Chapter 5 - Thermochemistry

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1 Chapter 5 - Thermochemistry Study of energy changes that accompany chemical rx s. I) Nature of Energy Energy / Capacity to do work Mechanical Work w = F x d Heat energy - energy used to cause the temperature of an object to inc. 1

2 A) Units of Energy w = F x d = ( m x a) x d = (kg m/s 2 ) m \ = (kg C m 2 )/ s 2 = N m = joule, J (SI unit) calorie (cal) original def: amt. of energy reg. to raise temp. of 1g of water by 1 C, from 14.5 C to 15.5 C 1 cal = J Cal - nutritional calorie 1 kcal 2

3 B) Kinetic & Potential Energy 1) Kinetic Energy KE = ½ m v 2 Energy due to motion SI units: Energy = kg (m/s) 2 = J 3

4 2) Potential Energy Energy stored in an object by virtue of its position or composition Chemical energy is due to composition of substances Electrostatic P.E. Interaction between charged particles E el = 6 Q 1 Q 2 d Q = charge d = distance between particles 4

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6 C) System and Surroundings System = portion we single out for study - focus attention on changes which occur w/in definite boundaries Surroundings = everything else System : Contents of rx. flask Surround. : Flask & everything outside it Agueous soln. rx : System : dissolved ions & molecules Surround : H 2 O that forms the soln. 6

7 II) First Law of Thermodynamics Law of Conservation of Energy : Energy can be neither created nor destroyed but may be converted from one form to another. Energy lost = Energy gained by system by surroundings A) Internal Energy, E E = total energy of the system Actual value of E cannot be determined 7

8 )E, change in energy, can be determined ) = final state! initial state )E / E f! E i Sign of )E is important E f > E i, )E > 0 system gained energy E f < E i, )E < 0 system lost energy Systems tend to go to lower energy state - more stable products i.e. rx s in which )E < 0 8

9 B) Thermodynamic State & State Functions Thermodynamic State of a System defined by completely specifying ALL properites of the system - P, V, T, composition, physical st. 1) State Function prop. of a system determined by specifying its state. depends only on its present conditions & NOT how it got there )E = E final! E initial independent of path taken to carry out the change - Also is an extensive prop. 9

10 C) Relating )E to Heat & Work 2 types of energy exchanges occur between system & surroundings Heat & Work + q : heat absorbed, endothermic! q : heat evolved, exothermic + w : work done on the system! w : work done by the system 1) First Law )E = q + w 10

11 1) Exothermic Reactions heat is released 2 H 2 (g) + O 2 (g) 6 2 H 2 O(R) + heat 11

12 2) Endothermic Reactions heat is absorbed - reaction requires input of energy 2 H 2 O(R) + heat 6 2 H 2 (g) + O 2 (g) 12

13 III) Enthalpy In ordinary chem. rx., work generally arises as a result of pressure-volume changes Inc. vol. & system does work against pressure of the atmosphere has dimensions of work : = (F/A)V = m/ s m 2 ) m 3 = m 2 )/(s 2 ) = J Constant Pressure w =! P )V Negative because work done by system 13

14 )E = q! P )V A) )E at Constant Volume )E = q v B) )E at Constant Pressure : )E = q p! P )V q p = )E + P)V 14

15 C) Enthalpy, H H = E + PV Change in enthalpy at constant P is: )H = )E + P )V & )H = q p Can think of as heat content state fnc. & is extensive 15

16 IV) Enthalpies of Reaction )H rxn = H products! H reactants A) Exothermic Rx s H p < H r, )H rxn < O, exothermic Heat is evolved 2 H 2 (g) + O 2 (g) 6 2 H 2 O(R) )H =!572 kj Thermochemical eqn. Physical states are given and energy associated w. rx. written to right - MUST give physical states If product is H 2 O(g), )H =!484kJ 16

17 )H corresponds to molar quantities given in eqn. as written H 2 (g) + ½ O 2 (g) 6 H 2 O(R) )H =!286 kj B) Endothermic Rx s H p > H r, )H rxn > O, endothermic Heat is absorbed 2 H 2 O(R) 6 2 H 2 (g) + O 2 (g) )H = +572kJ Reverse of previous rx. 17

18 Enthalpy Diagram 2 H 2 (g) + O 2 (g) +572 kj!572 kj 2 H 2 O (R) 18

19 C) Guidelines 1) Enthalpy is extensive Multiply a rxn by some factor the )H is multiplied by that factor 2) )H reverse =! )H forward 3) Enthalpy is a state function )H depends on the states of reactants and products. 19

20 D) Determining )H for a Rx. Convenient sample sizes are reacted & conv. factors are used to obtain the heat energy 1) Ex 1: When 36.0g of Al reacts w. excess Fe 2 O 3 how much heat is released? 2 Al(s) + Fe 2 O 3 (s) 2 Fe(s) + Al 2 O 3 (s) )H rxn =!847 kj 1 mol Al 847 kj? kj = 36.0 g Al x x = kj = 565 kj g Al 2 mol Al 20

21 VII) Calorimetry Exp. method of obtaining )H & )E Heat evolved or absorbed by system will be reflected in the surroundings. Need surr. Heat Capacity, C C = q )T Quantity of heat required to raise the temp. of an object by 1 C Unit: (J/ C) or (J/K) 21

22 C m - molar heat capacity heat capacity per mole, J/molC C C s - specific heat heat capacity per gram, J/gC C C s of H 2 O = J/gC C q = C )T q = n C m )T q = m C s )T q gained =! q lost Calorimeter )H (q p ) Bomb Calorimeter )E (q v ) 22

23 23

24 A) Ex 1: What amt. of heat has been absorbed by kg of water if its temp. inc. from C to C? q = m C s )T = ( g) (4.184 J/gC C)(22.73!18.22) = 18, J = 18.9 kj (3 s.f.) C 24

25 B) Ex 2: A g sample of graphite is placed in a bomb calorimeter & ignited in the presence of excess O 2 at C & 1 atm. The temp. of the calorimeter rises to C. The heat capacity of the calorimeter & contents is 20.7 kj/ C. What is )H at C and 1 atm? C(graphite) + O 2 (g) 6 CO 2 (g) q (lost by rxn) =! q (gained by calor. & contents) q rxn =! C cal )T =! (20.7 kj/ C)(25.89 C C) =! 18.4 kj (q v or )E for g) 25

26 Want kj/mol, kj! 18.4 kj 12.0 g C? = x mol g C 1 mol C =! 3.93 x 10 2 kj/mol ΔE =! 3.9 x 10 2 kj/mol since no change in moles of gas ΔH = ΔE 26

27 VI) Hess s Law )H is a state fnc. Same whether the process occurs as a single step or as a series of steps The )H rxn is the sum of the )H s for the individual steps. )H rx = G )H steps Steps * Add chem. eqn s for steps to get overall rxn. * Add )H steps )H rxn 27

28 A) Ex 1 : What is )H for Ca(OH) 2 (s) + SO 3 (g) 6 CaSO 4 (s) + H 2 O(g) We know the following: Ca(OH) 2 (s) 6 CaO (s) + H 2 O(g) CaO(s) + SO 3 (g) 6 CaSO 4 (s) )H 1 = kj )H 2 =! 401 kj Ca(OH) 2 (s) + CaO(s) + SO 3 (g) 6 )H rxn =!292kJ CaSO 4 (s) + CaO(s) + H 2 O(g) )H rxn = )H 1 + )H 2 28

29 B) Ex 2 :Want )H for rxn. C 2 H 2 (g) + 5N 2 O(g) 6 2CO 2 (g) + H 2 O(g) + 5N 2 (g) Have: 2 C 2 H 2 (g) + 5 O 2 (g) 6 4 CO 2 (g) + 2 H 2 O(g) )H 1a =!2512 kj N 2 (g) + ½ O 2 (g) 6 N 2 O(g) )H 2a = kj Adjust eqn s so they are in proper amt s and the correct directions so they add up to the desired eqn. ALL substances NOT appearing in desired eqn. MUST cancel. 29

30 Divide eqn. 1a by 2 (also )H ) C 2 H 2 (g) + 5/2 O 2 (g) 6 2 CO 2 (g) + H 2 O(g) )H 1b =! 1256 kj Reverse eqn. 2a and multiply by 5 5 N 2 O(g) 6 5 N 2 (g) + 5/2 O 2 (g) )H 2b =! 408 kj Add & Cancel C 2 H 2 (g) + 5N 2 O(g) 6 2CO 2 (g) + H 2 O(g) + 5N 2 (g) )H rxn = )H 1b + )H 2b =! 1256 kj +! 408 kj =! 1664 kj 30

31 C) Note: In using Hess s Law: 1) If an eqn. is multiplied by a factor, )H is multiplied by the same factor. 2) If an eqn. is reversed, sign of )H changes 3) All substances NOT appearing in desired eqn. MUST cancel 31

32 VII) Enthalpy of Formation Enthalpy change for the formation of a compound from its elements )H f A) Standard enthalpy change Enthalpy change when all reactants and and products are in their standard states )H 32

33 1) Standard State Most stable state of a substance in its pure form under standard pressure (1 atm) & some specified temp. of interest (usually 25 C) 2) Thermochemical Standard States A) solid or liquid Pure substance at 1 atm b) gas pressure of 1 atm c) species in solution Conc. of 1 M 33

34 B) Standard Enthalpy of Formation )H for the rxn in which 1 mole of a cmpd. is formed from its elements with ALL substances in their standard states (in kj/mol) )H f Note: )H f = 0 for an element in its standard state )H f 1/2 N 2 (g) + 3/2 H 2 (g) 6 NH 3 (g)! 46.2 Na(s) + 1/2 Cl 2 (g) 6 NaCl(s)! C(graphite) 6 C(diamond)

35 35

36 C) Determine )H rxn from )H f )H rxn = 3 n )H f! 3 m )H f prod. react. n = coef. in bal. eqn. for each product m = coef. in bal. eqn. for each reactant 36

37 1) Ex1 : Find )H rxn for the following rx. using Hess s Law and )H f. 2 H 2 S(g) + 3 O 2 (g) 6 2 H 2 O(R) + 2 SO 2 (g) )H (kj/mol) (a) H 2 (g) + S(s) 6 H 2 S(g)! 20.2 (b) H 2 (g) + ½ O 2 (g) 6 H 2 O(R)! (c) S(s) + O 2 (g) 6 SO 2 (g)! Need to: Rev. eqn. (a) and 2 Add eqn. (b) 2 Add eqn. (c) 2 37

38 2H 2 S(g) 6 2H 2 (g) + 2S(s) )H rxn =!2C(!20.2) = kj 2H 2 (g) + O 2 (g) 6 2H 2 O (R) )H rxn = 2(!285.8) =! kj 2S(s) + 2O 2 (g) 6 2SO 2 (g) )H rxn = 2(!296.9) =! kj 2H 2 S + 2H 2 + 2S +3O 2 6 2H 2 + 2S + 2H 2 O + 2SO 2 2H 2 S + 3O 2 6 2H 2 O + 2SO 2 )H rx = (+ 40.4) + (!571.6) + (!593.8) =! 1125 kj 38

39 a) Use )H f instead )H rxn = [ 2 )H f (H 2 O) + 2 )H f (SO )] 2! [ 2 )H f (H 2 S) + 3 )H f (O 2 )] = [2 (!285.8) + 2 (!296.8)]! [2 (!20) + 3 (0)] =! 1125 kj 39

40 2) Ex 2: Useful when considering organic cmpds. for which )H f can not be determined directly. What is )H f for benzene? 6 C(s) + 3 H 2 (g) 6 C 6 H 6 (R) )H f =? This rx. does not happen. Use of exp. heat of combustion C 6 H 6 (R) + 15/2 O 2 (g) 6 6 CO 2 (g) + 3 H 2 O(R) )H rxn =! 3271 kj 40

41 )H rxn = [ 6 )H f (CO 2 ) + 3 )H f (H 2 O)]! [ )H f (C 6 H 6 ) + 15/2 )H f (O 2 )]!3271 kj = [ 6 (!393.5) + 3 (!285.8) ]! [ )H f (C 6 H 6 ) + 15/2 (0) ]!3271 kj = [! ]! [ )H f (C 6 H 6 ) ] )H f (C 6 H 6 ) = [! ]! (! 3271) = kj 41

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