1. Zumdahl • Zumdahl • DeCoste
World of
CHEMISTRY

2. Chapter 10
Energy

3. Goals of Chapter 10 General properties of energy Temperature & Heat
Direction of energy flow as heat
How energy flow affects internal energy
How to measure heat
Heat (enthalpy) of chemical reactions
Hess’s Law
Changes in quality of energy as it’s used
World’s energy resources
Energy as driving force for natural processes
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4. The importance of energy
Huge abundance of fossil fuels
Society with huge appetite for energy
We have become dependent on oil
Has led to tension between countries
Supplies are dwindling, prices are rising
Need to find alternatives to oil
Use relationship between chemistry & energy to find these alternatives
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5. Energy: the ability to do work or produce heat
Potential Energy
Energy due to position or composition
Examples: water behind a dam, gasoline
Kinetic Energy
Energy of motion – depends on mass & velocity (KE = ½mv2)
Examples: car driving, thrown baseball
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6. Law of Conservation of Energy
Energy can be converted from one form to another but cannot be created or destroyed
Energy of the universe is constant
Can convert from one form to another
Example: roller coasters
State function: property that is independent of pathway
Example: Ball bearing in roller coaster; PE is same at top of first hill regardless of path to bottom
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7. Temperature & Heat
Temperature: measure of the random motions of the components of a substance (warm water molecules move faster than cold water molecules)
Heat: flow of energy due to a temperature difference (heat will flow from warmer water to cooler water)
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8. Figure 10.2: Equal masses of hot and cold water.
Each side contains
1 kg of water
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9. Figure 10.3: H2O molecules in hot and cold water.
Water molecules move more rapidly in hot water
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10. Figure 10.4: H2O molecules in same temperature water.
Heat is transferred from the hot water to the cold water until both are the same temperature
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11. Final temperature is the average of the two original temperatures Change in temp (hot) ΔT = 90°C – 50°C = 40°C Change in temp (cold) ΔT = 50°C – 10°C = 40°C
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12. Exothermic vs. Endothermic
System: part of universe on which we wish to focus
Surroundings: everything else in universe except system
Exothermic: evolution of heat, energy flows out of system
Endothermic: absorbs energy from surroundings, heat flows into system
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13. Which is it - Endothermic or Exothermic?
Your hand gets cold when you touch ice. (with respect to your hand)
The ice melts when you touch it
Ice cream melts
Propane burning in a propane torch
After swimming water drops evaporate from your skin
Two chemicals mixing in a beaker give off heat
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14. What happens in an exothermic chemical reaction?
Energy is conserved
Reactants have potential energy
Energy gained by surroundings must equal energy lost by the system
In any exothermic reaction, some of the potential energy stored in the chemical bonds is converted to thermal energy via heat (random kinetic energy)
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15. Figure 10.5: The energy changes accompanying the burning of a match.
Reactants have > PE than products, difference is heat
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16. Thermodynamics – the study of energy
First Law of Thermodynamics = Law of Conservation of Energy (energy of the universe is constant)
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17. Internal Energy (E)
Sum of kinetic and potential energies of all “particles” in a system
Can be changed by flow of work, heat, or both
ΔE = q + w; change in energy = heat + work
Sign reflects systems point of view
Endothermic – energy flows into system = +q (energy is increasing)
Exothermic – energy flows out of system = -q (energy is decreasing)
Same rules apply to work (w)
+w = work flows into system (surroundings do work)
-w = work flows out of system (system does work)
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18. Measuring Energy Changes
Calorie: amount of energy (heat) required to raise the temperature of one gram of water by one degree Celsius
Food “calorie” = kilocalorie = 1000 cal
Joule: SI unit of energy
1 calorie = J
It takes J of energy to raise the temperature of one gram of water by 1°C
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19. E = specific heat x mass x temp change
How much energy is required to raise the temperature of 7.40 g of H2O from 29.0°C to 46.0°C?
E = specific heat x mass x temp change
E = (4.184 J/g°C)x(7.40 g)x(46°C – 29°C)
E = 526 J
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20. Thermochemistry (Enthalpy)
H = Enthalpy: energy function that indicates how much energy is produced or absorbed in a reaction
ΔHp = energy that flows as heat
ΔH: the change in enthalpy
p: indicates process has occurred under constant pressure
The enthalpy change is the same as the heat of reaction
See Example 10.5 & Self-Check 10.5 (pg. 302)
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21. Figure 10.6: A coffee-cup calorimeter.
Calorimeter: device used to determine the heat associated with a chemical reaction
Run reaction – observe temperature change
Heat capacity of calorimeter enables us to calculate the heat energy released/absorbed by reaction
Determine ΔH for reaction & calculate ΔH for other reactions
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22. Enthalpy is a state function – it is independent of the pathway
Hess’s Law
Enthalpy is a state function – it is independent of the pathway
When going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or a series of steps
See example on page 304
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23. Two characteristics of ΔH for a reaction:
If a reaction is reversed, the sign of ΔH is also reversed
The magnitude of ΔH is directly proportional to the quantities of reactants and products in a reaction. If the coefficients in a balanced reaction are multiplied by an integer, the value of ΔH is also multiplied by that integer
See examples on page 304 & 305
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24. Figure 10.7: Energy sources used in the United States.
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25. Figure 10.8: The earth’s atmosphere.
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26. Figure 10.9: The atmospheric CO2 concentration over the past 1000 years.
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27. Energy as a Driving Force
Two driving forces
Energy spread: concentrated energy is dispersed widely
Matter spread: molecules of a substance are spread out and occupy a larger volume
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28. Entropy Designated by letter S Measure of disorder or randomness
More disorder (or entropy) means:
More energy spread
More matter spread
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29. Figure 10.10: Comparing the entropies of ice and steam.
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30. Second Law of Thermodynamics
The entropy of the universe is always increasing
All processes lead to a net increase in the disorder of the universe
We are plunging slowly toward total randomness – the heat death of the universe
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