinitial
If ∆T > 0, the q will become a positive number (heat is gained).
If ∆T < 0, the q will become a negative number (heat is lost).
Try This: Calculate the missing values. Using , q = mcΔT
Substance Heat
Mass Specific
Initial
T
Exchanged (g)
heat (J /
Temperature
(oC)
o
(J)
g · C)
(oC)
Water
+1530
20.0
Aluminum
14
95
+3600
20
14
Steam
-1500
-25
Mercury
+825
152
42.0
Final
Temperature
(oC)
23.2
26
48
116
It is useful in thermochemistry to define a system as the part of the universe on which we
focus our attention. For example, a system might be the chemicals that will undergo
reaction and the surroundings a water bath that surrounds the flask that contains the
chemicals. Together, the system and surroundings constitute the universe. The changes
that are of interest in thermochemistry are the flow of heat from the system to the
surroundings or to the system from the surroundings.
The first law of thermodynamics is that the energy of the universe remains constant.
No energy is created or destroyed as a result of these heats of transfers.
Char MacDonald
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