Energy Relationships in Chemical Reactions

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Energy Relationships in Chemical Reactions

• Chapter 6

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• Energy is the capacity to do work
• Thermal energy is the energy associated with the
  random motion of atoms and molecules
• Chemical energy is the energy stored within the
  bonds of chemical substances
• Nuclear energy is the energy stored within the
  collection of neutrons and protons in the atom
• Electrical energy is the energy associated with
  the flow of electrons
• Potential energy is the energy available by
  virtue of an objects position

6.1
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Thermochemical Definitions
Heat (q) Is the energy transferred between a
system and its surroundings as
result in the differences in
their temperatures only! Work (w) The energy
transferred when an object is moved by
a force.
Thermochemistry is the study of heat change in
chemical reactions.
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E is a state function, q and w are not.
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Energy Changes in Chemical Reactions
Heat is the transfer of thermal energy between
two bodies that are at different temperatures.
Temperature is a measure of the thermal energy.
greater thermal energy
6.2
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Exothermic process is any process that gives off
heat transfers thermal energy from the system
to the surroundings.
Endothermic process is any process in which heat
has to be supplied to the system from the
surroundings.
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Specific Heat
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The specific heat (s) of a substance is the
amount of heat (q) required to raise the
temperature of one gram of the substance by one
degree Celsius.

• in the SI system has units of J/g?C.
• in the metric system has units of cal/g?C.

Heat (q) absorbed or released
q m x s x Dt
Calculate the
Temperature Change (DT)
q C x Dt
Dt tfinal - tinitial
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Heat Capacity
The heat capacity (C) of a substance is the
amount of heat (q) required to raise the
temperature of a given quantity (m) of the
substance by one degree Celsius.
C m x s Heat Capacity mass x specific
heat
Heat Capacity
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Heat Equation

• Rearranging the specific heat expression gives
  the
• heat equation.
• Heat g x C x J J
• gC
• The amount of heat lost or gained by a substance
  is calculated from the
• mass of substance (g).
• temperature change (?T).
• specific heat of the substance (J/gC).

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cal/gC 0.214 0.0920 0.0308 0.108 0.0562 0.125 0.
488 0.588 0.207 0.100
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s of Fe 0.444 J/g 0C
Dt tfinal tinitial 50C 940C -890C
q msDt
869 g x 0.444 J/g 0C x 890C
-34,000 J
6.4
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Constant-Pressure Calorimetry
Reaction at Constant P
qrxn - (qwater qcal)
qwater msDt
qcal CcalDt
No heat enters or leaves!
6.4