Calculations of
Thermochemistry
May 21st 2015
Prior Knowledge
• Two main forms of energy
• Kinetic energy (energy of motion)
• Ex. Thermal energy (Heat)
• Potential Energy (energy of position)
• Ex. Chemical Potential Energy
• Law of conservation of energy
• System vs Surrounding
• Endothermic and exothermic Reactions
• Temperature vs Heat
• Units of measuring energy (cal, Cal, J)
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Objectives
• Conversions between different units of energy
• Define specific heat of a substance
• Relate chemical potential energy to the heat lost or gained in
chemical reactions
• Calculate the amount of heat absorbed or released by a substance as
its temperature changes
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Units defined
Metric System of energy
• 1 calorie (cal) is the amount of energy required to increase the
temperature of 1g of pure water by 1 degree Celcius (oC).
• Nutritional calorie (Cal) = 1000 calories = 1kcal
SI Unit of Energy
• Joule (J)
 1 cal = 4.184 J
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1.
2.
3.
4.
1.
2.
3.
Convert 100 cal to Cal
Convert 100 cal to J
Convert 100 J to cal
Convert 300 Cal to J
100 cal x
1 𝐶𝑎𝑙
1000 𝑐𝑎𝑙
100 cal x
100 J x
4. 300 Cal x
4.184 𝐽
1 𝑐𝑎𝑙
= 0.100 𝑐𝑎𝑙
= 418.4 𝐽
0.2390 𝑐𝑎𝑙
1𝐽
= 23.90 𝑐𝑎𝑙
1000 𝑐𝑎𝑙
1 𝐶𝑎𝑙
4.184 𝐽
1 𝑐𝑎𝑙
x
= 1255200
= 1.26 x 106
Practice problems 1,2 and 3
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Specific Heat ©
Definition:
Amount of heat (J) required to raise the temperature of one gram (1g)
of pure substance by one degree Celsius (1 .oC).
The specific heat of water is 1 calorie/gram °C = 4.186 joule/gram °C
which is higher than any other common substance.
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Calculating Heat Absorbed
p.520 : 5.00 x 103 g (5kg) block of concrete sidewalk increased by 6.0
oC. How much heat is absorbed?
qconcrete = 0.84
c = 0.84 J/g.oC
g.oC
x 5.00 x 103 g x 6.0 oC
= 25000 J = 25 kJ
m = 5.00 x 103 g
qconcrete = ???
𝐽
qH2O = 4.184
𝐽
g.oC
x 5.00 x 103 g x 6.0 oC
= 130000 J = 130 kJ
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Calculating heat…
• We can use the same equation to calculate heat released in cooling
down
• Use example problem 15.2 as a guide and solve practice problems 4,5
and 6.
• Try P.522 Section 15.1 assessment
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15.2 Heat – Determining Specific Heat
Objectives
• Describe how a calorimeter is used to measure energy that is
absorbed or released
• Explain the meaning of enthalpy change in chemical reactions and
processes
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Calorimeter
An insulated device used for measuring the amount of heat absorbed or released
during a chemical or physical process.
p.523
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Determining Specific Heat
• p.524 Example
• P.525 Example problem use this as a guide
• Practice problems 12-15
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