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1 . Therm odynam ics is n o t concern ed ab ou t______.

(i) energy cha nges involved in a chemical reaction.

(ii) the extent to which a chemical reaction proceeds.

(iii) the ra te a t which a react ion proceeds .

(iv) the feasibility of a ch emical reaction.

2 . Wh ich of th e followin g sta tem ent s is corr ect?

(i) The presence of reacting species in a covered beaker is an examp le of open system.

(ii) There is an exchan ge of energy as well as m atter between the systemand the su r roun dings in a c losed sys tem.

(iii) The presence of reactants in a c losed vessel ma de up of copper is an

examp le of a closed s ystem.

(iv) The presence of reactants in a therm os flask or any other closed ins u lated

vess el is a n example of a closed system .

3 . The s tate of a gas can be described by qu oting th e relations hip b etween___.

(i) p r es su r e , vo lum e , tem pe ra tu r e

(ii) t em pe ra tu r e, a m oun t , p r es su r e

(iii) a m oun t , volum e , t em pe ra tu r e

(iv) p r es su r e , volum e , t em pe ra tu r e, a m oun t

4 . The volume of gas is reduced to half from its original volume. The specific

h ea t will be ______.

(i) r ed u ce t o h a lf

(ii) b e dou b led

I. Multiple Choice Questions (Type-I)

U nitU nitU nitU nitU nit

66THERM ODYNAM ICSTHERM ODYNAM ICSTHERM ODYNAM ICSTHERM ODYNAM ICSTHERM ODYNAM ICSTHERM ODYNAM ICSTHERM ODYNAM ICSTHERM ODYNAM ICSTHERM ODYNAM ICSTHERM ODYNAM ICS

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(iii) r em a in c on s t a n t

(iv) inc re ase fou r t im e s

5 . Du ring complete comb u stion of one mole of bu tan e, 2658 kJ of hea t is released.Th e th ermochem ical reaction for ab ove cha n ge is

(i) 2C4H

1 0(g) + 13O

2(g) 8CO

2(g) + 10H

2O(l )

cH = 2658.0 kJ mol

1

(ii) C4H

1 0(g) +

1 3

2O

2(g) 4CO

2(g) + 5H

2O (g)

cH= 1329 .0 kJ mol

1

(iii) C4H

1 0(g) +

1 3

2O

2(g) 4CO

2(g) + 5H

2O (l )

cH= 2658.0 k J mol

1

(iv) C4H

10(g) +

1 3

2O

2(g) 4 CO

2(g) + 5 H

2O (l)

cH= +2658.0 kJ mol

1

6 . fU

V

of formation of CH4

(g) at certain temperature is 393 kJ mol1. Thevalue of

fH

Vis

(i) zero

(ii) < f

UV

(iii) > f

UV

(iv) equ a l to f

UV

7 . In an adiabatic process, n o tran sfer of heat takes p lace between system a nd

su rrou n dings. Choose th e correct option for free expa ns ion of an ideal gasu n der a diaba tic con dition from th e following.

(i) q = 0 , T 0, w = 0

(ii) q 0, T = 0, w = 0

(iii) q = 0 , T = 0, w = 0

(iv) q = 0 , T < 0, w 0

8 . The pressure-volume work for an ideal gas can be calculated by using the

expression = wf

i

V

ex

V

p dV . Th e work can also be calcu lated from th epV plot

by us ing the area u nd er the cu rve with in the sp ecified limits. Wh en an ideal

gas is comp res sed (a) reversibly or (b) irrevers ibly from volum e Vito V

f. choose

th e correct option.

(i) w (reversible) = w (irreversible)

(ii) w (revers ible) < w (irreversible)

(iii) w (revers ible) > w (irreversible)

(iv) w (reversible) = w (irreversible) +pe x

.V

6 9 Therm odynam ics

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7 0Exemplar Problems, Chemis t ry

9 . The en tropy cha nge can be calcu la ted b y us ing th e expression S =rev

q

T.

When water freezes in a glas s bea ker, choose the correct statem ent a mon gst

th e following :

(i) S (system ) decreases bu t S (su rroun dings) remains th e same.

(ii) S (system ) increases bu t S (surroundings) decreases.

(iii) S (system ) decreases b u t S (surroundings) increases.

(iv) S (system ) decreases an d S (su rrou n dings) also decreas es.

1 0 . On t h e bas is of th erm ochem ical equ at ion s (a), (b) an d (c), find ou t wh ich of th e

algebr ic relation sh ips given in op tion s (i) to (iv) is corr ect.

(a ) C (gr a ph it e) + O2

(g) CO2

(g) ; rH=x kJ m ol

1

(b ) C (gr ap h it e) +1

2O

2(g) CO (g) ;

rH=y kJ m ol

1

(c) CO (g) +1

2O

2(g) CO

2(g) ;

rH=z kJ m ol

1

(i) z =x + y

(ii) x =y z

(iii) x =y +z

(iv) y = 2z x

1 1 . Consider th e reaction s given b elow. On t h e bas is of th ese reactions fin d ou t

wh ich of th e algebric relat ions given in op tion s (i) to (iv) is corre ct?

(a ) C (g) + 4 H (g) CH4

(g); rH=x k J m ol

1

(b ) C (gr a ph it e,s ) + 2 H2

(g) CH4

(g); rH=y kJ m ol

1

(i) x =y

(ii) x = 2 y

(iii) x >y

(iv) x

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1 3 . Enth alpy of su blima tion of a s u bsta nce is equal to

(i) enth alpy of fu sion + enth alpy of vapou risa t ion

(ii) e n tha lpy o f fu s ion

(iii) en tha lpy of vapour isa t ion

(iv) twice the entha lpy of vapourisa t ion

1 4 . Which of th e followin g is n o t correct?

(i) G is zero for a reversible react ion

(ii) G is positive for a s pont an eous reaction

(iii) G is negative for a s pont an eous reaction

(iv) G is positive for a n on-sp onta neou s rea ction

II. Multiple Choice Questions (Type-II)In th e fol lowing quest ions two or more opt ions may be c orrect .

1 5 . Th ermodyna mics m ainly deals with

(i) interre la t ion of var iou s forms of energy and th eir t ran sformation f rom

one for m to an other.

(ii) energy cha nges in th e processes which depend only on ini t ia l and fina l

sta tes of the m icroscopic system s containing a few molecules.

(iii) how and a t wha t ra te these energy t rans format ions a re ca r r ied out .

(iv) the system in equilibr ium sta te or moving from one equ ilibr iu m s ta te toan other equ ilibrium s tate.

1 6 . In an exothermic reaction, heat is evolved, and system loses heat to the

su r rou nding. For su ch sys tem

(i) qp

will be n egat ive

(ii) rHwill be n egat ive

(iii) qp

will be p osit ive

(iv) rH will be p osit ive

1 7 . The sp onta neity mean s, ha ving the potential to proceed with out th e assista nce

of external agency. Th e processes which occur s pon tan eous ly are

(i) flow of heat f rom colder to warm er body.

(ii) gas in a conta iner cont rac t ing in to one corner.

(iii) gas expanding to fill the available volu me.

(iv) bu rning carbon in oxygen to give carbon d ioxide.

1 8 . For an ideal gas, th e work of reversible expan sion u nd er isoth erma l cond ition

can b e calculated by u sing the express ion w = nRTlnf

i

V

V

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A sa mp le con taining 1.0 m ol of an ideal gas is expan ded isotherm ally and

reversibly to ten times of its original volum e, in two sepa rate experimen ts.

Th e expan sion is carried out a t 30 0 K an d at 600 K respectively. Choose th ecorrect option .

(i) Work done a t 600 K is 20 t imes the work done a t 300 K.

(ii) Work done a t 300 K is twice the work done a t 600 K.

(iii) Work done a t 600 K is twice the work done a t 30 0 K.

(iv) U= 0 in b oth cases.

1 9 . Consider th e following reaction b etween zinc an d oxygen an d ch oose the correct

option s ou t of th e option s given b elow :

2 Zn (s) + O2

(g) 2 ZnO (s ) ; H = 693.8 kJ mol1

(i) The enth alpy of two moles of ZnO is less th an the tota l enth alpy of two

moles of Zn an d on e mole of oxygen by 693 .8 kJ .

(ii) The enth alpy of two moles of ZnO is more than the total ent ha lpy of two

moles of Zn an d on e mole of oxygen by 693 .8 kJ .

(iii) 6 93 .8 kJ m ol1

en ergy is evolved in th e reaction .

(iv) 6 93 .8 kJ m ol1

energy is abs orbed in the reaction.

III. Short Answer Type

2 0 . 18.0 g of water completely vapou rises a t 100 C an d 1 bar pressu re an d th e

enth alpy chan ge in th e process is 40.79 kJ mol1

. Wha t will be the ent h alpycha nge for vapou risin g two moles of water u nd er th e sam e conditions? Wh at

is the s tan dard enth alphy of vapou risa t ion for water?

2 1 . On e mole of aceton e requ ires less h eat to vapou rise th an 1 m ol of water. Wh ichof th e two liqu ids h as higher ent ha lpy of vap our isation?

2 2 . Stan dard molar enth alpy of formation, fH

V

is ju st a s pecial case of enth alpyof r eaction,

rH

V. Is th e

rH

Vfor th e following r eaction s am e as

fH

V? Give

reason for your an swer .

CaO(s) + CO2(g) CaCO

3(s);

fH

V

= 178.3 kJ mol1

2 3 .Th e valu e of

fH

V

for NH3 is 91.8 kJ mol

1

. Calculate ent ha lpy cha n ge for thefollowing rea ction :

2 NH3(g) N

2(g) + 3 H

2(g)

2 4 . En th alpy is a n exten sive prop erty. In gen eral, if ent h alpy of an overall reaction

AB along one rou te is rHa n d

rH

1,

rH

2,

rH

3. . . .. represent en th alpies of

intermediate rea ctions leadin g to produ ct B. Wha t will be th e relation b etween

rHfor overall reaction a n d

rH

1,

rH

2..... etc. for in term ediate rea ctions.

2 5 . The enthalpy of a tomisation for the reaction CH4(g) C(g) + 4H (g) is

1665 kJ m ol1 . Wha t is th e bond energy of CH bon d?

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2 6 . Use th e followin g data to calculat e lattice

HV for NaBr.

sub

HV for sodiu m m etal = 108.4 kJ mol

1

Ionizat ion enth alpy of sodium = 496 k J mol1

Electron gain enth alpy of bromine = 325 kJ mol1

Bond dissociation en th alpy of brom in e = 192 k J mol1

f

HV

for NaBr (s) = 36 0.1 kJ m ol1

2 7 . Given th at H= 0 for m ixing of two gases. E xplain wheth er th e diffu sion of these

gases into each other in a closed conta iner is a sp ontan eou s process or not?

2 8 . Heat h as ra ndomising influ ence on a system an d temperature is the m easure

of average ch aotic motion of particles in t he s ystem. Write th e ma th ema ticalrelation which r elates th ese three para meters.

2 9 . Increase in enth alpy of the s u rroun dings is equ al to decrease in enth alpy of

the system. Will the temperatu re of system an d su rroun dings be the sam ewhen t hey are in th ermal equilibr ium ?

3 0 . At 2 98 K. Kp

for th e reaction N2O

4(g) U 2NO

2(g) is 0.98 . Predict wheth er th e

reaction is spontan eou s or not .

3 1 . A sample of 1.0 mol of a monoatomic ideal gas is taken through a cyclicprocess of expan sion an d compres sion as sh own in Fig. 6.1. What will be the

value ofH for th e cycle as a wh ole?

Fig . : 6 .1

3 2 . The standard molar entropy of H2O (l) is 70 J K1 m ol1 . Will the standard

m olar en trop y of H2O(s) be more, or less th an 70 J K1 m ol1?

3 3 . Iden tify the s tate fun ctions an d p ath fu nctions out of th e followin g :

enth alpy, entr opy, hea t, temp eratu re, work, free energy.

3 4 . Th e molar enth alpy of vap our isation of aceton e is less th an th at of water. Wh y?

3 5 . Which qu an tity out of rG a nd

rGV will be zero at equ ilibriu m?

3 6 . Predict the ch an ge in int erna l energy for an isolated system a t const an t volum e.

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3 7 . Although heat is a path function but heat absorbed by the system under

certain s pecific condition s is indepen den t of pat h . Wh at a re th ose con ditions ?

Explain.

3 8 . Expan sion of a gas in vacuu m is ca lled free expan sion. Calcu late the workdone and the change in internal energy when 1 litre of ideal gas expandsisotherm ally int o vacu u m u n til its total volu me is 5 litre?

3 9 . Heat cap acity (Cp

) is an extens ive propert y bu t s pecific heat (c) is an inten sive

property. What will be t h e relation between Cp

an d c for 1 m ol of water?

4 0 . The difference between CP

and CV

can be derived using the empirical relation

H= U+pV. Calculate th e difference between CPand C

Vfor 10 m oles of an ideal gas.

4 1 . If th e comb u stion of 1g of graph ite produ ces 20.7 k J of hea t, what will be

m olar en th alpy cha n ge? Give th e sign ifican ce of sign also.

4 2 . The net enthalpy change of a reaction is the amount of energy required tobreak a ll the bonds in reactant m olecu les minu s a mou nt of energy required

to form all the bonds in the product molecules. What will be the enthalpych an ge for th e followin g reaction.

H2(g) + Br

2(g) 2HBr(g)

Given th at Bond en ergy of H2, Br

2and HBr is 435 kJ mol

1, 192 kJ mol

1a n d

368 kJ m ol1

respectively.

4 3 . The enthalpy of vapourisation of CCl4

is 30 .5 kJ mol1

. Calcula te the h eatrequ ired for th e vap ou risa tion of 28 4 g of CCl

4at cons tan t press u re . (Molar

ma ss of CCl4

= 154 g mol1

).

4 4 . Th e enth alpy of react ion for th e reaction :

2H2(g) + O

2(g) 2H

2O(l) is

rHV = 572 k J mol

1.

What will be standard enthalpy of formation ofH

2O (l ) ?

4 5 . Wh at will be the work d one on an ideal gas en closed

in a cylind er , when it is compressed by a cons tan texternal pressu re ,p

ex tin a s ingle s tep as sh own in

Fig. 6.2. Exp lain grap h ically.

4 6 . How will you calculate work done on an ideal gas

in a compress ion , when change in pressure i scarried out in infinite steps?

4 7 . Represent t he potential energy/ enth alpy chan ge in

th e followin g process es gra ph ically.

(a ) Throwing a s tone from the grou nd to roof.

(b)1

2H

2(g) +

1

2Cl

2(g) U HCl(g)

rH

V= 92.32 kJ mol1

In which of the processes potentia l energy/ enth alpy cha nge is contr ibut ing

factor to th e spon tan eity?

Fig . : 6 .2

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5 2 . Match th e following p rocesses with entropy ch an ge:

Re ac t io n En tro py c h ange

(i) A liqu id va pou rises (a) S = 0

(ii) Rea ction is n on -spon ta n eou s (b ) S = posit ive

at a l l temperatu res and H

is pos itive

(iii) Revers ib le expa n s ion of a n (c) S = negat ive

ideal gas

5 3 . Match th e following p ara meters with description for sp onta neity :

(Param e t e rs ) Desc ript ion

r

HVVVVV

r

SVVVVV

r

GVVVVV

(i) + + (a) Non -spon ta n eou s a t h igh

temperature.

(ii) + a t h igh T (b ) S pon ta ne ous a t a l l t em pe ra tu r e s

(iii) + (c) Non -s pon ta n eou s a t a lltemperatures

5 4 . Match th e followin g :

(i) En tropy of va pou risa t ion (a ) decrea ses

(ii) Kfor s pon ta n eou s proces s (b ) is a lwa ys pos it ive

(iii) Crys ta llin e solid s ta te (c) lowes t en tropy

(iv) Uin a d ia ba tic expa n s ion (d )va p

b

H

T

of idea l gas

V. Assert ion and Reas on Type

In th e following ques tion s a st atem ent of Ass ertion (A) followed by a state me nt

of Reason (R) is given. Choose t he c orrect opt ion o ut of the c hoice s given

below each quest ion.

5 5 . As s er t ion (A): Combustion of a l l organic compounds is an exothermic

reaction.

Rea s on (R) : The en th alpies of all elemen ts in th eir sta nd ard sta te are zero.

(i) Both A and R a re t rue an d R is th e cor rect explana t ion of A.

(ii) Both A an d R are true bu t R is n ot the correct explan ation of A.

(iii) A is t rue but R is fa lse .

(iv) A is fa ls e bu t R is t r ue .

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5 6 . As s ert ion (A) : Spontan eou s p rocess is an ir reversible process an d m ay be

reversed b y some externa l agen cy.

Rea s on (R) : Decrease in en th alpy is a con tribu tory factor for spon tan eity.

(i) Both A and R a re t rue an d R is th e cor rect explana t ion of A.(ii) Both A an d R are true bu t R is n ot the correct explan ation of A.



5 7. As s ert ion (A) : A liquid crystallises into a solid and is accompanied bydecrease in entropy.

Re a s on (R) : In crysta ls, m olecules organ ise in a n ordered ma n ner.

(i) Both A and R a re t rue an d R is th e cor rect explana t ion of A.

(ii) Both A an d R are true bu t R is not th e correct explanation of A.



VI. Long Answe r Type

5 8 . Derive the relationsh ip between H a n d Ufor an ideal gas . Explain each

term involved in t he equa tion .

5 9 . Extensive properties depen d on th e qua nt ity of ma tter bu t intens ive propertiesdo not . Explain wheth er th e followin g properties ar e exten sive or in ten sive.

Mas s, intern al energy, press u re, heat capacity, molar hea t capa city, density,mole fraction, specific heat, t empera tu re a nd molarity.

6 0 . The lattice en th alpy of an ion ic compou n d is th e enth alpy wh en on e mole of

an ionic compou nd presen t in its gaseous sta te, diss ociates into its ions . It isimp ossible to determ ine it directly by experimen t. Su ggest a n d explain an

indirect met h od to mea su re lattice en th alpy of NaCl(s).

6 1 . G is net en ergy availab le to do u sefu l work an d is th u s a mea su re of free

energy. Sh ow mat hem atically tha t G is a m easu re of free energy. Fin d th e

un it of G. If a rea ction h as positive enth alpy chan ge an d pos itive entropychan ge, u nder wh at condit ion will the reaction be s pontan eou s?

6 2 . Graph ically show the total work done in an expan sion when t he st ate of anideal gas is cha n ged reversibly an d isoth erm ally from (p

i, V

i) to (p

f, V

f). With

the help of a pV plot compare the work done in the above case with that

carr ied out agains t a constan t external pressu re pf

.

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ANSWERS

I. Mult ip le Ch oice Qu es t ions (T y p e-I)

1. (iii) 2. (iii) 3. (iv) 4. (iii) 5 . (iii) 6. (ii)

7 . (iii)

Jus tifica tion : free expa n sion w = 0

ad iab atic process q = 0

U= q + w = 0, this m eans tha t internal energy remains

consta nt . Therefore, T= 0 .

In ideal gas there is no intermolecular attraction. Hence when such a gas

expan ds u nder a diabatic cond it ions into a vaccu m n o heat is abs orbed or

evolved sin ce no externa l work is don e to sepa rat e th e molecu les.

8 . (ii) w (rever s ible) < w (irre vers ible)

Jus tifica tion : Ar e a unde r the c u r ve i s a lwa ys m or e in i r r e ve r s ib l ecompres sion a s ca n be s een from Fig. 6.5 (a) and (b).

(a ) Revers ib le com press ion (b) Ir revers ib le com pres s ion

Fig . : 6 .5

9. (iii)

Jus tifica tion : Freezing is exotherm ic process. The h eat released in creasesthe en tropy of su rroun ding.

1 0. (iii)

11 . (iii)

Jus tifica tion : Same bonds are formed in reaction (a) and (b) but bonds

between reactan t m olecules are b roken on ly in reaction (b).

12 . (iii ) 13 . (i ) 14 . (ii )

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II. Mul t ip le Ch oice Qu es t ion s (Ty p e-II)

15. (i), (iv) 16 . (i), (ii) 17 . (iii), (iv)

18 . (iii), (iv)

Jus tif ica tion :600K

300K

1 01 R 6 00 lnw 60 01= 2

1 0w 30 01 R 30 0 ln

1

K

K

= =

For isoth erma l expan sion of ideal gas es, U= 0

since temperatu re is consta nt this mean s th ere is n o chan ge in interna l energy.Therefore, U= 0

19. (i), (iii)

III. S h or t An s w er Ty p e

20. + 81 .58 kJ , va p

HV

= + 40.79 kJ mol1

2 1 . Water

2 2 . No, sin ce Ca CO3

ha s been formed from oth er compou nd s a nd not from itscons titu ent elements .

2 3 . rH

V = +91.8 kJ mol1

2 4 . r

H= r

H1

+ r

H2

+ r

H3

.....

2 5 .1 6 6 5

4k J m o l

1= 416.2 kJ mol

1

2 6. +735.5 kJ m ol1

2 7 . It is s p o n t a n e ou s p r oc es s . Alt h o u g h e n t h a l p y c h a n g e is ze r o b u t

randomness or disorder ( i .e . , S) increases. Therefore, in equationG = H TS, the term TS will be n egative. Hen ce, G will be n egat ive.

2 8 . S=re v

q

T

2 9 . Yes

3 0 . Th e r ea ct ion is s pon t a n eou s

rGV

= RTln Kp

31. H(cycle) = 0

32 . Le ss , bec a use ic e is m ore or der ed tha n H2O (l ).

33 . S ta te Func t ions : Entha lpy, Ent ropy, Tempera ture , F ree energy

Path Fu n ctions : Heat, Work

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3 4 . B ec a u s e o f s t r o n g h yd r o ge n b o n d in g in w a t e r, it s e n t h a l p y o f

vapourisa tion is more.

3 5 . rG will always be zero.

rGV is zero for K= 1 becaus e GV = RT lnK, GV will be non zero for

oth er values of K.

36 . For isola ted system, there is no tran sfer of energy as h eat or as work i.e .,

w=0 a nd q=0. According to th e first law of th ermodyn am ics.U = q + w

= 0 + 0 = 0

U = 0

3 7 . At constant volume

By first law of th erm odyna mics:

q = U+ (w)

(w) = p V

q = U+ pV

V = 0, s ince volum e is cons tan t .

qV

= U+ 0

qV

= U= cha nge in int erna l energy

At co nst ant press ure

qp

= U+ pV

But , U+ pV = H

qp

= H = chan ge in enth alpy.

So, a t a constan t volum e and a t cons tan t pressu re heat chan ge is a s ta tefu nction becaus e i t is equ al to chan ge in intern al energy and chan ge in

enth alpy respectively wh ich are s tate fun ctions.

3 8 . (w) =pex t

(V2V

1) = 0 (5 1 ) = 0

For isoth erma l expan sion q = 0

By first law of th erm odyna mics

q = U+ (w)

0 = U+ 0 so U= 0

39. For wa te r, hea t capac ity = 18 spec ific hea t

or Cp

= 18 c

Specific h eat = c = 4.18 J g1

K1

Heat cap acity = Cp

=18 4 .18 J K

1=

75 .3 J K

1

4 0 . CP

CV

= nR

= 10 4.184 J

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4 1 . Mola r en th a lp y

cha n ge of graph ite= enth alpy chan ge for 1 g carbon molar m as s of carbon

= 20.7 kJ g1

12g mol1

H = 2.48 102k J m ol

1

Negat ive valu e ofH exoth ermic reaction .

4 2 . rH

V= Bond energy of H

2+ Bond en ergy of Br

2 2 Bond en ergy of HBr

= 435 + 192 (2 368) kJ mol1

rH

V= 1 09 kJ m o l

1

4 3 . qp

= H = 30.5 kJ mol1

Heat requ ired for vapour isation of 284 g of CCl4 = 12 8 4 g

30 .5154g mol

kJ m ol

1

= 56.2 kJ

44 . According to the definition of stan dard enth alpy of formation, the enth alpych an ge for th e followin g reaction will be st an da rd en th alpy of form at ion

of H2O (l)

H2(g) +

1

2O

2(g) H

2O(l ).

or the s tan dar d en th alpy of forma tion of H2O(l ) will be h alf of th e en th alpy

of th e given equ at ion i.e., rH

V

is also h alved.

fH

V

H2O(l ) =

1

2

rH

V=

1

572 kJ mol

2= 286 kJ / m ol.

45 . Work done on an idea l gas can be ca lcula ted from p-V graph sh own inFig. 6.6. Work done is equal to the sh ad ed area ABV

IV

II.

Fig . : 6 .6

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46 . The work done ca n be c a lc u la t ed with the he lp o f pVplot. ApVplot of

th e work of compres sion which is carr ied out b y cha n ge in press u re in

infin ite steps , is given in Fig. 6.7. Sha ded ar ea represen ts t he work doneon the gas .

Fig . : 6 .7

4 7 .

Fig . : 6.8 Enthalpy change in processes (a) and (b)

4 8 . No.

En th alpy is on e of th e contribut ory factors in deciding spon tan eity bu t it

is not t he on ly factor. One m u st look for contribu tion of an other factori.e., entr opy also, for gettin g the correct resu lt.

49 . It is c lear from the figu re that the process ha s been carr ied out in infini testeps , h ence it is isoth ermal reversible expans ion.

w = 2.303 nRTlogV

V

2

1

But , p1V

1= p

2V

2

V

V

2

1

=p

p

1

2

=2

1= 2

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w = 2.303 nRT logp

p

1

2

= 2.303 1 mol 8.314 J mol1 K1 298 K1 log 2

= 2.303 8.314 298 0.3010 J = 1717.46 J

5 0 . w = pe x

(Vf

Vi) = 2 4 0 = 80 L ba r = 8 k J

The nega t ive s ign shows tha t work i s done by the sys tem on the

su rr oun ding. Work don e will be more in t h e reversible expan sion becau seintern al pressure an d exerna l pressu re are a lmost s am e at every step.

IV. Ma t ch ing T y p e

51. (i) (e) (ii) (d ) (iii) (f) (iv) (a)

(v) (g), (k), (l) (vi) (b ) (vii) (c) (viii) (j)

(ix) (h ) (x) (i) (xi) (a ), (l), (m ) (xii) (g), (k)

52 . (i) (b ) (ii) (c) (iii) (a)

53 . (i) (c) (ii) (a ) (iii) (b)

54 . (i) (b), (d) (ii) (b ) (iii) (c) (iv) (a)

V. Asser t ion and Reason Type

55 . (ii) 56 . (ii) 57 . (i)

VI. Long Answer Type

5 9 . Hint : Ratio of two extens ive proper ties is always int en sive

Extensive= In tens ive

Extensive.

e.g., Mole fra ction =Mole s (Extens ive )

=To ta l n u m b er of m o le s (Ext e ns ive )

6 0. Na (s ) +1

2Cl

2(g) Na +(g) + Cl

(g) ;

latticeH

V

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Bonn - Haber Cycle

Steps to mea su re lattice ent ha lpy from Bonn - Haber cycle

Sub lima tion of sodiu m metal

(1 ) Na (s ) Na (g) ; s ub

HV

(2) Ionisa t ion of sodium a toms

Na(g) Na +(g) + e(g) ; tH

Vi.e. , ionisation en th alpy

(3) Dissocia t ion of chlor ine m olecule

1

2Cl

2(g) Cl(g) ;

1

2

b o n dH

Vi.e., On e-ha lf of bon d diss ociat ion

enthalpy.

(4 ) C l(g) + e(g) Cl(g) ; egHV i .e. , electron gain ent ha lpy.

6 1 . STotal = Ssy s + Ssurr

STotal

= Ssy s

+sys

- H

T

TSTotal

= TSsys

Hsy s

For spontan eou s chan ge, Stotal

> 0

T Ssy s

Hsy s

> 0

(Hsy s

T Ssy s

) > 0

iH

V

Na(g) +1

2 Cl2(g)

NaCl(s)

s ub

HV

Na(s) +1

2Cl

2(g)

e gH

V

Na +(g) + Cl(g)

lattice

HV

Na+(g) +

1

2Cl

2(g)

1

2

bondH

V

Na+

(g) + Cl (g)

fH

V

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But , Hsys

T Ssy s

= Gsys

Gsy s

> 0

Gsys = Hsy s TSs y s < 0

Hsys

= Enth alpy cha n ge of a reaction.

TSsys

= Energy which is n ot available to do u sefu l work.

Gsys

= En ergy available for doing u seful work.

Unit ofG is J oule

The reaction will be sponta neous a t h igh temperatu re .

62 .

Fig . : 6 .9

(i) Reversible Work is r epresented by the combined areas an d .(ii) Work aga ins t cons tan t pressu re,p

fis represented by the a rea

Wor k (i) > Wor k (ii)

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