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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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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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 = ½mv 2 )
• Examples: car driving, thrown baseball
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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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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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Figure 10.2: Equal masses of hot and cold water.
Each side contains
1 kg of water
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Figure 10.3: H
2
O molecules in hot and cold water.
Water molecules move more rapidly in hot water
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Figure 10.4: H
2
O 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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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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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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Which is it - Endothermic or Exothermic?
1.
Your hand gets cold when you touch ice.
(with respect to your hand)
2.
The ice melts when you touch it
3.
Ice cream melts
4.
Propane burning in a propane torch
5.
After swimming water drops evaporate from your skin
6.
Two chemicals mixing in a beaker give off heat
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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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Figure 10.5: The energy changes accompanying the burning of a match.
Reactants have > PE than products, difference is heat
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Thermodynamics
• Thermodynamics – the study of energy
• First Law of Thermodynamics = Law of
Conservation of Energy (energy of the universe is constant)
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• 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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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 = 4.184 J
• It takes 4.184 J of energy to raise the temperature of one gram of water by 1 °C
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How much energy is required to raise the temperature of 7.40 g of H
2
46.0
°C?
O from 29.0
°C to
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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Thermochemistry (Enthalpy)
• H = Enthalpy: energy function that indicates how much energy is produced or absorbed in a reaction
• ΔH p
= 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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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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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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Two characteristics of Δ H for a reaction:
• If a reaction is reversed, the sign of Δ H is also reversed
• The magnitude of Δ H 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 Δ H is also multiplied by that integer
• See examples on page 304 & 305
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Figure 10.7: Energy sources used in the
United States.
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Figure 10.8: The earth’s atmosphere.
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Figure 10.9: The atmospheric CO
2 concentration over the past 1000 years.
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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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• Designated by letter S
• Measure of disorder or randomness
• More disorder (or entropy) means:
• More energy spread
• More matter spread
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Figure 10.10: Comparing the entropies of ice and steam.
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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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