Chapter 6 Thermodynamics

Question 1. Thermodynamics is not concerned about______.

(i) energy changes involved in a chemical reaction.

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

(iii) the rate at which a reaction proceeds.

(iv) the feasibility of a chemical reaction.

Question 2. Which of the following statements is correct?

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

(ii) There is an exchange of energy as well as matter between the system and the surroundings in a closed system.

(iii) The presence of reactants in a closed vessel made up of copper is an example of a closed system.

(iv) The presence of reactants in a thermos flask or any other closed insulated vessel is an example of a closed system.

Question 3. The state of a gas can be described by quoting the relationship between___.

(i) pressure, volume, temperature

(ii) temperature, amount, pressure

(iii) amount, volume, temperature

(iv) pressure, volume, temperature, amount

Question 4. The volume of gas is reduced to half from its original volume. The specific heat will be ______.

(i) reduce to half

(ii) be doubled

(iii) remain constant

(iv) increase four times

Question 5. During complete combustion of one mole of butane, 2658 kJ of heat is released. The thermochemical reaction for above change is

(i) $2C_4H_{10}(g) + 13O_2(g) \rightarrow 8CO_2(g) + 10H_2O(l) \quad \Delta_cH = -2658.0 \text{ kJ mol}^{-1}$

(ii) $C_4H_{10}(g) + \frac{13}{2}O_2(g) \rightarrow 4CO_2(g) + 5H_2O(g) \quad \Delta_cH = -1329.0 \text{ kJ mol}^{-1}$

(iii) $C_4H_{10}(g) + \frac{13}{2}O_2(g) \rightarrow 4CO_2(g) + 5H_2O(l) \quad \Delta_cH = -2658.0 \text{ kJ mol}^{-1}$

(iv) $C_4H_{10}(g) + \frac{13}{2}O_2(g) \rightarrow 4CO_2(g) + 5H_2O(l) \quad \Delta_cH = +2658.0 \text{ kJ mol}^{-1}$

Question 6. $\Delta_fU^\ominus$ of formation of $CH_4(g)$ at certain temperature is $–393 \text{ kJ mol}^{-1}$. The value of $\Delta_fH^\ominus$ is

(i) zero

(ii) < $\Delta_f U^\ominus$

(iii) > $\Delta_f U^\ominus$

(iv) equal to $\Delta_f U^\ominus$

Question 7. 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.

(i) $q = 0, \Delta T \neq 0, w = 0$

(ii) $q \neq 0, \Delta T = 0, w = 0$

(iii) $q = 0, \Delta T = 0, w = 0$

(iv) $q = 0, \Delta T < 0, w \neq 0$

Question 8. The pressure-volume work for an ideal gas can be calculated by using the expression $w = -\int_{V_i}^{V_f} p_{ex} dV$. The work can also be calculated from the pV– plot by using the area under the curve within the specified limits. When an ideal gas is compressed (a) reversibly or (b) irreversibly from volume $V_i$ to $V_f$. choose the correct option.

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

(ii) w (reversible) < w (irreversible)

(iii) w (reversible) > w (irreversible)

(iv) w (reversible) = w (irreversible) + $p_{ex}.\Delta V$

Question 9. The entropy change can be calculated by using the expression $\Delta S = \frac{q_{rev}}{T}$. When water freezes in a glass beaker, choose the correct statement amongst the following :

(i) $\Delta S$ (system) decreases but $\Delta S$ (surroundings) remains the same.

(ii) $\Delta S$ (system) increases but $\Delta S$ (surroundings) decreases.

(iii) $\Delta S$ (system) decreases but $\Delta S$ (surroundings) increases.

(iv) $\Delta S$ (system) decreases and $\Delta S$ (surroundings) also decreases.

Question 10. On the basis of thermochemical equations (a), (b) and (c), find out which of the algebric relationships given in options (i) to (iv) is correct.

(a) $C(graphite) + O_2(g) \rightarrow CO_2(g); \quad \Delta_rH = x \text{ kJ mol}^{-1}$

(b) $C(graphite) + \frac{1}{2}O_2(g) \rightarrow CO(g); \quad \Delta_rH = y \text{ kJ mol}^{-1}$

(c) $CO(g) + \frac{1}{2}O_2(g) \rightarrow CO_2(g); \quad \Delta_rH = z \text{ kJ mol}^{-1}$

(i) z = x + y

(ii) x = y – z

(iii) x = y + z

(iv) y = 2z – x

Question 11. Consider the reactions given below. On the basis of these reactions find out which of the algebric relations given in options (i) to (iv) is correct?

(a) $C(g) + 4H(g) \rightarrow CH_4(g); \quad \Delta_rH = x \text{ kJ mol}^{-1}$

(b) $C(graphite,s) + 2H_2(g) \rightarrow CH_4(g); \quad \Delta_rH = y \text{ kJ mol}^{-1}$

(i) x = y

(ii) x = 2y

(iii) x > y

(iv) x < y

Question 12. The enthalpies of elements in their standard states are taken as zero. The enthalpy of formation of a compound

(i) is always negative

(ii) is always positive

(iii) may be positive or negative

(iv) is never negative

Question 13. Enthalpy of sublimation of a substance is equal to

(i) enthalpy of fusion + enthalpy of vapourisation

(ii) enthalpy of fusion

(iii) enthalpy of vapourisation

(iv) twice the enthalpy of vapourisation

Question 14. Which of the following is not correct?

(i) $\Delta G$ is zero for a reversible reaction

(ii) $\Delta G$ is positive for a spontaneous reaction

(iii) $\Delta G$ is negative for a spontaneous reaction

(iv) $\Delta G$ is positive for a non-spontaneous reaction

Question 15. Thermodynamics mainly deals with

(i) interrelation of various forms of energy and their transformation from one form to another.

(ii) energy changes in the processes which depend only on initial and final states of the microscopic systems containing a few molecules.

(iii) how and at what rate these energy transformations are carried out.

(iv) the system in equilibrium state or moving from one equilibrium state to another equilibrium state.

Question 16. In an exothermic reaction, heat is evolved, and system loses heat to the surrounding. For such system

(i) $q_p$ will be negative

(ii) $\Delta_rH$ will be negative

(iii) $q_p$ will be positive

(iv) $\Delta_rH$ will be positive

Question 17. The spontaneity means, having the potential to proceed without the assistance of external agency. The processes which occur spontaneously are

(i) flow of heat from colder to warmer body.

(ii) gas in a container contracting into one corner.

(iii) gas expanding to fill the available volume.

(iv) burning carbon in oxygen to give carbon dioxide.

Question 18. For an ideal gas, the work of reversible expansion under isothermal condition can be calculated by using the expression $w = - nRT \ln \frac{V_f}{V_i}$.

A sample containing 1.0 mol of an ideal gas is expanded isothermally and reversibly to ten times of its original volume, in two separate experiments. The expansion is carried out at 300 K and at 600 K respectively. Choose the correct option.

(i) Work done at 600 K is 20 times the work done at 300 K.

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

(iii) Work done at 600 K is twice the work done at 300 K.

(iv) $\Delta U = 0$ in both cases.

Question 19. Consider the following reaction between zinc and oxygen and choose the correct options out of the options given below :

$2Zn(s) + O_2(g) \rightarrow 2ZnO(s); \quad \Delta H = – 693.8 \text{ kJ mol}^{-1}$

(i) The enthalpy of two moles of ZnO is less than the total enthalpy of two moles of Zn and one mole of oxygen by 693.8 kJ.

(ii) The enthalpy of two moles of ZnO is more than the total enthalpy of two moles of Zn and one mole of oxygen by 693.8 kJ.

(iii) 693.8 kJ mol–1 energy is evolved in the reaction.

(iv) 693.8 kJ mol–1 energy is absorbed in the reaction.

Question 20. 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?

Question 21. One mole of acetone requires less heat to vapourise than 1 mol of water. Which of the two liquids has higher enthalpy of vapourisation?

Question 22. Standard molar enthalpy of formation, $\Delta_f H^\ominus$ is just a special case of enthalpy of reaction, $\Delta_r H^\ominus$. Is the $\Delta_r H^\ominus$ for the following reaction same as $\Delta_f H^\ominus$? Give reason for your answer.

$CaO(s) + CO_2(g) \rightarrow CaCO_3(s); \quad \Delta_f H^\ominus = –178.3 \text{ kJ mol}^{-1}$

Question 23. The value of $\Delta_f H^\ominus$ for $NH_3$ is $– 91.8 \text{ kJ mol}^{-1}$. Calculate enthalpy change for the following reaction :

$2NH_3(g) \rightarrow N_2(g) + 3H_2(g)$

Question 24. Enthalpy is an extensive property. In general, if enthalpy of an overall reaction A → B along one route is $\Delta_r H$ and $\Delta_r H_1, \Delta_r H_2, \Delta_r H_3$ ..... represent enthalpies of intermediate reactions leading to product B. What will be the relation between $\Delta_r H$ for overall reaction and $\Delta_r H_1, \Delta_r H_2$ ..... etc. for intermediate reactions.

Question 25. The enthalpy of atomisation for the reaction $CH_4(g) \rightarrow C(g) + 4H(g)$ is $1665 \text{ kJ mol}^{-1}$. What is the bond energy of C–H bond?

Question 26. Use the following data to calculate $\Delta_{lattice}H^\ominus$ for NaBr.

$\Delta_{sub}H^\ominus$ for sodium metal = $108.4 \text{ kJ mol}^{-1}$

Ionization enthalpy of sodium = $496 \text{ kJ mol}^{-1}$

Electron gain enthalpy of bromine = $– 325 \text{ kJ mol}^{-1}$

Bond dissociation enthalpy of bromine = $192 \text{ kJ mol}^{-1}$

$\Delta_f H^\ominus$ for $NaBr(s) = – 360.1 \text{ kJ mol}^{-1}$

Question 27. Given that $\Delta H = 0$ 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?

Question 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.

Question 29. 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?

Question 30. At 298 K. $K_p$ for the reaction $N_2O_4(g) \rightleftharpoons 2NO_2(g)$ is 0.98. Predict whether the reaction is spontaneous or not.

Question 31. A sample of 1.0 mol of a monoatomic ideal gas is taken through a cyclic process of expansion and compression as shown in Fig. 6.1. What will be the value of $\Delta H$ for the cycle as a whole?

Question 32. The standard molar entropy of $H_2O(l)$ is $70 \text{ J K}^{-1} \text{mol}^{-1}$. Will the standard molar entropy of $H_2O(s)$ be more, or less than $70 \text{ J K}^{-1} \text{mol}^{-1}$?

Question 33. Identify the state functions and path functions out of the following : enthalpy, entropy, heat, temperature, work, free energy.

Question 34. The molar enthalpy of vapourisation of acetone is less than that of water. Why?

Question 35. Which quantity out of $\Delta_rG$ and $\Delta_rG^\ominus$ will be zero at equilibrium?

Question 36. Predict the change in internal energy for an isolated system at constant volume.

Question 37. 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.

Question 38. 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?

Question 39. Heat capacity ($C_p$) is an extensive property but specific heat (c) is an intensive property. What will be the relation between $C_p$ and c for 1 mol of water?

Question 40. The difference between $C_P$ and $C_V$ can be derived using the empirical relation $H = U + pV$. Calculate the difference between $C_P$ and $C_V$ for 10 moles of an ideal gas.

Question 41. 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.

Question 42. The net enthalpy change of a reaction is the amount of energy required to break all the bonds in reactant molecules minus amount of energy required to form all the bonds in the product molecules. What will be the enthalpy change for the following reaction.

$H_2(g) + Br_2(g) \rightarrow 2HBr(g)$

Given that Bond energy of $H_2$, $Br_2$ and HBr is $435 \text{ kJ mol}^{-1}$, $192 \text{ kJ mol}^{-1}$ and $368 \text{ kJ mol}^{-1}$ respectively.

Question 43. The enthalpy of vapourisation of $CCl_4$ is $30.5 \text{ kJ mol}^{-1}$. Calculate the heat required for the vapourisation of 284 g of $CCl_4$ at constant pressure. (Molar mass of $CCl_4 = 154 \text{ g mol}^{-1}$).

Question 44. The enthalpy of reaction for the reaction :

$2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$ is $\Delta_rH^\ominus = – 572 \text{ kJ mol}^{-1}$.

What will be standard enthalpy of formation of $H_2O(l)$ ?

Question 45. What will be the work done on an ideal gas enclosed in a cylinder, when it is compressed by a constant external pressure, $p_{ext}$ in a single step as shown in Fig. 6.2. Explain graphically.

Question 46. How will you calculate work done on an ideal gas in a compression, when change in pressure is carried out in infinite steps?

Question 47. Represent the potential energy/enthalpy change in the following processes graphically.

(a) Throwing a stone from the ground to roof.

(b) $\frac{1}{2}H_2(g) + \frac{1}{2}Cl_2(g) \rightarrow HCl(g) \quad \Delta_rH^\ominus= –92.32 \text{ kJ mol}^{-1}$

In which of the processes potential energy/enthalpy change is contributing factor to the spontaneity?

Question 48. Enthalpy diagram for a particular reaction is given in Fig. 6.3. Is it possible to decide spontaneity of a reaction from given diagram. Explain.

Question 49. 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.

Question 50. An ideal gas is allowed to expand against a constant pressure of 2 bar from 10 L to 50 L in one step. Calculate the amount of work done by the gas. If the same expansion were carried out reversibly, will the work done be higher or lower than the earlier case?

(Given that 1 L bar = 100 J)

Question 51. Match the following :

A

(i) Adiabatic process

(ii) Isolated system

(iii) Isothermal change

(iv) Path function

(v) State function

(vi) $\Delta U = q$

(vii) Law of conservation of energy

(viii) Reversible process

(ix) Free expansion

(x) $\Delta H = q$

(xi) Intensive property

(xii) Extensive property

B

(a) Heat

(b) At constant volume

(c) First law of thermodynamics

(d) No exchange of energy and matter

(e) No transfer of heat

(f) Constant temperature

(g) Internal energy

(h) $p_{ext} = 0$

(i) At constant pressure

(j) Infinitely slow process which proceeds through a series of equilibrium states.

(k) Entropy

(l) Pressure

(m) Specific heat

Question 52. Match the following processes with entropy change:

Reaction

(i) A liquid vapourises

(ii) Reaction is non-spontaneous at all temperatures and $\Delta H$ is positive

(iii) Reversible expansion of an ideal gas

Entropy change

(a) $\Delta S = 0$

(b) $\Delta S = \text{positive}$

(c) $\Delta S = \text{negative}$

Question 53. Match the following parameters with description for spontaneity :

Parameters			Description
$\Delta_rH^\ominus$	$\Delta_rS^\ominus$	$\Delta_rG^\ominus$
(i) +	–	+	(a) Non-spontaneous at high temperature.
(ii) –	–	+ at high T	(b) Spontaneous at all temperatures
(iii) –	+	–	(c) Non-spontaneous at all temperatures

Question 54. Match the following :

(i) Entropy of vapourisation

(ii) K for spontaneous process

(iii) Crystalline solid state

(iv) $\Delta U$ in adiabatic expansion of ideal gas

(a) decreases

(b) is always positive

(c) lowest entropy

(d) $\frac{\Delta_{vap}H}{T_b}$

Question 55. 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.

Question 56. 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.

Question 57. 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.

Question 58. Derive the relationship between $\Delta H$ and $\Delta U$ for an ideal gas. Explain each term involved in the equation.

Question 59. Extensive properties depend on the quantity of matter but intensive properties do not. Explain whether the following properties are extensive or intensive.

Mass, internal energy, pressure, heat capacity, molar heat capacity, density, mole fraction, specific heat, temperature and molarity.

Question 60. 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).

Question 61. $\Delta G$ is net energy available to do useful work and is thus a measure of “free energy”. Show mathematically that $\Delta G$ is a measure of free energy. Find the unit of $\Delta G$. If a reaction has positive enthalpy change and positive entropy change, under what condition will the reaction be spontaneous?

Question 62. Graphically show the total work done in an expansion when the state of an ideal gas is changed reversibly and isothermally 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 carried out against a constant external pressure $p_f$.