Chapter 6 - Thermodynamics

ΔG is net energy available to do useful work and is thus a measure of “free
energy”. Show mathematically that ΔG is a measure of free energy. Find the
unit of ΔG. If a reaction has positive enthalpy change and positive entropy
change, under what condition will the reaction be spontaneous?

The lattice enthalpy of an ionic compound is the enthalpy when one mole of an ionic compound present in its gaseous state, dissociates into its ions. It is impossible to determine it directly by experiment. Suggest and explain an indirect method to measure lattice enthalpy of NaCl(s).

Assertion (A) : A liquid crystallises into a solid and is accompanied by decrease in entropy.
Reason (R) : In crystals, molecules organise in an ordered manner.
(i) Both A and R are true and R is the correct explanation of A.
(ii) Both A and R are true but R is not the correct explanation of A.
(iii) A is true but R is false.
(iv) A is false but R is true.

Assertion (A) : Spontaneous process is an irreversible process and may be reversed by some external agency.

Reason (R) : Decrease in enthalpy is a contributory factor for spontaneity.
(i) Both A and R are true and R is the correct explanation of A.
(ii) Both A and R are true but R is not the correct explanation of A.
(iii) A is true but R is false.
(iv) A is false but R is true.

In the following questions a statement of Assertion (A) followed by a statement of Reason (R) is given. Choose the correct option out of the choices given below each question.

Assertion (A): Combustion of all organic compounds is an exothermic reaction.
Reason (R) : The enthalpies of all elements in their standard state are zero.
(i) Both A and R are true and R is the correct explanation of A.
(ii) Both A and R are true but R is not the correct explanation of A.
(iii) A is true but R is false.
(iv) A is false but R is true.

Match the following processes with entropy change:
Reaction Entropy change

(i) A liquid vapourises                                                      (a) ΔS = 0
(ii) Reaction is non-spontaneous                                          (b) ΔS = positive
at all temperatures and ΔH is positive
(iii) Reversible expansion of an                                         (c) ΔS = negative
ideal gas

In the following questions more than one correlation is possible between
options of both columns.

Match the following :
A B
(i) Adiabatic process                                                                              (a) Heat
(ii) Isolated system                                                                 (b) At constant volume
(iii) Isothermal change                                                   (c) First law of thermodynamics
(iv) Path function                                                 (d) No exchange of energy and matter
(v) State function                                                (e) No transfer of heat
(vi) ΔU = q                                                           (f) Constant temperature
(vii) Law of conservation of energy                           (g) Internal energy
(viii) Reversible process                                          (h) pext = 0
(ix) Free expansion                                                (i) At constant pressure
(x) ΔH = q

                                                       (j) Infinitely slow process which
proceeds through a series of equilibrium states.
(xi) Intensive property                                            (k) Entropy
(xii) Extensive property                                             (l) Pressure
(m) Specific heat

1.0 mol of a monoatomic ideal gas is
expanded from state (1) to state (2) as shown in
Fig. 6.4. Calculate the work done for the expansion
of gas from state (1) to state (2) at 298 K.

If the combustion of 1g of graphite produces 20.7 kJ of heat, what will be
molar enthalpy change? Give the significance of sign also.

Expansion of a gas in vacuum is called free expansion. Calculate the work
done and the change in internal energy when 1 litre of ideal gas expands
isothermally into vacuum until its total volume is 5 litre?

Although heat is a path function but heat absorbed by the system under
certain specific conditions is independent of path. What are those conditions?
Explain.

Predict the change in internal energy for an isolated system at constant volume.

The molar enthalpy of vapourisation of acetone is less than that of water. Why?

State Functions : Enthalpy, Entropy, Temperature, Free energy
Path Functions : Heat, Work

Increase in enthalpy of the surroundings is equal to decrease in enthalpy of
the system. Will the temperature of system and surroundings be the same
when they are in thermal equilibrium?

Heat has randomising influence on a system and temperature is the measure
of average chaotic motion of particles in the system. Write the mathematical
relation which relates these three parameters.

Given that ΔH = 0 for mixing of two gases. Explain whether the diffusion of these
gases into each other in a closed container is a spontaneous process or not?

The enthalpy of atomisation for the reaction CH4(g)→ C(g) + 4H (g)
is 1665 kJ mol–1. What is the bond energy of C–H bond?

18.0 g of water completely vapourises at 100°C and 1 bar pressure and the
enthalpy change in the process is 40.79 kJ mol–1. What will be the enthalpy
change for vapourising two moles of water under the same conditions? What
is the standard enthalphy of vapourisation for water?

The DH and DS for 2Ag₂ O(s)→4Ag(s) +O₂ (g) are given + 61.17kJ mol-1 and +

132 Jk^-1mol^-1 respectively. Above what temperature will the reaction be

spontaneous?

Explain how is enthalpy related to spontaneity of a reaction?

Predict the sign of the entropy change for each of the following changes of

state.

(a) Hg(l)→Hg(g)

(b) AgNO₃ (S)→ AgNO₃ (aq)

(c) I2(g) →I₂ (S)

(d) C(graphite)→C(diamond)

Two ideal gases under same pressure and temperature are allowed to mix in
an isolated system – what will be sign of entropy change?

Predict the sign of DS for the following reaction

 CaCo3 (s) ¾D¾→CaO₅ +CO₂  (g).

What is the sign of enthalpy of formation of a highly stable compound?

Define non-spontaneres process.

Define spontaneous process.

The reaction C (graphite) + O₂ (g) →CO₂(g) + 393.5 kJ mol-1 represents the
formation of CO₂ and also combustion of carbon. Write the DH^O values of the
two processes.

What is the relation between the enthalpy of reaction and bond enthalpy?

Give two examples of reactions which are driven by enthalpy change.

Calculate the heat of combustion of ethylene (gas) to from CO₂ (gas) and H₂O

 (gas) at 298k and 1 atmospheric pressure. The heats of formation of CO₂, H₂O

 and C₂H₄ are – 393.7, - 241.8, + 52.3 kJ per mole respectively.

The standard heat of formation of Fe₂O₃ (s) is 824.2kJ mol-1 Calculate heat

change for the reaction.

4Fe(s) + 30₂ (g) →2Fe₂O₃(s)

Define reaction enthalpy.

A 1.25g sample of octane (C18 H18) is burnt in excess of oxygen in a bomb
calorimeter. The temperature of the calorimeter rises from 294.05 to 300.78K.
If heat capacity of the calorimeter is 8.93 KJ/K. find the heat transferred to
calorimeter.

Show that for an ideal gas, the molar heat capacity under constant volume
conditions is equal to 3/2 R.

Show that for an ideal gas Cp- Cv = R

It has been found that 221.4J is needed to heat 30g of ethanol from 15⁰C to

18⁰C. calculate (a) specific heat capacity, and (b) molar heat capacity of

ethanol.

Give the mathematical expression of heat capacity.

Define specific heat.

Define Heat capacity [

Give the expression for
(i) isothermal irreversible change, and
(ii) isothermal reversible change.

What is an extensive and intensive property?

Give the relationship between DU and DH for gases.

Why is the difference between DH and DU not significant for solids or
liquids?

When is enthalpy change (DH ) -
(i) positive (ii) negative.

With the help of first law of thermodynamics and H = U + pv, prove DH = qp

Give the mathematical expression of enthalpy.

Define enthalpy.

Change in internal energy is a state function while work is not, why?

How may the state of thermodynamic system be defined?

From thermodynamic point of view, to which system the animals and plants
belong?

What will happen to internal energy if work is done by the system?

Name the different types of the system.

Give an example of an isolated system.

What kind of system is the coffee held in a cup?

State the first law of thermodynamics.

Define surroundings.

Define a system.

Calculate the entropy change in surroundings when 1.00 mol of H₂O_(l)is formed under standard conditions. Δ_fH^θ= -286 kJ mol^-1.

The equilibrium constant for a reaction is 10. What will be the valueof ΔG^θ? R = 8.314 JK^-1mol^-1, T= 300 K.

For the reaction

2A_(g) + B_(g) → 2D_(g)

ΔU^θ = -10.5 kJ and ΔS^θ= -44.1 JK^-1.

Calculate ΔG^θ for the reaction, and predict whether the reaction may occur spontaneously.

For the reaction,

2Cl_(g) → Cl_2(g),what are the signs of ΔH and ΔS ?

For the reaction at 298 K,

2A + B → C

ΔH = 400 kJ mol^-1and ΔS = 0.2 kJ K^-1mol^-1

At what temperature will the reaction become spontaneous considering ΔH and ΔS to be constant over the temperature range?

For an isolated system, ΔU = 0, what will be ΔS?

Calculate the enthalpy change for the process

CCl_4(g) → C_(g) + 4Cl_(g)

and calculate bond enthalpy of C-Cl in CCl_4(g).

Δ_vapH^θ (CCl₄) = 30.5 kJ mol^-1.

Δ_fH^θ (CCl₄) = -135.5 kJ mol^-1.

Δ_aH^θ (C) = 715.0 kJ mol^-1, where Δ_aH^θ is enthalpy of atomisation

Δ_aH^θ (Cl₂) = 242 kJ mol^-1

Enthalpies of formation of CO_(g), CO_2(g), N₂O_(g) and N₂O_4(g) are –110 kJ mol^–1, – 393 kJ mol^–1, 81 kJ mol^–1and 9.7 kJ mol^–1respectively. Find the value of Δ_rH for the reaction:

N₂O_4(g)+ 3CO_(g) N₂O_(g)+ 3CO_2(g)

Enthalpy of combustion of carbon to CO₂is -393.5 kJ mol^-1. Calculate the heat released upon formation of 35.2 g of CO₂from carbon and dioxygen gas.

Calculate the enthalpy change on freezing of 1.0 mol of water at 10.0°C to ice at -10.0°C. Δ_fusH = 6.03 kJ mol^-1 at 0°C.

C_p[H₂O(l)] = 75.3 J mol^-1 K^-1

C_p[H₂O(s)] = 36.8 J mol^-1 K^-1

Calculate the number of kJ of heat necessary to raise the temperatureof 60.0 g of aluminium from 35°C to 55°C. Molar heat capacity of Al is 24 J mol^-1 K^-1.

The reaction of cyanamide, NH₂CN_(s),with dioxygen was carried out in a bomb calorimeter, and ΔU was found to be -742.7 kJ mol^-1at 298 K. Calculate enthalpy change for the reaction at 298 K.

In a process, 701 J of heat is absorbed by a system and 394 J ofwork is done by the system. What is the change in internal energy for the process?

A reaction, A + B → C + D + qis found to have a positive entropy change. The reaction will be

(i) possible at high temperature

(ii) possible only at low temperature

(iii) not possible at any temperature

(iv) possible at any temperature

The enthalpy of combustion of methane, graphite and dihydrogen at 298 K are, -890.3 kJ mol-1 -393.5 kJ mol-1, and -285.8 kJ mol-1 respectively. Enthalpy of formation of CH4(g) will be

(i) -74.8 kJ mol-1             (ii) -52.27 kJ mol-1

(iii) +74.8 kJ mol-1           (iv) +52.26 kJ mol-1.

ΔUθof combustion of methane is - XkJ mol-1. The value of ΔHθis

(i) = ΔUθ

(ii) > ΔUθ

(iii) < ΔUθ

(iv) = 0

The enthalpies of all elements in their standard states are:

(i) unity

(ii) zero

(iii) < 0

(iv) different for each element

For the process to occur under adiabatic conditions, the correct condition is:

(i) ΔT = 0

(ii) Δp = 0

(iii) q = 0

(iv) w= 0

Choose the correct answer. A thermodynamic state function is a quantity

(i) used to determine heat changes

(ii) whose value is independent of path

(iii) used to determine pressure volume work

(iv) whose value depends on temperature only.