Chapter 6 - lecture 1

Chapter 6 - lecture 1 - General Chemistry I Fall 2007 Joann...

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Unformatted text preview: General Chemistry I Fall 2007 Joann S. Monko Chemistry 9th ed. Raymond Chang Di r ecti onal i ty of H eat Tr ansfer Heat always transfers from a hotter object to a cooler one. EXOthermic: heat transfers from SYSTEM to SURROUNDI NGS. T(system) goes down T(surr) goes up Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Di r ecti onal i ty of H eat Tr ansfer Heat always transfers from a hotter object to a cooler one. ENDOthermic: heat transfers from SURROUNDI NGS to the SYSTEM . T(system) goes up T (surr) goes down Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Ener gy & Chemi str y All of thermodynamics depends on the law of CONSERVATI ON OF ENERGY. Total energy is unchanged in a chemical rxtn. If PE of products is less than reactants, the difference is released as KE ... you feel a temperature increase. PE Reactants Kinetic Energy Products Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. UNI TS OF ENERGY 1 calorie = heat required to raise temp. of 1.00 g of H2O by 1.0 oC. 1000 cal = 1 kilocalorie = 1 kcal 1 kcal = 1 Calorie (a food "calorie") But we use the unit called the JOULE or KILOJOULE 1 J = 1 kg m2/s2 1 cal = 4.184 joules Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. James Joule 1818-1889 Speci fi c H eat Capaci ty How much energy is transferred due to T difference? The heat (q) "lost" or "gained" is related to a) b) c) sample mass change in T and specific heat capacity, SH Specific heat capacity = heat lost or gained by substance (J) (mass, g)(T change, K) SH = q . m T q = m SH T Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. T = Tfinal - Tinitial Speci fi c H eat Capaci ty Substance Spec. Heat (J/gK) H2O 4.184 Ethylene glycol 2.39 Al 0.897 glass 0.84 Aluminum Which has the larger heat capacity? Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Speci fi c H eat Capaci ty If 25.0 g of Al cool from 310 oC to 37 oC, how many joules of heat energy are lost by the Al? heat gain/lose = q = (SH)(mass)(T) where T = Tfinal - Tinitial q = (0.897 J/gK)(25.0 g)(310 - 583)K q = - 6120 J Notice -the negative sign on q signals heat "lost by" or transferred OUT of Al. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Cl i cker Questi on A one square mile lake is approx. 10 m deep = 2.6 x 1013 grams. What quantity of heat (in kJ) must be transferred to the lake water to raise the temperature by 1 C? The specific heat of water is 4.184 J/gK) Reading the Signs T = Tfinal - Tinitial q = m SH T T of System Sign Sign Direction of Process of of q Heat Transfer T + + From surroundings Endothermic to system From system to Exothermic surroundings Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Increase Decrease - - A 15.5 g piece of chromium, heated to 100C, is dropped into 55.5 g of water at 16.5C. The final temp. of the metal and water is 18.9C. What is the specific heat capacity of chromium? (Assume no heat is lost to the container or to the surrounding air.) The sum of thermal energy changes is zero. qwater and qmetal are = but opposite signs qwater = -qmetal qwater + qmetal = 0 (SH m T)water + (SH m T)metal = 0 (4.184 J/gK)(55.5 g)(18.9-16.5)K + SH(15.5 g)(18.9-100)K = 0 (4.184 J/gK)(55.5 g)(2.4K) + SH(15.5 g)(-81.1K) = 0 557 J + SH(-1257gK) = 0 557 J = SH(1257gK) Kotz &Treichel Chemistry & Chemical Reactivity SH of the metal = 0.443 J/gK 5th ed. Clicker Question Determine the specific heat capacity of a piece of metal A 55.0 g sample was heated to 99.8 C and then put into 225 g of water at 21 C. The final temperature of the water and metal was 23.1 C. (Answer to 3 decimal places) ENTH AL PY Most chemical reactions occur at constant P, so Heat transferred at constant P = qp qp = H where H = enthalpy and so E = H + w (and w is usually small) H = heat transferred at constant P E H = change in heat content of the system H = Hfinal - Hinitial If Hfinal > Hinitial Then H is + Process is Endothermic If Hfinal < Hinitial Then H is Process is Exothermic Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. USI NG ENTH AL PY Consider the formation of water H2(g) + 1/2 O2(g) --> H2O(g) + 241.8 kJ Exothermic reaction = heat is a "product" H = 241.8 kJ Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. USI NG ENTH AL PY Making liquid H2O from H2 + O2 involves two exothermic steps. H2 + O2 gas H2O vapor Liquid H2O Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. USI NG ENTH AL PY Making H2O from H2 involves two steps. H2 (g) + 1/2 O2 (g) ---> H2O (g) + 242 kJ H2O (g) ---> H2O (l) + 44 kJ --------------------------------------------------------H2 (g) + 1/2 O2 (g) --> H2O (l) + 286 kJ Example of H ESS'S L AW: I f a r xn. i s the sum of 2 or mor e other s, the net H i s the sum of the H 's of the other r xns. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. H ess's L aw & Ener gy L evel Di agr ams Forming H2O can occur in a single step or in a two steps. Htotal is the same no matter which path is followed. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. H ess's L aw & Ener gy L evel Di agr ams Forming CO2 can occur in a single step or in a two steps. Htotal is the same no matter which path is followed. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. H along one path = H along another path This equation is valid because H is a STATE FUNCTI ON These depend only on the state of the system and not how it got there. V, T, P, energy -- and your bank account! Unlike V, T, and P, one cannot measure absolute H. Can only measure H. Chang Chemistry, 8th ed. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Standard Enthalpy Values Most H values are labeled Ho Measured under standar d condi ti ons P = 0.9869 atm (1 bar) Concentration = 1 mol/L T = usually 25 oC with all species in standard states: e.g., C = graphite and O2 = gas Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Enthal py Val ues Depend on how the reaction is written and on phases of reactants and products. 2 2 2 H (g) + 1/2 O (g) --> H O(g) H = -242 kJ 2 H2(g) + O2(g) --> 2 H2O(g) H = -484 kJ Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Revi ewi ng H ess's L aw I f a r xn. i s the sum of 2 or mor e other s, the net H i s the sum of the H 's of the other r xns. 4 Fe (s) + 3 O2 (g) 2 Fe2O3 (s) H = ? 2 Fe (s) + 6 H2O (l) 2 Fe(OH)3 (s) + 3 H2 (g) 2 H2 (g) + O2 (g) 2 H2O (l) kJ Fe2O3 (s) + 3 H2O (l) 2 Fe(OH)3 (s) 321.8 kJ - 571.7 288.6 kJ Find the H for the reaction: 2 NH3 (g) + 3 N2O (g) 4 N2 (g) + 3 H2O (l) H (kJ) 4 NH3 (g) + 3 O2 (g) 2 N2 (g) + 6 H2O (l) N2O (g) + H2 (g) N2 (g) + H2O (l) H2 (g) + O2 (g) H2O (l) -1531 -367.4 -285.9 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. CAL ORI M ETRY M easur i ng H eats of Reacti on Constant Volume "Bomb" Calorimeter Burn combustible sample. Measure heat evolved in a reaction. Derive E for reaction. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Cal or i metr y qrxtn = -(qwater + qbomb) Some heat from reaction warms water qwater = (SH)(water mass)(T) Some heat from reaction warms "bomb" qbomb = heat capacity (J/K) T Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. M easur i ng H eats of Reacti on CAL ORI M ETRY Calculate heat of combustion of octane. C8H18 + 25/2 O2 --> 8 CO2 + 9 H2O Burn 1.00 g of octane Temp rises from 25.00 to 33.20 oC T = (33.20 25.00) = 8.20 K Calorimeter contains 1200 g water Heat capacity of bomb = 837 J/K Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Step 1 Calc. heat transferred from reaction to water. qwater = SHm T qwater = (4.184 J/gK)(1200 g)(8.20 K) = 41,170 J Step 2 Calc. heat transferred from reaction to bomb. q = (bomb heat capacity)(T) = (837 J/K)(8.20 K) = 6860 J Step 3 Total heat evolved qrxtn + qbomb+ qwater = 0 qrxtn = - (qbomb + qwater) = - (6860 J + 41,170 J) Heat of combustion of 1.00 g of octane = - 48,030 J - 48.0 kJ Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Coffee Cup Calorimeter Constant Pressure H = qrxtn Need to know the Mass Specific Heat Constant Amount of reactants Measure a change in temp. qrxtn = - (qsoln' + qcal) qsoln' = mSHT Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Cal cul ate the enthal py change for di ssol vi ng NH 4NO3 (s) i n water i n kJ / mol . Addi ng 5.44 g of NH 4NO3 (s) to 150.0g of water i n a coffee cup cal or i meter (wi th sti r r i ng to di ssol ve the sal t) r esul ted i n a temper atur e decr ease fr om 18.6 C to 16.2 C. The sol uti on has a speci fi c heat of 4.2 J / gK. The cal or i meter constant i s 76.5 J / K. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. ...
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