Presentation on theme: "Thermochemistry First Law of Thermodynamics"— Presentation transcript:
1Thermochemistry First Law of Thermodynamics System, surrounding, and thermodynamic universeHeat (q), work (w) and internal energy (E)Calculation of heat gained or lost by systemState FunctionsEnthalpy of reactionsCalorimetry; determination of enthalpy of reactionsHess’s law of enthalpy summation
2Energy, Work, Heat & Enthalpy Energy – the capacity to do work or to produce heatPotential Energy:energy associated with the relative position of a substance in a force field, such as gravitational attraction, chemical bonds, electrostatic, nuclear force, etc., or on the chemical compositionKinetic Energy:energy associated with the translational motion of an object. Kinetic energy depends on the mass and speed of the object. EK = ½mv2
3Energy, Work, Heat & Enthalpy energy associated with temperature change – heat flows from a hot object to a cold one.Heat gained or lost: q = m.s.Dtwhere m = mass; s = specific heat capacity of the substance, and Dt = change in temperatureEnthalpy change (DH):Heat gained or lost during a chemical reaction at constant pressure.
4First Law of Thermodynamics Energy is not created nor destroyed during chemical or physical processesThe change in the internal energy of a system (DE) depends only on the amount of heat (q) gained or lost by the system and the work (w) done on or by the system.DE = q + wFor processes that involve gas expansion or compression, w = -pDVDE = q – pDV; q = DE + pDV
6Heat Capacity of Calorimeter To determine the heat capacity of coffee cup calorimeter:A 25.0-g sample of warm water at 40.0oC was added to a 25.0-g sample of water in a Styrofoam coffee cup calorimeter initially at 20.0oC. The final temperature of the mixed water and calorimeter was 29.5oC and the specific heat capacity of water is J/g.oC. Calculate the heat capacity, Ch, of the calorimeter.
7Specific Heat Capacity To determine the specific heat capacity of a metal using coffee cup calorimeter:A 55.0-g sample of hot metal initially at 99.5oC was added to 40.0 g of water in a Styrofoam coffee cup calorimeter. The water and calorimeter were initially at 21.0oC. If the final temperature of mixture was 30.5oC, calculate the total heat lost by metal and the specific heat capacity of the metal. The specific heat of water is J/(g.oC) and heat capacity of calorimeter is 10.0 J/oC.
8Heat of Neutralization To determine the molar enthalpy of acid-base reaction using coffee cup calorimeter.50.0 mL of 2.0 M HCl was reacted with 50.0 mL of 2.0 M NaOH in a coffee cup calorimeter. The reaction was exothermic, which caused the temperature of the solution to increase from 22.0oC to 35.6oC. Assume the density of solution as 1.0 g/mL, its specific heat capacity as 4.18 J/g.oC, and the heat capacity of calorimeter as 10.J/oC. Calculate the total amount of heat produced by the reaction. Calculate the enthalpy change (DH, in kJ/mol) for the following reaction:HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
9Calculation the enthalpy of reaction using Styrofoam cup calorimeter To determine the enthalpy of reaction using coffee cup calorimeter.Suppose 100. mL of 1.0 M HCl solution is placed in a Styrofoam coffee cup calorimeter. The initial temperature of HCl solution is 22.5oC. A g sample of magnesium ribbon is cut to short pieces and added to the acid solution in which the following exothermic reaction occurred.Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)The heat produced by the above reaction is completely absorbed by the solution and calorimeter, which attained the highest temperature of 34.2oC. Assume the acid solution has a density of 1.0 g/mL and its specific heat capacity as 4.0 J/g.oC, and the calorimeter has a heat capacity of 10. J/oC. Calculate the molar enthalpy change (DH, in kJ/mol) for the above reaction.
10Calculating the enthalpy of reaction using Styrofoam cup calorimeter Reaction: Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g);Calculations:
12Heat Capacity of Calorimeter To determine the heat capacity of bomb calorimeter:When a g sample of glucose, C6H12O6, was completely combusted in a bomb calorimeter, the temperature of the calorimeter assembly increased by 4.48oC. If the combustion of glucose produces 14.0 kJ/g of energy, how much heat energy is absorbed by the calorimeter. Calculate the heat capacity, Ch, of the calorimeter. (Assume that all of the heat produced by the combustion of glucose is absorbed by the calorimeter.)
13Enthalpy changes of exothermic and endothermic reactions
14Heat of CombustionTo calculate the enthalpy of combustion using bomb calorimeterWhen a g sample of sucrose (cane sugar) is completely combusted in a bomb calorimeter, the temperature of the calorimeter was increased by 4.50oC. If the heat capacity of calorimeter is 3.75 kJ/oC, how much heat was absorbed by the calorimeter? Calculate the molar enthalpy of combustion of sucrose according to the following equation:C12H22O11(s) O2(g) 12CO2(g) + 11H2O(l)
15Hess’s Law of Enthalpy of Reactions According to Hess’s law:The net enthalpy change of a given process is independent of the number of steps taken to complete the process.If a reaction can be broken down into several steps, then the overall enthalpy for the reaction is equal to the sum of enthalpies of individual steps.
16Hess’s Law of Enthalpy of Summation According to Hess’s law:For a given reaction, the overall enthalpy change is equal to the difference between the algebraic sum of enthalpies of formation of products and the algebraic sum of enthalpies of formation of reactants.DHrxn = S(npDHf[products]) – S(nrDHf[reactants])
17Enthalpy of ReactionsApplying Hess’s law to calculate enthalpy of reactions.Given:C(s) + O2(g) CO2(g); DHo = -394 kJ (1)CO(g) + ½O2(g) CO2(g); DHo = -283 kJ (2)Calculate DH for the following reaction:C(s) + ½O2(g) CO(g)
22Heat of Combustion Reaction To calculate heat of reaction using molar enthalpy of formation:Given: DHf[C3H8(g)] = -104 kJ/mol;DHf[H2O(g)] = -242 kJ/mol, andDHf[CO2(g)] = -394 kJ/mol;Calculate DH for the following combustion of C3H8(g):C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)
26Reactions in Aqueous Solution Enthalpy change for reactions in aqueous solution:Standard condition for gas: P = 1 atm at 25oCStandard condition for solution: 1 M at 25oCUnder standard condition: DHf[H3O+(aq)] = 0.0 kJGiven:DHf[Mg2+(aq)] = -467 kJ/mol;DHf[OH-(aq)] = -230 kJ/mol, and DHf[Mg(OH)2(s)] = -925 kJ/mol,Calculate DH for the reaction:MgCl2(aq) + 2NaOH(aq) Mg(OH)2(s) + 2NaCl(aq)
27Enthalpy of Ionic Reactions Given the following enthalpy of formation (in kJ/mol):DHf[Ba2+(aq)] = ; DHf[CO32-(aq)] = ;DHf[BaCO3(s)] = -1219; DHf[BaSO4(s)] = -1465;DHf[OH-(aq)] = -230; DHf[H2SO4(aq)] = andDHf[H2O(l)] = kJ/molCalculate enthalpy changes for the following reactions in aqueous solution:(1) BaCl2(aq) + Na2CO3(aq) BaCO3(s) + 2NaCl(aq)(2) HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)(3) Ba(OH)2(aq) + H2SO4(aq) BaSO4(s) + 2H2O(l)
28Global Energy Resources Biomass (mainly wood) – major sources of energy in many under-developed countries;Coal was once the major source of energy in U.S.A. and industrialized European countries;Petroleum replaces coal in the middle of 20th Century as the major source of energy for power plants and transportation;Hydroelectric power and nuclear energy are used in certain developed countries. Geothermal energy is used as a secondary source of energySolar energy is a secondary source of energy, but mainly for household heating.
29Comparison of Enthalpy of Combustion ————————————————————————Substances Energy (kJ/g fuel)Hydrogen gas (H2) 120Natural gas (CH4) 50GasolineCrude petroleum 43Animal fatCoalCharcoalEthanolMethanolPaperDry biomass (wood)
30Petroleum and Natural Gas Origin of petroleum and natural gas:most likely from fossilized remains of marine organisms that lived approximately 500 millions years agoPetroleum –thick, dark liquid composed of mixture of hydrocarbonsComposition varies from one location to another, but mostly hydrocarbon compounds containing C5 to > C25Natural GasConsists mostly methane (CH4, >90%) and some ethane (C2H6), propane (C3H8) and butane (C4H10)
31Petroleum Refining Petroleum refining Fractional distillation of crude petroleum yields the following fractions:Gasoline (C5 – C10);Kerosene & jet fuel (C10 – C18);Diesel fuel, heating, and lubricating oil (C15 – C25),Asphalt (>C25)More gasoline is produced by pyrolytic (high temperature) cracking of larger HC compounds (> C25)
32CoalFormed from fossilized plant remains that have been subjected to high temperature and pressure for many millions yearsCoal matures through 4 stages:Lignite, subbituminous, bituminous, and anthracite;Composition by mass%:Lignite: 71% C, 4% H, 23% O, 1% N, and 1% S;Subbituminous: 77% C, 5% H, 16% O, 1% N, and 1% S;Bituminous: 80% C, 6% H, 8% O, 1% N, and 5% S;Anthracite: 92% C, 3% H, 3% O, 1% N, and 1% S.The relative carbon content increases and those of hydrogen and oxygen decrease as coal matures.
33Coal as Energy SourceCoal furnishes about 23% of energy needs in U.S.A.Underground coal mining is dangerousStrip mining destroys lands and the environmentsCoal contains sulfur and burning coal causes severe air pollution due to:Air particulate matters, CO2, CO, and SO2 (from sulfur in coal)In atmosphere, SO2 is oxidized to SO3, which yields acid rain (SO3 forms H2SO4 when mixed with rain water)
34Processing Coal Coal gasification - converting coal into gaseous fuel Treating coal with air and steam at high temperature produces mixture of CO, H2, and CH4. Some CO2 and SO2 are also formedMixture of CO and H2 is also called syngasReactions in coal gasification:C(s) + O2(g) CO2(g); DH = -394 kJC(s) + ½O2(g) CO(g); DH = -111 kJC(s) + H2O(g) CO(g) + H2(g); DH = 131 kJC(s) + 2H2(g) CH4(g); DH = -75 kJNote: both exothermic and endothermic reactions occur. An energy balance can be maintained by controlling the temperature, the rate of coal feed, and the flow of air and steam.
35Coal Gasification Process Coal + Steam + AirHeatCH4 + CO, CO2, H2, H2O(+ sulfur containing impurities)SeparateTreatmentCO + H2O CO2 + H2CO + 3H2 CH4 + H2OAfter removal of CO2 and H2O, the remaining mixture contains CH4 and syngas (CO + H2)
36Coal to Coal SlurryCoal is pulverized and mixed with water to form a thick slurryCoal slurry burns like residual oil, with less CO and SO2 produced
37Hydrogen Fuel Combustion of H2: H (g) + ½O2(g) H2O(l); DH = -286 kJ H2(g) + ½O2(g) H2O(g); DH = -242 kJCombustion of H2 produces 2.5 times more energy per gram than natural gasCombustion of H2 only produces water.However, production, storage and transportation of the gas pose major problems
38Problems in Hydrogen Production H2 does not exist in the free form (like N2 or O2)All existing methods to produce H2 gas involve endothermic reactions or high temperature:Steam reformation:CH4(g) + H2O(g) CO(g) + 3H2(g); DH = 206 kJElectrolysis: H2O(l) H2(g) + ½O2(g); DH = 286 kJThermochemical decomposition of H2O:2HI(g) H2(g) + I2(s); (425oC)2H2O(l) + SO2(g) + I2(s) H2SO4(aq) + 2HI(aq); (90oC)H2SO4(aq) SO2(g) + H2O(l) + ½O2(g); (825oC)Net Reaction: H2O(l) H2(g) + ½O2(g);
39Problems in Storage & Transportation of H2 H2 decomposes to H-atoms on metal surfaceH-atoms are small enough to be absorbed into the metal lattices and makes the metal to become brittleH2 requires a much larger container for storage and transportationStorage or transportation of H2 under high pressure (say as liquefied gas) poses high explosion hazardEnergy produced per unit volume by H2 is only one-third that produced by natural gas under similar conditionsAn alternative method suggested for H2 storage/transportation is to convert it into solid metal hydrides, MH2.
40Alternative Fuels Hydrogen gas is most efficient and clean fuel It produces the most energy per gram,Transportation and storage are difficultIt requires a much large fuel tank than gasoline;May be compressed into liquid form, but poses explosion hazardAn alternative way is to convert it into a solid metal hydride (MH2) – one that is able to release H2 when heated.Most likely the hydrogen will be used to power fuel cells that will be installed in automobiles
41Alternative Fuels Oil shale Shale rocks contain complex HC called kerogenHuge deposits in western states, especially Colorado;Kerogen is not fluid like petroleum - cannot be pumped;Rocks containing fuel must be heated to >250oC to decompose the kerogen into smaller HC molecules;Process produces large quantities of waste rocks – a negative environmental impact.
42Alternative Fuels Ethanol: Produced by fermentation of sugar (from sugar cane or corn)Pure ethanol produces about 27 kJ/g of energyUse as fuel supplementAdded to gasoline as gasohol (which contains ca. 10% ethanol)Pure ethanol not suitable for motor fuel in USA, especially in winter, because it does not vaporize easily in cold climate.Pure ethanol is widely used as motor fuel in Brazil where the climate is warmer.
43Alternative Fuels Methanol: Methanol produces about 20 kJ/g of energy when completely combustedIt has been used in race cars as a mixture of 85% methanol and 15% gasoline.California is evaluating methanol as motor fuelArizona and Colorado are considering similar step.