5. Internal energy: The total energy with a system.

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1 CAPTER 6 TERMODYNAMICS Brie Summary o the chapter:. Thermodynamics: Science which deals with study o dierent orms o energy and quantitative relationship.. System & Surroundings: The part o universe or study is called system and remaining portion is surroundings. 3. State o system & state unction: State o system is described in terms o T, P, V etc. The property which depends only on state o system not upon path is called state unction eg. P, V, T, E,, S etc. 4. Extensive & Intensive Properties: Properties which depends on quantity o matter called extensive prop. eg. mass, volume, heat capacity, enthalpy, entropy etc. The properties which do not depends on matter present depends upon nature o substance called Intensive properties. eg. T,P, density, reractive index, viscosity, bp, p, mole raction etc. 5. Internal energy: The total energy with a system. i.e. U = E e + E n + E c + E p + E k U = U U or U P U R & U is state unction and extensive properly. I U > U energy is released. 6. eat (q): It I a orm o energy which is exchanged between system and surrounding due to dierence o temperature. Unit is Joule (J) or Calorie ( Calorie = 4.8 J). 7. First Law o Thermodynamics: It is law o conservation energy. Energy can neither be created not destroyed, it may be converted rom one rom into another. Mathematically U = q + w, w = p. V (work o expansion) U = q p. V or q = U + p. V, q,w are not state unction. 8

2 But U is state unction. 8. Enthalpy (): At constant volume V =, q v = U So = U + p. V, q p = = = U + P. V. 9. Relationship between q p, q v i.e. & U It is = U+ ng.rt or q p = q v + ng.rt. Exothermic and Endothermic reactions: = Ve or exothermic and = +Ve or endothermic reaction i.e. evolution and absorption o heat. Eg C+O N + O CO KJ, = KJ (exothermic) NO 8.7 KJ, = 8.7 KJ (Endothermic). Enthalpy o reaction ( r ): The amount o heat evolved or absorbed when the reaction is completed.. Standard Enthalpy o reaction ( r ) at bar pressure and speciic temp. (9K) i.e. standard state. 3. Dierent types o Enthalpies o reactions: (i) Enthalpy o combustion ( c ), (ii) Enthalpy o ormation ( ) (iii) Enthalpy o neutralization (iv) Enthalpy o solution (v) Enthalpy o atomization( a ), (vi)enthalpy o Ionisation ( i ) (vii) Enthalpy o ydration ( hyol. ) (viii) Enthalpy o usion ( us. ) (ix) Enthalpy o vaporization ( vap. ) (x)enthalpy o sublimation ( sub. ) ( sub. ) = us.() - vap() 4. ess s Law o constant heat summation: The total amount o heat change is same whether the reaction takes place in one step or in several steps. i.e. =

3 5. Bond enthalpy: It is amount o energy released when gaseous atoms combines to orm one mole o bonds between them or heat absorbed when one mole o bonds between them are broken to give ree gaseous atoms. Further r = B.E. (Reactants) - B.E. (Products) 6. Spontaneous & Non Spontaneous Processes: A process which can take place by itsel is called spontaneous process. A process which can neither take place by itsel or by initiation is called non Spontaneous. 7. Driving orces or spontaneous process: (i) Tendency or minimum energy state. (ii) Tendency or maximum randomness. 8. Entropy (S): It is measure o randomness or disorder o system. i.e. Gas>Liquid>Solid. q (rev.) Entropy change ( S) = J.K.mol T 9. Spontaneity in term o ( S) S (total) = S (universe) = S (system) + S (surrounding) I S (total) is +ve, the process is spontaneous. I S (total) is ve, the process is non spontaneous.. Second Law o thermodynamics: In any spontaneous process, the entropy o the universe always increases. A spontaneous process cannot be reversed.. Gibb s ree energy (G): deined as G = T.S & G = T. S (Gibb s elmholts equation) it is equal useul work i.e. - G = W (useul) = W (max.) I G = ve, process is spontaneous.. Eects o T on spontaneity o a process: G = T. S. (i) For endothermic process may be non spontaneous at law temp. 84

4 (ii) For exothermic process may be non spontaneous at high temp. and spontaneous at law temp. 3. Calculation o ( r G ) rg = G (p) - G (r) 4. Relationship between ( r G ) & equilibrium constant (k) G = G + RTlnQ & G =.33RT logk. 5. Calculation o entropy change: rs = S (p) - S (r) ONE MARK QUESTIONS:. State First Law o thermodynamics.. What is a thermodynamic state unction? 3. Give enthalpy () o all elements in their standard state. 4. From thermodynamic point to which system the animals and plants belong? 5. Predict the sign o S or the ollowing reactions. CaCO 3(s) + CO (g) heat CaO (s) + CO (g) 6. For the reaction Cl (g) Cl (g), What will be the sign o and S? 7. State ess s Law or constant heat summation? 8. What is Gibb s elmhaltz equation? 9. Deine extensive properties.. Give relationship between, U or a reaction in gaseous state. 85

5 ANSWERS FOR ONE MARK QUESTIONS. Energy can neither be created nor destroyed. The energy o an isolated system is constant. U = q + w.. A unction whose value is independent o path. eg. P, V, E, 3. In standard state enthalpies o all elements is zero. 4. Open system. 5. S is positive (entropy increases) 6. : ( ve) b/c energy is released in bond ormation and S: ( ve) b/c atoms combines to orm molecules. 7. The change o enthalpy o reaction remains same, whether the reaction is carried out in one step or several steps. 8. G = T. S 9. Properties which depends upon amount o substance called extensive properties. Volume, enthalpy, entropy.. = U + ng. RT. TWO MARKS QUESTIONS:- Q. In a process, 7J heat is absorbed and 394J work is done by system. What is change in Internal energy or process? Q. Given: N (g) + 3 (g) N 3(g), r = 9.4KJ.mol. What is the standard enthalpy o ormation o N 3(g). Q.3 Calculate entropy change in surroundings when. mol o O(l) is ormed under standard conditions? Given = 86KJmol. Q.4 Give relationship between entropy change and heat absorbed/evolved in a reversible reaction at temperature T. Q.5 What is spontaneous change? Give one example. 86

6 Q.6 A real crystal has more entropy than an Ideal Crystal. Why? Q.7 Under what condition, the heat evolved/absorbed in a reaction is equal to its ree energy change? Q.8 Predict the entropy change in- (i) (ii) A liquid crystallizes into solid Temperature o a crystallize solid raised rom OK to 5K Q.9 What is bond energy? Why is it called enthalpy o atomization? Q. Calculate entropy change or the ollowing process. O (s) O (l), is 6. KJ mol - at C. ANSWER FOR TWO MARKS QUESTIONS:. q = 7J, w = 394J, so U = q + w = = 37J N 3(g) = 46.KJ.mol 3. q (rev.) = = 86 KJmol - = 86Jmol - S q (rev.) T 86 J.mol 98K 959 J.K mol 4. S = q (rev) T 5. A process which can take place o its own or initiate under some condition. eg. Common salt dissolve in water o its own. 6. A real crystal has some disorder due to presence o deects in their structural arrangement, and Ideal crystal does not have any disorder. 7. In G = T. S, when reaction is carried out at OK or S =, then G =. 87

7 8. (i) Entropy decreases b/c molecules attain an ordered state. (ii) entropy increase b/c rom OK to 5K particles begin to move. 9. It is the amount o energy required to dissociate one mole o bonds present b/w atoms in gas phase. As molecules dissociates into atoms in gas phase so bond energy o diatomic molecules is called enthalpy o atomization.. O (s) O (l) at C, us = 6KJ mol - = 6J mol - T = C = + 73 = 73K Do uss = us 6 J.mol.98J.K mol T 73K TREE MARKS QUESTIONS: Q. For oxidation o iron, 4Fe (s) + 3O (g) Fe O 3(s) S is 549.4J.K - mol -, at 98K. Inspite o ve entropy change o this reaction, Why the reaction is spontaneous? ( r = 648x 3 J.mol - ) Q. Using the bond energy o r = 435 KJ mol -, Br = 9 KJ mol -, Br = 368 KJmol -. Calculate enthalpy change or the reaction (g) + Br (g) Br (g) Q.3 Enthalpies o ormation o CO (g), CO (g), N O (g) and N O 4(g) and, 393, 8 and 9.7 KJ mol - respectively. Find value r or the reaction N O 4(g) + 3CO (g) Q.4 For the reaction at 98K, A+B C, = 4 KJ mol -, S =. KJ mol - K -. At what temperature will the reaction become spontaneous, considering, S be constant at the temp. Q.5 The equilibrium constant or a reaction is. What will be the value o G? R = 8.34J.K - mol - T = 3K. Q.6 What do you understand by state unction? Neither q nor w is a state unction but q + w is a state unction? Explain. Q.7 Justiy the ollowing statements: 88

8 (i) (ii) An endothermic reaction is always thermodynamically spontaneous. The entropy always increases on going rom liquid to vapour state at any temperature T. Q.8 Calculate the temperature above which the reduction reaction becomes spontaneous: PbO (s) + C (s) Pb (s) + CO (g), given [ = 8.4 KJ mol -, S = 9J.K - mol - ]. Q.9 From the data given below at 98K or the reaction: C 4(g) + O (g) CO (g) + O (l) Calculate enthalpy o ormation o C 4(g) at 98K. Given:[ r = -89.5KJ, [ 393.5KJ.mol, (CO ) (O) 86.KJ.mol ] Q. For the reaction N 4 Cl (s) N 3(g) + Cl (g) at 5 C enthalpy change = 77KJ.mol - and S = 85J.K - mol -. Calculate ree energy change G at 5 C and predict whether the reaction is spontaneous or not. ANSWER TO TREE MARKS QUESTIONS: 3 r C 648 x J.mol ) Ans. (surr.) = 553 JK mol. T 98 K & S (system) = QJK - mol -. rs (total) = = J.K - mol - Since rs (total) is +ve, so the reaction is spontaneous. Ans. r = bond enthalpies (rect.) - bond enthalpies (prod.) = Br Br Br = [435 9] [ x 368] KJ mol - = = 9KJ. Mol - Ans. Ans.3 r = (prod.) - r (rect) = (N O) 3 (CO ) NO4 3 (CO) = [8 + 3( 393)] [9.7 + ( )] 89

9 = [8 + 79] [9.7 33] = 777.7KJ Ans.4 = 4 KJ mol -, S =. KJK - mol -. G = T. S O = 4. x T ( G = at equilibrium) 4 T =. K, so reaction will be spontaneous above K. Ans.5 rg =.33 RT logk =.33 x 8.34 x 3 x log = 9.47 x 3 x = 5744.J rg = 5.744KJ.mol - Ans.6 The property whose value depends upon state o system and is independent o path. q + w = U, which is a state unction as value o U does not depends upon path. Ans.7 (a) It is alse, exothermic reaction is not always spontaneous. I S = +ve and T. S>. The process will be non spontaneous even it. It is endothermic. (b) The entropy o vapour is more than that o liquid, so entropy increases during vaporization. Ans.8 G = T. S, at equilibrium G =, = T. S T = S 8.4x 9 J.K 3 J.mol mol K So the reaction will be spontaneous above 57.5K, as above this temperature G will be ve. Ans.9 r = (CO ) C 4(g) (O ) 89.5KJ = 393.5KJ + v 86 (C 4 ) O = (C 4 ) = 75. KJ.mol -. = 75.KJ. mol (C ) 4. Ans. = 77 KJ mol -, S = 85 JK - mol - 98x85 G = T. S = 77KJ KJ = 77 KJ KJ = 9.7 KJ.mol -. Since G is +ve, so the reaction is non spontaneous. 9

10 FIVE MARKS QUESTIONS:- Q. What is entropy? Why is the entropy o a substance taken as zero at K? Calculate the rg or the reaction? N (g) + 3 (g) N 3(g) at 98K The value o equilibrium constant (K) is 6.6x 5, R = 8.34JK - mol -. Ans: It is measure o randomness or disorder o system. Because at O K there is complete order in the system. G =.33 RT logk =.33 x 8.34 x 98 x log6.6 x 5 = [log6.6 + log 5 ] = 575.8[ ] = J = J G = 33.5 KJ mol -. Q. (i) What are extensive property and intensive properties? (ii)calculate the value o equilibrium constant (K) at 4K or NOCl (g) NO (g) + Cl (g). = 77.KJ.mol -, S = J.K - mol - at 4K, R = 8.34 J.K - mol -. Ans. (i) An extensive property is a property whose value depends on the quantity or size o matter present in the system Those properties which do not depend on the quantity or size o matter present are known as intensive properties (ii) = T. S 4x = 77.KJ - KJ.mol = = 8.4 KJ mol - and G =.33 RT logk. 84 =.33 x 8.34 x 4 log K. logk = 7..33x K 4 K antilog ( 4.99 ).95x Ans. 9

11 Q.3 Deine standard enthalpy o ormation. Calculate the enthalpy o ormation o benzene rom data C 6 6(l) 5 O (g) 6CO (g) 3 O (l), c 366.KJ C (s) + O (g) CO (g) 393.KJ (g) + O(g) O (l) 86. KJ Ans. Ans. The standard enthalpy change or the ormation o one mole o a compound rom its elements in their most stable states o aggregation (also known as reerence states) is called Standard Molar Enthalpy o Formation. c 6 ( co ) 3 ( O) ( C ) ( O ) = 366KJ = 6 x x 86 - = -38 kj/mol 6 6 (C ) OTS QUESTIONS. Why standard entropy o an elementary substance is not zero whereas standard enthalpy o ormation is taken as zero? Ans. A substance has a perectly ordered arrangement only at absolute zero. ence, entropy is zero only at absolute zero. Enthalpy o ormation is the heat change involved in the ormation o one mole o the substance rom its elements. An element ormed rom it means no heat change.. The equilibrium constant or a reaction is one or more i G or it is less than zero. Explain Ans. G = RT ln K, thus i G is less than zero. i.e., it is negative, then ln K will be positive and hence K will be greater than one. 3. Many thermodynamically easible reactions do not occur under ordinary conditions. Why? Ans. Under ordinary conditions, the average energy o the reactants may be less than threshold energy. They require some activation energy to initiate the reaction. 9