1. Calculations of Thermochemistry
May 21st 2015

2. Prior Knowledge Two main forms of energy Law of conservation 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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3. 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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4. 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 = J
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5. Convert 100 cal to Cal
Convert 100 cal to J
Convert 100 J to cal
Convert 300 Cal to J
cal x 1 𝐶𝑎𝑙 1000 𝑐𝑎𝑙 =0.100 𝑐𝑎𝑙
cal x 𝐽 1 𝑐𝑎𝑙 =418.4 𝐽
J x 𝑐𝑎𝑙 1 𝐽 =23.90 𝑐𝑎𝑙
300 Cal x 𝑐𝑎𝑙 1 𝐶𝑎𝑙 x 𝐽 1 𝑐𝑎𝑙 =
= 1.26 x 106
Practice problems 1,2 and 3
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6. 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 = joule/gram °C which is higher than any other common substance.
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8. Calculating Heat Absorbed
p.520 : x 103 g (5kg) block of concrete sidewalk increased by 6.0 oC. How much heat is absorbed?
qconcrete = 𝐽 g.oC x 5.00 x 103 g x 6.0 oC
= J = 25 kJ
c = 0.84 J/g.oC
m = 5.00 x 103 g
qconcrete = ???
qH2O = 𝐽 g.oC x 5.00 x 103 g x 6.0 oC
= J = 130 kJ
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9. 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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10. 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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11. 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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12. Determining Specific Heat
p.524 Example
P.525 Example problem use this as a guide
Practice problems 12-15
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