Unit 07 - Thermodynamics
Calculating Energy Changes in
One Substance:
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When energy is given off in the form of heat, this
value can be measured or calculated
Recall that the base unit for energy is the joule (J)
If the amount of heat given off by a substance is to
be measured, its specific heat capacity must also
be known.
Specific heat capacity is defined as the amount
of heat required to raise 1 g of a substance 1˚C.
Therefore the units for a substance’s specific heat
capacity is J/g˚C.
The specific heats of several substances will be
provided for you (Page 42 in your text has a few)
Calculating Energy Changes:
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There is a WAY COOL FORMULA we can use
to calculate the amount of energy absorbed or
released from a substance when the
temperature changes. It is:
Q = mc∆t
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Q = energy (J) absorbed or released
m = mass of the substance (g)
c = specific heat capacity (J/g°C)
∆t = change in temperature (°C)
 tfinal - tinitial
Calculating Energy Changes:
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Please note:
 When using the Q = mc∆t formula:
○ If heat is absorbed, Q will be positive.
○ If heat is released, Q will be negative.
 The value of Q itself will always be positive.
You need to understand mentally if Q is
representing heat being absorbed or heat
energy being released; the negative sign is
simply representing if it’s absorbed or
released
Example 1:
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What quantity of heat is required to raise
the temperature of 100.g of liquid water
10.°C?
Example 2:
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What quantity of heat is released when
420.0g of lead cools from 85°C to 25°C?
The specific heat capacity of lead is
0.13J/g°C.