1 Marks Question

Question 11 Mark

Why for predicting the spontaneity of a reaction, free energy criteria is better than the entropy criteria?

Answer

Criteria of free energy change is better because it requires free energy change of the system only whereas the entropy change requires the total entropy change of the system and the surroundings.

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Question 21 Mark

When is the entropy of a perfectly crystalline solid zero?

Answer

The entropy of a perfectly crystalline solid is zero at absolute zero temperature, i.e. 0K or -273.15°C because there is perfect order and no disorder at this temperature.

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Question 31 Mark

In the following Questions, the Assertion and Reason have been put forward. Read the statements carefully and choose the correct alternative from the following:

Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
Assertion is true but Reason is false.
Both Assertion and Reason are false.

Assertion: Heat capacity is the amount of heat required to raise the temperature of a body by 1K.
Reason: Heat capacity is an extensive property and It depends upon the size of the body.

Answer

Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.

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Question 41 Mark

If enthalpy of fusion and enthalpy of vaporisation of sodium metals are 2.6 and 98.2kJ mol^-1 respectively, what is the enthalpy of sublimation of sodium.

Answer

$\Delta_\text{sub}\text{H}^\circ=\Delta_\text{fus}\text{H}^\circ+\Delta_\text{vap}\text{H}^\circ$

$=2.6+98.2=100.8\text{kJ mol}^{-1}$

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Question 51 Mark

Write an expression in the form of chemical equation for the standard enthalpy of formation $(\Delta_\text{f}\text{H})$ of CO(g).

Answer

$\text{C(s)}+\frac{1}{2}\text{O}_2(\text{g})\rightarrow\text{CO(g)}$

1 mole of compound is funned from constituting elements in their standard states enthalpy change is $\Delta_\text{f}\text{H}^\circ.$

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Question 61 Mark

Give two examples of state functions.

Answer

$\Delta\text{H}$ (Enthalpy change) and $\Delta\text{U}$ (Internal energy change) are state functions as they depend upon initial and final state and not on the path.

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Question 71 Mark

What happens to the internal energy of the system if:

Work is done on the system?
Work is done by the system?

Answer

If work is done on the system, internal energy will increase.

$\because\Delta\text{U}=\text{q}+\text{w}$

If work is done by the system, internal energy will decrease.

$\because\Delta\text{U}=\text{q}-\text{w}$

[work is -ve when done by the system.]

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Question 81 Mark

How is change in entropy during melting of solid related to its melting point?

Answer

$\Delta\text{S}_\text{fusion}=\frac{\Delta\text{H}_\text{fusion}}{\text{Melting point in kelvin}}$

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Question 91 Mark

How is standard free energy change related to equilibrium constant?

Answer

$\Delta\text{G}^\circ=-2.303\text{RT}\log\text{K}$ where $\Delta\text{G}^\circ$ is standard free energy change, R is gas constant, T is temperature in Kelvin, 'K' is equilibrium constant.

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Question 101 Mark

If the gibbs free energy is negative than reaction will be?

Always positive
Sometimes negative
Non-spontaneous
Spontaneous
None of the above

Answer

Spontaneous

Explanation:

(D): Always negative.

ΔG = 0, it means the reaction is equilibrium at standard conditions.

A negative value of ΔG, means spontaneous.

A positive value of ΔG, means non-spontaneous.

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Question 111 Mark

Determine the sign of entropy change in N₂(g) + O₂(g) → 2NO(g).

Answer

$\Delta\text{S}=+\text{ve}$ because it is endothermic process and spontaneous. $\Delta\text{G}^\circ$ will -ve only if $\Delta\text{S}$ is +ve.

$\because\Delta\text{G}^\circ=\Delta\text{H}^\circ-\text{TDS}^\circ$

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Question 121 Mark

Name the thermodynamic system to which following belong:

Human body.
Milk in thermos flask.

Answer

Human body is an open system because it can exchange matter as well as energy with the surroundings.
Milk in thermos flask is an isolated system which can neither exchange matter nor energy with the surroundings.

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Question 131 Mark

What is the value of internal energy for 1 mole of a monoatomic gas?

Answer

$\text{U}=\frac{3}{2}\text{RT}$ for monatomic gas.

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Question 141 Mark

Name the state variables which remain constant in:

Isobaric process.
Isothermal process.

Answer

Isobaric process: In this process, pressure remains constant i.e. $\Delta\text{p}=0.$
Isothermal process: In this process, temperature remains constant i.e. $\Delta\text{T}=0.$

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Question 151 Mark

Choose the correct answer.
The enthalpies of all elements in their standard states are:

Unity
Zero
< 0
Different for each element.

Answer

Zero

Explanation:

The enthalpy of all elements in their standard state is zero.

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Question 161 Mark

In the following Questions, the Assertion and Reason have been put forward. Read the statements carefully and choose the correct alternative from the following:

Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
Assertion is true but Reason is false.
Both Assertion and Reason are false.

Assertion: An isolated system is the one which can neither exchange matter nor energy with the surroundings.

Reason: It should be noted that every system is perfectly isolated.

Answer

Assertion is true but Reason is false.

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Question 171 Mark

When 430J of work was done on a system, it lost 120J of energy as heat. Calculate the value of internal energy change $(\Delta\text{U})$ for this process.

Answer

$\Delta\text{U}=\text{q}+\text{w}$ $\text{q}=-120\text{J}$

$\Delta\text{U}=-120+430$ $\text{w}=+430\text{J}$

$\Delta\text{U}=310\text{J}$

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Question 181 Mark

The energy released in the neutralisation of H₂SO₄ and KOH is 57.1kJ mol^-1. Therefore, calculate the value of $\Delta\text{H}$ for the reaction:
H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O

Answer

$\Delta\text{H}=-2\times57.1=-114.2\text{kJ}$ because 2 moles of H⁺ from H₂SO₄, are reacting with 2 moles of OH^- from 2KOH to form 2 moles of water.

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Question 191 Mark

What are the ways by which the internal energy of a system can be changed?

Answer

Exchanging heat with the surroundings.
Work done on the system or by the system.

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Question 201 Mark

Predict the sign of entropy change for the change.
H₂O(g) → H₂O(l)

Answer

$\Delta\text{S}=-\text{ve}$ because disorder or randomness is less in liquid water than steam.

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Question 211 Mark

Identify the species and physical state for which $\Delta_\text{f}\text{H}^\circ=0$ at 298K.

Br₂, Cl₂, CH₄

Answer

$\Delta_\text{f}\text{H}^\circ\text{Br}_2(\text{l})=0\ \ \Delta_\text{f}\text{H}^\circ\text{Cl}_2(\text{g})=0$

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Question 221 Mark

Predict $\Delta\text{H}>\Delta\text{U}$ or $\Delta\text{H}$ is less than $\Delta\text{U}$ or $\Delta\text{H}=\Delta\text{U}.$

C(graphite) + O₂(g) → CO₂(g)
PCl₅(g) → PCl₃(g) + Cl₂(g)

Answer

$\Delta\text{H}=\Delta\text{U}$ because $\Delta\text{n}=0$ i.e. number of moles of gaseous reactants and products are equal.
$\Delta\text{H}>\Delta\text{U}$ because $\Delta\text{n}=1\ [\Delta\text{H}=\Delta\text{U}+\Delta\text{nRT}]$

$\because$ Numbers of moles of gaseous products are more than gaseous reactants.

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Question 231 Mark

Predict the sign of $\Delta\text{S}^\circ$ for the following reaction:
2H₂S(g) + 3O₂(g) → 2H₂O(g) + 2SO₂(g)

Answer

$\Delta\text{S}^\circ,$ i.e. entropy decreases during the reaction because 5 moles of gaseous reactants change into 4 moles of gaseous products. Hence, the sign of $\Delta\text{S}^\circ$ will be negative in the given reaction.

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Question 241 Mark

If $\Delta\text{H}$ for a reaction has a negative value, how would you know the sign requirement of $\Delta\text{S}$ for it so that the reaction is spontaneous at low temperatures?

Answer

$\Delta\text{S}$ must be positive, then only $\Delta\text{G}$ will be negative and reaction will spontaneous.
$\because\Delta\text{G}^\circ=\Delta\text{H}^\circ-\text{T}\Delta\text{S}^\circ$

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Question 251 Mark

At 1atm will the $\Delta_\text{f}\text{H}^\circ$ be zero for Cl₂(g) and Br₂(g)? Explain.

Answer

$\Delta_\text{f}\text{H}^\circ$ for Cl₂(g) will be zero but $\Delta_\text{f}\text{H}^\circ$ for Br₂(g) will not be zero because liquid bromine is its elementary state and not gaseous bromine.

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Question 261 Mark

Under what conditions $\Delta\text{H}$ and $\Delta\text{U}$ are equal?

Answer

When $\Delta\text{n}=0,\ \Delta\text{H}=\Delta\text{U}$

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Question 271 Mark

What is the value of $\Delta\text{G}$ when ice and water are in equilibrium?

Answer

$\Delta\text{G}=0$ at equilibrium.

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Question 281 Mark

State a chemical reaction in which $\Delta\text{H}$ and $\Delta\text{U}$ are equal.

Answer

H₂(g) + I₂(g) → 2HI(g)

Since $\Delta\text{n}=0,\ \therefore\Delta\text{H}=\Delta\text{U}.$

$\because$ Number of moles of gaseous reactants and products are equal.

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Question 291 Mark

Predict the sign of $\Delta\text{S}$ for the reaction:
2NaHCO₃(s) → Na₂CO₃(s) + CO₂(g) + H₂O(g)

Answer

$\Delta\text{S}=+\text{ve}.$ because more number of products are being formed and solid reactant is changing to solid and gas.

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Question 301 Mark

For the same increase in volume, why work done is more if the gas is allowed to expand reversibly at higher temperature?

Answer

For isothermal reversible expansion, $\text{W}=-\text{p}_\text{int}\times\Delta\text{V}.$
At higher temperature, internal pressure of the gas is more so work done is more.

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Question 311 Mark

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

Answer

Water has higher enthalpy of vapourisation. $(\Delta\text{H}_\text{r})_\text{water}>(\Delta\text{H}_\text{r})_\text{acetone}$

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Question 321 Mark

For the reaction, N₂(g) + 3H₂(g) → 2NH₃(g), predict whether the work is done on the system or by the system.

Answer

Volume is decreasing, therefore, work is done on the system.

$\because\ \text{w}=-\text{P}\Delta\text{V,}$

$\Delta\text{V}=-\text{ve}$

$\text{w}=+\text{ve}$

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Question 331 Mark

Air contains about 99% of N₂ and O₂ gases. Why do not they combine to form NO under the standard conditions? Standard Gibbs energy of formation of NO(g) is 86.7kJ mol^-1.

Answer

Sol. For the combination of N, and O₂ to form NO, the standard Gibbs energy of formation, is +ve

$\frac{1}{2}\text{N}_2(\text{g})+\frac{1}{2}\text{O}_2(\text{g})\overrightarrow{\ \ \ \ \ \ }\ \text{NO(g)}$ $\big(\Delta_\text{f}\text{G}^\circ_\text{NO}=+86.7\text{kJ mol}^{-1}\big)$

Therefore, this reaction is non-spontaneous under the standard conditions and hence N₂ and O₂ do not combine.

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Question 341 Mark

Neither q nor W is a state function but q + W is state function. Explain why?

Answer

Both q and W are not state functions but q + W is equal to $\Delta\text{U}$ which is a state function.

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Question 351 Mark

Under what condition, the heat evolved or absorbed in a reaction is equal to its free energy change?

Answer

As $\Delta\text{G}=\Delta\text{H}-\text{T}\Delta\text{S}.$ Thus, $\Delta\text{G}=\Delta\text{H}$ only when either the reaction is carried out at 0K or the reaction is not accompanied by any entropy change, i.e. $\Delta\text{S}=0.$

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Question 361 Mark

Will the change in enthalpy of the system be zero in an adiabatic process?

Answer

Yes, In adiabatic process, $\Delta\text{H}=0$ because q = 0 and $\Delta\text{H}=\text{q}_\text{p}=0$

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Question 371 Mark

Why is standard enthalpy of formation of diamond not zero although it is an element?

Answer

It is because diamond is not standard state of element. Standard state of carbon is graphite whose $\Delta_\text{f}\text{H}^\circ=0.$

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Question 381 Mark

Give an example of a spontaneous process which is endothermic.

Answer

N₂(g) + O₂(g) → 2NO(g)
It is endothermic as well as spontaneous because entropy is increasing and $\Delta\text{G}=-\text{ve.}$

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Question 391 Mark

Why does NH₄NO₃ dissolve in water spontaneously even when this process is endothermic?

Answer

NH₄NO₃dissolves in water spontaneously even when this process is endothermic because entropy increasing due to free movement of ions on dissolving. $\Delta\text{S}=+\text{ve}$ favours the process and makes it spontaneous.

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Question 401 Mark

Classify the following as extensive and intensive properties:
Molar heat capacity, Temperature, Enthalpy and Volume.

Answer

Extensive property: Volume, Enthalpy because these depend upon amount of substances.
Intensive property: Temperature, Molar heat capacity because these do not depend upon amount of substances.

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Question 411 Mark

Water can be lifted into the water tank at the top of the house with the help of a pump. Then why is it not considered to be spontaneous?

Answer

A spontaneous process should occur continuously by itself after initiation. But this is not so in the given case because water will go up as long as the pump is working.

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Question 421 Mark

How can you say that universe is going towards chaos?

Answer

Most of the naturally occurring processes are accompanied by increase of randomness. Hence, randomness of the universe is continuously increasing. Thus, we are going towards chaos.

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Question 431 Mark

H₂(g) + Cl₂(g) → 2HCl(g) + 185kJ. State whether this reaction Is exothermic or endothermic and why.

Answer

It is an exothermic reaction because heat is being evolved.

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Question 441 Mark

Which of the following is an intensive property?
Surface tension, Mass, Volume, Enthalpy, Density.

Answer

Surface tension and density are intensive properties because these do not depend upon amount of substance.

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Question 451 Mark

What is the limitation of first law of thermodynamics?

Answer

It cannot tell us the direction of the process i.e. feasibility of Process.

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Question 461 Mark

Which one of the following is not extensive state function?
Enthalpy change, Internal energy change and Pressure.

Answer

Pressure is not a state function because it depends upon path.

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Question 471 Mark

Why is entropy of a solution higher than that of pure liquid?

Answer

In solution, there is more disorderness than in pure liquid, so entropy is more.

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Question 481 Mark

What is the effect of temperature on entropy?

Answer

Entropy increases with increase in temperature because disorder or randomness increases.

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Question 491 Mark

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

Answer

The standard molar entropy of H₂O(l) is 70JK^-1mol^-1. The solid form of H₂O is ice. In ice, molecules of H₂O are less random than in liquid water. Thus, molar entropy of H₂O(s) < molar entropy of H₂O(l). The standard molar entropy of H₂O(s) is less than 70JK^-1mol^-1.

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Question 501 Mark

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l); $\Delta\text{H}=-890\text{kJ mol}^{-1}$
What is the calorific or fuel value of 1kg of CH₄?

Answer

Calorific value/ kg $=\frac{890}{16}\times1000=55625\text{kJ/kg}$

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Question 511 Mark

Which have more entropy, real crystal or Ideal crystal and why?

Answer

Real crystal has more entropy because it has more disorderness or randomness.

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Question 521 Mark

In the equation, $\text{N}_2(\text{g})+3\text{H}_2(\text{g})\rightleftharpoons2\text{NH}_3(\text{g})$ would be the sign of work done?

Answer

The sign of work done will be positive, i.e. work will be done on the system due to decrease in volume.

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Question 531 Mark

Why does entropy increase on mixing of two gases?

Answer

The disorder increases when two gases are mixed together, that is why entropy increases.

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Question 541 Mark

State why heat changes in physical and chemical processes are indicated by enthalpy changes and not by internal energy changes.

Answer

Most of the processes are carried out in open container, at constant pressure. Therefore, enthalpy changes indicate heat changes in processes and not internal energy changes. Enthalpy changes are measured at constant pressure where as internal energy change is measured at constant volume.

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Question 551 Mark

Why in some reactions heat is evolved while some reactions take place only on absorption of heat?

Answer

Every substance has energy stored in it in the form of heat content. If heat content of reactants (H_R) is greater than that of products (H_p), heat is evolved. If H_R < H_p, heat is absorbed.

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Question 561 Mark

Which of the following has the highest entropy?
H₂(g), F₂(g), Cl₂(g), Br₂(g), I₂(g)

Answer

I₂(g) has maximum entropy due to presence of largest number of electrons.

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Question 571 Mark

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

Answer

State functions: Enthalpy, entropy, temperature, free energy Path functions: Heat, work.

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Question 581 Mark

When an ideal gas expands into vacuum, there is neither absorption nor evolution of heat. Why?

Answer

In an ideal gas, there are no intermolecular forces of attraction. Hence, no energy is required to overcome these forces. Moreover, when a gas expands against vacuum, work done is zero (because Pe_xt = 0). Hence, internal energy of the system does not change, i.e. there is neither absorption nor evolution of heat.

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Question 591 Mark

Define standard enthalpy of formation.

Answer

It is defined as enthalpy change when I mole of substance is fanned from constituenting elements in their standard states.

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Question 601 Mark

The fact that enthalpy is a state function forms the basis of a very useful law. Name the law.

Answer

Hess's law of heat summation.

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Question 611 Mark