3Some thermodynamic terms chemists use: System: the portion of the universe that we are consideringopen system: energy & matter can transferclosed system: energy transfers onlyisolated system: no transfers
4Surroundings: everything else besides the system Isothermal: a system that is kept at a constant temperature by adding or subtracting heat from the surroundings.
5Heat Capacity: the amount of heat energy required to raise the temperature of a substance by 1°C (or 1 K). (little c in equations)
6Heat Capacity: Specific Heat Capacity: 1 g by 1°C Capital “C” Molar Heat Capacity: 1 mole by 1°C“Cmol”
7c C Cmolar Symbol Name formula units heat capacity q =c*m*∆T J/g*K specific heat capacityq =C*m*∆TCmolarMolar heat capacityamount of heat to raise 1 mole of a substance 1Kq =Cmol*m*∆TJ/mol*Ks
8Calorie: the amount of heat required to raise the temperature of 1g of water by 1°C. specific heat of water = 1 cal/g °C1 calorie = 4.18 joules
9Substance Specific Heat (J/°C•g) Molar Heat (J/°C•mol)Al0.9024.3Cu0.3824.4Fe0.4525.1CaCO30.8483.8Ethanol2.43112.0Water4.1875.3Air1.00~ 29important to:EngineersChemist
10Heat Energy Heat energy = (specific heat)(mass)(ΔT) q =(C)(m)(Tf-Ti) Note that ΔT can be ºC or K, but NOT ºF. When just T is being used in a scientific formula it will usually be kelvin (K).)
11EXAMPLE:How many joules of energy are needed to raise the temperature of an iron nail (7.0 g) from 25°C to 125°C?The specific heat of iron is 0.45 J/°C•g.Heat energy = (0.45 J/°C•g)(7.0 g)(125-25°C)= 315 J
12Problem:How much energy does it take to raise the body temperature 2.5ºC (a fever of just over 103ºF) for someone who weighs 110 pounds (50. kg).Assume an average body specific heat capacity of 3.0 J/ºC•g.380,000 J
13-30oC to OoC – kinetic energy increases 0 o C to 0oC – phase change – potential energy change – solid to liquid0 o C to 100oC – kinetic energy change100 o C to 100oC – phase change – potential energy change – liquid to gas100 oC to 125oC – kinetic energy change
15Phase Change Calculations How much energy and time is needed to take 40.0g of ice from -30oC to 125oCGivenCice = The specific heat capacity of ice = 0.50 cal/g oCLf = The Latent heat of fusion of ice or energy needed to melt = 80 cal/gCwater = The specific heat capacity of water = 1.0 cal/g oCLv = The latent heat of vaporization of water = 540 cal/gCsteam = The specific heat capacity of steam = 0.48 cal/g oCRate of heating = 12 cal/ s
16Phase Change Calculations How much energy and time is needed to take 40.0g of ice from -30oC to 125Q=mcT= 40.0g (0.50 cal/g oC) ( 0oC – (-30.0 oC)) = 600 calTime = 600 cal | 12 cal = 50s1 sQ= mLf = 40.0g (80 cal )= 3200 calgTime = 3200 cal | 1 s = 267 s12 cal
17Phase Change Calculations How much energy and time is needed to take 40.0g of ice from -30oC to 125oCQ=m Ttc= 40.0g (1.0 cal/g oC) ( 100oC – 0 oC) = 4000 calTime = 4000 cal / 12 cal / s = 333sQ= mLv = 40.0g (540 cal /g ) = 21,600 calTime = 21,600 cal / 12 cal / s = 1800sQ=m c T= 40.0g (0.48 cal/g oC) ( 125oC – 100 oC) = 480 calTime = 480 cal / 12 cal / s = 40s
18Phase Change Calculations How much energy and time is needed to take 40.0g of ice from -30oC to 125oCQprocess = 600cal cal cal cal +21,600 cal = 29880calS process = 50 sec sec s s + 40 s = 2490 sIt takes cal and will heat in 2490 seconds
19A calorimeterA calorimeter is used to measure the transfer of heat during a physical or chemical change. The formula for heat transfer calculations is:q = m(ΔT)CpAmount of heat transferred =mass x change in temperature x specific heatWhere q = heat transferred,ΔT = the change in temperature andCp = the specific heat. Know the C H2O = 4.18J/(g*K)
20Heat loss by metal = heat gained by water - q = q - mCpT = mCpT If a mass of cadmium of 65.6g and a temperature of 100oC is dropped into 25.0 cm3 of water at a temperature of 23.0oC what will the final temperature be of the water?Cp = J/oC*g CadmiumHeat loss by metal = heat gained by water- q = q- mCpT = mCpTTf= 33.07oC
21Phileas Fogg, the character who went around the world in 80 days, was very fussy about his bathwater temperature. It had to be exactly 38.0o C. You are his butler, and one morning while checking his bath temperature, you notice that it’s 42.0oC. You plan to cool the kg of water to the desired temperature by adding an aluminum -duckie originally at freezer temperature (-24.0oC). Of what mass should the Al-duckie be?[Specific heat of Al = J/(goC); density of water =1 .00 g/ml]. Assume that no heat is lost to the air.33.89 Kg for duck (Don’t forget) –q=q
22Specific Heat Capacity of Copper Calorimeters are designed to be well-insulated, so no heat is gained from or lost to the surroundings. If no heating element is used to introduce heat in the system, the total heat transferred (q) for the entire calorimeter system must equal zero. The total heat can be split into heats for each component in the system.Imagine an experiment in which a hot copper ball is dropped into a calorimeter containing water at room temperature. The copper ball will lose heat, which will be absorbed by the calorimeter and water. Because no heat enters or leaves the system the heat balance for this experiment is0 = q = qCu + qcal + qw
23Specific Heat Capacity of Copper In this case qCu < 0, because the copper ball will lose heat to the calorimeter and water. Similarly qcal > 0 and qw > 0, because both the calorimeter and the water will gain heat. In chemistry the thermodynamic sign convention is things entering the system a positive in value, and things leaving the system are negative in value.
24Specific Heat Capacity of Copper The basic strategy in calorimetry is to use a temperature change and a heat capacity to determine a heat flow. In this experiment, all substances have the same final temperature (Tf), but not all substances have the same initial temperature. The copper ball is initially at temperature TCu while the calorimeter and water are initially at temperature Ti.qCu = mCu sCu ( Tf - TCu ) qcal = Ccal ( Tf - Ti ) qw = mw sw ( Tf - Ti )
25Specific Heat Capacity of Copper The heat capacity of the calorimeter must be obtained from a separate calibration experiment (for example, a heating element can be used to introduce a known amount of heat). The specific heat capacity of water is known (4.184 J oC-1), and the temperatures TCu, Ti, and Tf can be measured experimentally. The masses of the copper and water (mCu and mCu) can also be measured experimentally. The only unknown property in the above equations is the specific heat capacity of the copper.sCu = - (Ccal + mw sw) (Tf - Ti) mCu (Tf - TCu)
26The temperature of a piece of copper with a mass of 95 The temperature of a piece of copper with a mass of 95.4g increases from 25oC to 48.0oC when the metal absorbs 849J of heat. What is the specific heat of copper?What do we know?C = qm x Tmcu = T =q=Don’t know:Ccu = ? Jg x oCTo solveCcu =95.4 goC849 J849 J= J/g·oC95.4 g *23oC
27Energy:"The capacity to do work and/or transfer heat”
28Forms of Energy: Kinetic (Ekinetic = ½mv2) Potential Heat Light (Electromagnetic)ElectricityChemicalNuclearMatter (E = mc2)
30First Law of Thermodynamics The total amount of energy (and mass) in the universe is constant. In any process energy can be changed from one form to another; but it can never be created nor destroyed.
32Enthalpy (Heats) of Reaction The amount of heat released or absorbed by a chemical reaction at constant pressure (as one would do in a laboratory) is called the enthalpy or heat of reaction. We use the symbol H to indicate enthalpy.
33Sign notation (EXTREMELY IMPORTANT!!): +ΔH indicates that heat is being absorbed in the reaction (it gets cold) endothermic−ΔH indicates that heat is being given off in the reaction (it gets hot) exothermic
34Pressure 1 atm (760 torr) Concentration 1.0 M Standard Enthalpy = ΔH° (° is called a “not”)Occurring under Standard Conditions:Pressure 1 atm (760 torr)Concentration 1.0 M
35Temperature is not defined or part of Standard Conditions, but is often measured at 298 K (25°C).
37Standard Enthalpy of Formation The amount of heat absorbed (endothermic) or released (exothermic) in a reaction in which one mole of a substance is formed from its elements in their standard states, usually at 298 K (25°C).Also called heat of formation.
38Also called heat of formation. ΔHf° = 0 for any element in its standard state (the natural elemental form at 1 atm or 1 M) at 298 K.
39C (graphite,s) + O2 (g) CO2 (g) ΔHrxn °= 0 kJ/mol + 0 kJ/mol - 393 C (graphite,s) + O2 (g) CO2 (g) ΔHrxn °= 0 kJ/mol + 0 kJ/mol kJ/mol elements in their product standard state (one mole) negative sign heat released - exothermic rxn ΔHf ° (CO2 ) = kJ/mol
402H2(g) + O2 (g) 2H2O (g) ΔHrxn °= 0 kJ/mol + 0 kJ/mol - 483 2H2(g) + O2 (g) 2H2O (g) ΔHrxn °= 0 kJ/mol + 0 kJ/mol kJ/2mol elements in their product standard state (TWO mole) negative sign heat released - exothermic rxn Divide by 2 to get to per mole level ΔHf ° (H2O) = kJ/mol
41Hess's Law -- Adding Reactions The overall heat of reaction (ΔHrxn) is equal to the sum of the ΔHf (products) minus the sum of the ΔHf (reactants): ∆Horxn = ∑ΔHof (products) -Σ ΔHof (reactants)
42So, by knowing ΔHf of the reactants and products, we can determine the ΔHrxn for any reaction that involves these reactants and products.
432Mg(s) + CO2 (g) 2MgO(s) + C(s) CO2 is used in certain kinds of fire extinguishers to put out simple fires. It works by smothering the fire with "denser" CO2 that replaces oxygen needed to maintain a fire. CO2 is not good, however, for more exotic electrical and chemical fires.2Mg(s) + CO2 (g) 2MgO(s) + C(s)ΔH°f = 0 kJ/mol -393 kJ/mol kJ/mol + 0 kJ/mol ΔH°rxn = Σ (#mol)ΔH°f (products) - (#mol)ΔH°f(reactants)ΔH°rxn =Σ(2)(-602 kJ/mol)+(1)(0 kJ/mol) - Σ(2)(0kJ/mol) + (1)(-393 kJ/mol)
45Calculate ΔHrxn for the following reaction: You can also add two reactions together to get the ΔHrxn for another new reaction:Calculate ΔHrxn for the following reaction:C2H4 (g) + H2O(l) C2H5OH(l) ΔHrxn = ?? Given these two reactions and thermodynamic data:2CO2 (g) + 3 H2O(l) C2H5OH(l) + 3O2(g) ΔH°rxn = kJ/mol2CO2 (g) + 3 H2O(l) C2H5OH(l) + 3O2(g) ΔH°rxn = kJ/molC2H5OH(l) + 3O2(g) 2CO2 (g) + 3 H2O(l) ΔH°rxn = kJ/molC2H4(g) + 3O2(g) 2CO2 (g) + 2 H2O(l) ΔH°rxn = kJ/moC2H5OH is on the product side of the first reaction -- so we want to switch equation a) around to get C2H5OH on the product side:Note that when we reverse the reaction, ΔH°rxn changes sign!!!
46C2H4 (g) + H2O(l) C2H5OH(l) ΔHrxn = ?? Now we can add the two reactions together to give us the desired net reaction:C2H4 (g) + H2O(l) C2H5OH(l) ΔHrxn = ??12CO2 (g) + 3 H2O(l) C2H5OH(l) + 3O2(g) ΔH°rxn = kJ/molC2H4(g) + 3O2(g) 2CO2 (g) + 2 H2O(l) ΔH°rxn = kJ/molC2H4(g) + H2O(l) C2H5OH(l)ΔH°rxn = kJ/mol kJ/molΔH°rxn = - 44 kJ/mol46
47Calculate the standard enthalpy change, ΔHo, for the formation of 1 mol of strontium carbonate (the material that gives the red color in fireworks) from its elements.Sr (s) + C (graphite) + 3/2 O2 (g) SrCO3 (s)The information available is:Sr (s) + 1/2 O2 (g) SrO (s) ∆Ho = -592 kJSrO (s) + CO2 (g) SrCO3 (s) ∆Ho = -234 kJC (graphite) + O2 CO2 (g) ∆Ho = -394 kJ∆Ho = kJ
482C(s) graphite + 2H2O (g) CH4 (g) + CO2 (g) The combination of cake and steam produces a mixture called coal gas, which can be used as a fuel or as a starting material for other reactions. The equation for coal gas is;2C(s) graphite + 2H2O (g) CH4 (g) + CO2 (g)Determine the standard enthalpy change for this reaction from the following informationC(s) graphite + 2H2O (g) 2H2 (g) + CO2 (g) ∆Ho = kJCO (g) + H2O (g) H2 (g) + CO2 (g) ∆Ho = kJCH4 (g) + H2O (g) 3H2 (g) + CO (g) ∆Ho = kJ∆Ho = 15.3 kJ
49If we have to multiply one (or more) of the reactions by some constant to get them to add correctly, then we also would have to multiply ΔHrxn for that reaction by the same amount.
51Calculate Enthalpy of Reaction from Bond Energies Because of the principle of conservation of energy the total energy before and after the reaction must not change. So, the energy of a reaction released or absorbed in the reaction must come from the difference in bond energies of the products and the reactants.
52Calculate Enthalpy of Reaction from Bond Energies ExampleThe bond energy (kJ) for H2, F2, and HF are 436, 158 and 568 kJ (assume one mole) respectively.Calculate the enthalpy (energy) of the reaction,H2(g) + F2(g) 2 HF
53H2(g) + F2(g) 2 HFSolution Based on the bond energies given, we haveH2 2H D = 436 kJ/molF2 2F D = 158 kJ/mol2H + 2F 2HF H = -568 kJ/mol * 2 = -1136Adding all three equations and energiesH2(g) + F2(g) = 2 HF ΔH = -542 kJ/equationNote that D represent bond dissociation energy, and H the enthalpy of the reaction as written. We use ΔH in the last equation to denote enthalpy of change of the overall reaction.