Question 1

What would be the amount of heat absorbed in the cyclic process shown below?

Accepted Answer

In a cyclic process, internal energy change =0 According to 1^ st law of thermodynamics U= q+ W As, U=0, q=- W From the graph, work done = W= Area under curve Here, area of circle = r^2 aligned & = ( 25-5 2 )^2= 10^2 \ q & =100 ~J aligned

Question 2

The bond dissociation energy of X_2, Y_2 and X Y are in the ratio of 1: 0.5: 1 . H for the formation of X Y is -200 ~kJ / mol . The bond dissociation energy of X_2 will be

Accepted Answer

Let the bond dissociation energy of X_2=a ~kJ / mol i.e. BE (X_2 )=a ~kJ / mol Then, BE ( Y _2 )=0.5 a and BE (X Y)=a ~kJ / mol Given, 1 2 X_2+ 1 2 Y_2 X Y, H=-200 ~kJ / mol aligned _r H & = BE ( Rreactants )- BE ( Product ) \ _r H & = [ 1 2 BE (X_2 )+ 1 2 BE (Y_2 ) ]- BE (X Y) \ -200 & = a 2 + 0.5 a 2 -a \ a & = 200 0.25 =800 ~kJ / mol aligned

Question 3

Which of the following relation is not correct?

Accepted Answer

H = U + PV (By definition) H = U + ( PV ) at constant pressure H= U+P V

Question 4

The standard Gibbs energy ( G^0 ) for the following reaction is aligned & A ( s )+ B ^ 2+ ( aq ) A ^ 2+ ( aq )+ B ( s ), K _ c =10^ 12 at \ & 25^ C aligned ( K _c= . equilibrium constant )

Accepted Answer

A + B ^ 2+ A ^ 2+ + B G ^ =- RT K _ c =-8.314 298 2.303 10^ 12 =68.47 ~kJ / mol

Question 5

The combustion of benzene (L) gives CO _2( ~g ) and H _2 O (L). Given that heat of combustion of benzene at constant volume is -3263.9 ~kJ ~mol ^ - ^1 at 25^ C ; heat of combustion (in kJ mol ^ -1 ) o

Accepted Answer

C _6 H _6( I )+ 15 2 O _2( ~g ) 6 CO _2( ~g )+3 H _2 O ( l ) n_g=6- 15 2 =- 3 2 H= U+ n_ g R T =-3263.9+ (- 3 2 ) 8.314 10^ -3 298 =-3263.9+(-3.71)=-3267.6 ~kJ ~mol ^ -1

Question 6

Choose the correct option for free expansion of an ideal gas under adiabatic condition from the following:

Accepted Answer

U=q+w For a adiabatic process, U = w =- p _ ex v For free expansion of ideal gas p _0=0 As U = C T =0

Question 7

Given that bond energies of H - H and Cl - Cl are 430 ~kJ ~mol ^ -1 and 240 ~kJ ~mol ^ -1 respectively and H _ f for HCl is -90 ~kJ ~mol ^ -1 , bond enthalpy of HCl is

Accepted Answer

1 2 H _2+ 1 2 Cl _2 HCl aligned & H_ HCl = B.E. of reactant - B.E. of products \ &-90= 1 2 430+ 1 2 240- B.E. of HCl \ & B.E. of HCl =215+120+90=425 ~kJ ~mol ^ -1 aligned

Question 8

If enthalpies of formation of C _2 H _4( ~g ), CO _2( ~g ) and H _2 O ( l ) at 25^ C and 1 ~atm pressure are 52,394 and -286 ~kJ / mol respectively, the change in enthalpy for combustion of C _2 H _4

Accepted Answer

Enthalpy of formation of C _2 H _4, CO _2 and H _2 O are 52,-394 and -286 ~kJ / mol respectively. (Given) The reaction is C _2 H _4+3 O _2 2 CO _2+2 H _2 O change in enthalpy, ( H)= H_ products - H_ reactants =2 (-394)+2 (-286)-(52+0) =-1412 ~kJ / mol .

Question 9

Standard entropy of X_2, Y_2 and X Y_2 are 60,40 and 50 JK ^ -1 ~mol ^ -1 , respectively. For the reaction, 1 2 X_2+ 3 2 Y_2 X Y_3, H=-30 ~kJ , to be at equilibrium, the temperature will be

Accepted Answer

S=S (X Y_3 )- 1 2 S (X_2 )- 3 2 S (Y_2 ) =50-30-60=-40 ~J ~mol ^ -1 ~K ^ -1 H=-30 ~kJ =-3000 ~J G= H-T S At equilibrium, G=0 T= H S = -3000 -40 =750 ~K

Question 10

In an adiabatic process, no transfer of heat takes place between system and surrounding. Choose the correct option for free expansion of an ideal gas under adiabatic condition from the following.

Accepted Answer

Free expansion, W=0 Adiabatic process, q=0 U=q+W=0, this means that internal energy remains constant. Therefore, T=0 in ideal gas there is no intermolecular attraction. Hence, when such a gas expands under adiabatic conditions into a vacuum, no heat is absorbed or evolved, since no external work is done to separate the molecules.

Question 11

Work out the heat change (cal) when 40 ~g of He gas at 27^ C undergoes isothermal and reversible compression from initial pressure of 1 ~atm to 10 ~atm ( R =2 cal K ^ -1 ~mol ^ -1 ).

Accepted Answer

aligned q &=- w _ rev =- (-2.303 nRT P _ 1 P _ 2 ) \ &=2.303 40 4 2 300 1 10 =-13.82 kcal aligned

Question 12

For which one of the processes represented by the following equations the enthalpy (heat) change is likely to be negative

Accepted Answer

Gaseous ions, when dissolved in water, get hydrated and heat is evolved (heat of hydration). Cl ^ - ( g )+ aq Cl ^ - ( aq ) is such reaction

Question 13

Acyclicprocess ABCD is shown in P-V diagram for an ideal gas. Which of the following diagram represents the same process?

Accepted Answer

AB : Isobaric expansion BC : Isothermal expansion CD : Isochoric DA: Isothermal compression

Question 14

For the combustion reaction at 298 ~K 2 Ag ( s )+1 / 2 O _ 2 ( ~g ) 2 Ag _ 2 O ( s ) Which of the following alternatives is correct?

Accepted Answer

H = U + nRT n = n _ p - n _ R Now, n =2- 5 2 =- 1 2 H = U - 1 2 RT Thus, U = H + 1 2 RT U > H

Thermodynamics (C) — Previous Year Questions & Solutions