# Thermodynamics

### Question:

Calculate the number of kj of heat necessary to raise the temperature of 60 g of aluminium from 35 °C to 55 °C. Molar heat capacity of Al is 24  J mol-1 K-1.

No. of moles of Al (m) = (60g)/(27 g mol-1)  = 2.22 mol
Molar heat capacity (C) = 24  J mol-1 K-1.
Rise in temperature (âˆ†T) = 55 – 35 = 20 °C = 20 K
Heat evolved (q) = C x m x T = (24  J mol-1 K-1) x (2.22 mol) x (20 K)
= 1065.6 J = 1.067 kj

### Thermodynamics

#### Q 1.

The difference between Cp and Cv can be derived using the empirical relation H = U + pV. Calculate the difference between Cp and Cv for 10 moles of an ideal gas.

#### Q 2.

What do you mean by entropy?

#### Q 3.

Predict the sign of  âˆ†S for the following reaction  heat
CaCO3 (s) ———> CaO(s) + CO2(g)

#### Q 5.

What is a spontaneous change? Give one example.

#### Q 6.

1 g of graphite is burnt in a bomb calorimeter in excess of oxygen at 298 K and 1 atmospheric pressure according to the equation C(graphite) + 02 (g) â€”> C02 (g) During the reaction, temperature rises from 298 K to 299 K. If the heat capacity of the bomb calorimeter is 20.7 kJ/K, what is the enthalpy change for the above reaction at 298 K and 1 atm?

#### Q 7.

At 298 K, Kp for the reaction N204(g)â‡Œ 2N02(g) is 0.98. Predict whether the reaction is spontaneous or not.

#### Q 8.

Thermodynamics mainly deals with
(a) interrelation of various forms of energy and their transformation front one from  to another.
(b) energy changes in the processes which depend only on initial and final states of the microscopic system containing a few molecules.
(c) how and at what rate these energy transformations are carried out.
(d) the system in equilibrium state or moving from one equilibrium state to another equilibrium state.

#### Q 9.

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

#### Q 10.

Calculate the enthalpy change on freezing of 1.0 mol of water at 10.0 °C to ice at – 10.0 °C. A, H = 6.03 KJ mot1 at 0 °C. Cp [H20(l)J = 75.3  J mol-1 K-1; Cp [H20(s)J = 36.8  J mol-1 K-1.

#### Q 11.

For the reaction; 2Cl(g) ———-> Cl2(g); what will be the signs of âˆ†H and âˆ†S?

#### Q 12.

Many thermodynamically feasible reactions do not occur under ordinary conditions. Why?

#### Q 13.

What are the units of entropy?

#### Q 15.

Give reason for the following:
(a)Neither q nor w is a state function but q + w is a state function.
(b)A real crystal has more entropy than an ideal crystal.

#### Q 16.

Enthalpy of sublimation of a substance is equal to
(a) enthalpy of fusion + enthalpy of vapourisation
(b) enthalpy of fusion
(c) enthalpy of vapourisation
(d) twice the enthalpy of vapourisation.

#### Q 17.

The reaction of cyanamide,NH2CN(s) with dioxygen was carried out in a bomb calorimeter and âˆ†U was found to be -742,7  KJ-1   mol-1  at 298 K. Calculate the enthalpy change for the reaction at 298 K.NH2CN  (S) + 3/202(g) —–>N2(g) + CO2(g) + H20(Z)

#### Q 18.

Calculate the standard enthalpy of formation of CH3OH. from the following data:
(i) CH3OH(l) + 3/2 02 (g) ———-> CO2 (g) + 2H20 (l); âˆ†rH = – 726kj mol-1
(ii) C(s) + 02(g) —————>C02 (g); âˆ†cH = -393 kj mol-1
(iii) H2(g) + 1/202(g) —————->H20 (l); âˆ†fH = -286 kj mol-1

#### Q 19.

What is free energy in terms of thermodynamics?

#### Q 20.

How are internal energy change, free energy change and entropy change are related to one another?

#### Q 21.

Define intensive properties.

#### Q 22.

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

The work can also be calculated from the pV

plot by using the area under curve within the specified limits. When an ideal gas is compressed (a) reversibly or (b) irreversibly from Vi to Vf, choose the correct option.
(a) w (reversible) = w (irreversible)
(b) w (reversible) < w (irreversible)
(c) w (reversible) > w (irreversible)
(d) w (reversible) = w (irreversible) + pex. âˆ†V

#### Q 23.

In an exothermic reaction, heat is evolved, and system loses heat to the surroundings. For such system
(a) qP will be negative
(b) âˆ†Î³Hwill be negative
(c) qp will be positive
(d) âˆ†Î³Hwill be positive.

#### Q 24.

The standard molar entropy of H2O(l) is 70 J K-1 mol-1. Will the standard molar entropy H20(s) be more, or less than 70 J K -1 mol-1?

#### Q 25.

When is bond energy equal to bond dissociation energy ?

#### Q 26.

Predict in which of the following, entropy increases/decreases.
(i) A liquid crystallizes into a solid
(ii) Temperature of a crystallize solid is raised from OK to 115 K
(iii) 2NaHCO3 (s) ———-> Na2 C03 (s) + C02 (g) + H2O (g)
(iv) H2(g)——>2H(g)

#### Q 27.

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 enthalpy of vapourisation for water?

#### Q 28.

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.

#### Q 29.

Given : N2(g) + 3H2(g) ————> 2NH3(g); âˆ†r H  = -92.4 kj mot-1  What is the standard  enthalpy of formation of NH3 gas?

#### Q 31.

Calculate the enthalpy change for the reaction: H2(g) + Cl2(g) ————-> 2HCl(g). Given that  bond energies ofH-H, Cl- Cl and H-Cl bonds are 433, 244 and 431 kj mol-1  respectively.

#### Q 32.

(a)What is a spontaneous process? Mention the conditions for a reaction to be spontaneous at constant temperature and pressure.
(b) Discuss the effect of temperature on the spontaneity of an exothermic reaction.

#### Q 33.

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

#### Q 34.

One mole of acetone requires less heat to vapourise than 1 mol of water. Which of the two liquids has higher enthalpy of vapourisation?

#### Q 35.

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

#### Q 36.

What is Gibbs Helmholtz equation?

#### Q 37.

The enthalpy of formation of methane at constant pressure and 300 K is – 78.84 kJ. What will be the enthalpy of formation at constant volume?

#### Q 38.

A sample of 1.0 mol of a monoatomic ideal gas is taken through a cyclic process of expansion and compression as shown in the figure. What will be the value of Î”Hfor the cycle as a whole?

#### Q 39.

What is the enthalpy of formation of the most stable form of an element in its standard state?

#### Q 40.

What is the enthalpy change for an adiabatic process?

#### Q 41.

Define extensive properties.

#### Q 43.

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 CHJg) 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

#### Q 44.

If âˆ†U = 0 how are q and w related to each other?

#### Q 45.

Define the following:
(i) First law of thermodynamics.
(ii) Standard enthalpy of formation.

#### Q 46.

Identify the state functions and path functions out of the following: enthalpy, entropy, heat, temperature, work, free energy.

#### Q 47.

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.

#### Q 48.

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.

#### Q 49.

Calculate the number of kj of heat necessary to raise the temperature of 60 g of aluminium from 35 °C to 55 °C. Molar heat capacity of Al is 24  J mol-1 K-1.

#### Q 50.

At what temperature entropy of a substance is zero?