What is energy??? The ability to do work or produce heat. Potential Energy (PE) energy due to position or composition

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1 Chapter 10: Energy

2 What is energy??? The ability to do work or produce heat. Potential Energy (PE) energy due to position or composition Kinetic Energy (KE) energy due to motion Law of Conservation of Energy energy neither created nor destroyed

3 Energy Simply Changes Form!

4 Some more definitions: Work = force acting over a distance State function = property of a system that changes independently of its path Ex: Change in Elevation Temperature = measure of the random motions of the components of a substance Heat is a common term, but not easy to define Heat = energy change in a system not identified as work; flow of energy due to temperature differences

5 Energy is a state function (independent of the pathway taken) BUT Work and heat are not state functions!!!

6 What will happen???

7 Heat flows from hot to cold!!!

8 Dividing the Universe Everything in the universe falls into 2 categories: 1. System = the part of the universe on which we are focusing 2. Surroundings = everything else in the universe

9 Exothermic Processes When a process results in the evolution of heat Energy flows out of the system Example: the combustion of a match

10 Where does the energy released as heat come from??? A change in potential energy In any exothermic rxn, some of the potential energy stored in the chemical bonds is converted to thermal energy via heat.

11 Endothermic Processes Processes that absorb energy from surroundings Heat flow moves into the system Example: boiling water to produce steam

12 Thermodynamics The study of energy First Law of Thermodynamics Law of Conservation of Energy The energy of the universe is constant Internal Energy (E) Equal to the sum of the kinetic and potential energies of all particles in a system

13 Change in Energy DE = q + w q = heat w = work Number gives magnitude Sign indicates direction of flow +q = endothermic -q = exothermic

14 Change in Energy Same applies for quantities of work - w = work done by the system on the surroundings + w = work done on the system by the surroundings

15 Measuring Energy Changes Common units of energy: calorie joule calorie (cal)= the amount of energy (heat) required to raise the temperature of 1 gram of water by 1 C joule (J) 1 cal = J

16 Important Note: Food calories are not the same as energy calories The energy content of 1 food calorie is actually 1 kilocalorie (1000 cal)!

17 Practice Problem Express 60.1 cal of energy in units of joules. Answer: 251 J

18 Consider the Following How does the amount of substance heated affect the energy required to change its temperature?? The larger the substance the more energy required to heat it The larger the temperature difference the more energy required

19 Another Important Factor The identity of the substance! Called specific heat capacity Energy needed to raise the temp of 1 g of substance by 1 C Specific heat capacity of water = J/g C

20 So we have: Q = C x m x DT Where: Q = energy (heat) required C = specific heat capacity m = mass of the sample in grams DT = change in temperature ( C) Only works when no change of state occurs!

21 Practice Problem Determine the amount of energy (heat) in joules required to raise the temperature of 7.40 g water from 29.0 C to 46.0 C. Answer: 526 J

22 Practice Problem What quantity of energy (in joules) is required to heat a piece of iron weighing 1.30 g from 25.1 C to 46.5 C? Answer: 12.6 J

23 Practice Problem A 1.6 gram sample of a metal that has the appearance of gold requires 5.8 J of energy to change its temperature from 23.0 C to 41.0 C. Is the metal pure gold? Answer: No

24 Practice Problem A 12.8 gram sample of aluminum at 19.3 o C has 33.9 joules of heat added to it. What is its final temperature? Answer: 22.3 o C

25 Mixing Temps When substances at two different temperatures are mixed, the hotter object loses heat and the cooler object gains heat Q gained by cool = - Q lost by hot The final temperature for both is the same

26 Practice Problem What is the final temperature when 35.4 grams of water at 68.9 o C is added to 44.1 grams of water at 22.4 o C? Answer = 43.1 o C

27 Practice Problem What mass of o C water must be added to g of o C water to raise its temperature to o C? Answer = grams

28 Practice Problem What is the final temperature when g of aluminum at o C is placed into g of water at o C? The specific heat capacity of aluminum is.890 J/g o C. Answer = o C

29 Practice Problem When 97.5 grams of a metal at 98.4 o C is placed into 52.1 grams of water at 24.7 o C, the final temperature settles at 31.3 o C. What is the specific heat capacity of this metal? Answer =.220 J/g o C

30 Enthalpy Symbolized by H For a reaction occurring at constant pressure, DH is equal to the enrgy that flows as heat. DH p = heat

31 Practice Problem When 1 mol of methane (CH 4 ) is burned at constant pressure, 890 kj of energy is released as heat. Calculated DH for a process in which a 5.8-g sample of methane is burned at constant pressure. Answer: -320 kj

32 Calorimetry Calorimeter A device used to determine the heat associated with a chemical rxn Temperature change observed Know heat capacity Can calculate DH

33 Hess Law Enthalpy is a state function Independent of path taken When going from a particular set of reactants to a particular set of products, DH is the same whether the rxn takes place in 1 step or in a series of steps Allow us to easily calculate DH values that are difficult to measure

34 Example N 2 (g) + 2O 2 (g) 2NO 2 (g) Same as the following 2 step process: 1. N 2 (g) + O 2 (g) 2NO (g) DH = 180 kj 2. 2NO 2 (g) 2NO (g) + O 2 (g) DH = 112 kj Net Reaction: N 2 (g) + 2O 2 (g) NO 2 (g) DH = 180 kj kj = 68 kj

35 Characteristics of DH If a reaction is reversed, the sign of DH is also reversed. The magnitude of DH 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 DH is multiplied by the same integer.

36 Practice Problem Two forms of carbon are graphite and diamond. Using the enthalpies of combustion for graphite (-394 kj/mol) and diamond (-396 kj/mol), calculate DH for the conversion of graphite to diamond. Answer: 2 kj/mol C graphite (s) C diamond (s)

37 Practice Problem Calculate DH for the following reaction 2Cu + O 2 2CuO given the information below. 4CuO 2Cu 2 O + O 2 DH = 288 kj Cu 2 O Cu + CuO DH = 11 kj Answer = -310 kj

38 Quality vs. Quantity of E Energy crisis not about quantity of energy but instead the quality Which energy is easiest to use for work? When we use energy the quality of that energy (its ease of use) is lowered i.e. using gasoline in a car

39 Solutions We need to conserve quality energy sources Petroleum fuel Alternative Sources Solar energy Wind energy Wave energy

40 Energy and Our World- Sun is Ultimate Source of E

41 Petroleum & Natural Gas Formed from remains of ancient marine organisms Petroleum = thick, dark liquid made of hydrocarbons Natural gas = gases associated with petroleum deposits (methane, ethane, propane, butane)

42 What s in a Standard 42 Gallon Barrel of Crude Oil? Other Products Liquefied Petroleum Gases (LPG) Jet Fuel Heavy Fuel Oil (Residual) Other Distillates (Heating Oil) Diesel Gasoline

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44 What s in a Standard 42 Gallon Barrel of Crude Oil? Other Products Liquefied Petroleum Gases (LPG) Jet Fuel Heavy Fuel Oil (Residual) Other Distillates (Heating Oil) Diesel Gasoline

45 Additives To Use or Not To Use Anti-knock Additive TEL (tetraethyl lead) Bad because of lead content Unleaded Gasoline MTBE & ETBE Contaminates Drinking Water Desulfurization Decreases Sulfur Emissions Also Decreases Lubricity

46 Coal Made from the remains of plants that were buried and subjected to high pressure and heat over long periods of time Four types Lignite Subbituminous Bituminous Anthracite

47 Anthracite = Most Valuable Lignite = Least Valuable

48 Negative Effects Expensive Dangerous to Mine Contributes to Acid Rain Produces Large Amounts of CO 2

49 Effects of CO 2 on Climate

50 Greenhouse Effect Greenhouse Gases CO 2 H 2 O Atmosphere Acts as Greenhouse Glass Traps Gases Cause Temperature to Rise

51 Energy as a Driving Force Why do things occur in a particular direction? Examples: Burning Wood Gas trapped in one end of vessel What makes some reactions spontaneous??

52 Two Major Driving Forces After years of analysis, 2 major driving forces have been identified: Energy Spread Matter Spread

53 Energy Spread In a given process, concentrated energy is dispersed widely Occurs with all exothermic processes Heat flow = more movement Increases random motion of molecules in the surroundings

54 Matter Spread The molecules of a substance spread out and occupy a larger volume Example: Separated Gas chamber

55 Is there ever opposition? Yes! When NaCl is dissolved in water. Matter spread = favorable Energy spread = endothermic, unfavorable Reaction takes place because: Matter Spread > Energy Spread

56 Entropy (S) Measure of disorder Increased Randomness = Increased S Example: Which has a higher S, ice or water?

57 How does this relate? Increases in energy spread and matter spread both correspond to increase in entropy Increased energy spread = faster random motions of the molecules in surroundings Increased matter spread = components of matter are dispersed, they occupy a larger V

58 Second Law of Thermodynamics The entropy of the universe is always increasing!! Always heading towards more disorder Sadly, we are plunging slowly but inevitably toward total randomness the heat death of the universe. But don t despair it won t happen soon