Chemical Thermodynamics(12th Grade > Chemistry ) Questions and answers for exam Preparation

12th Grade > Chemistry

CHEMICAL THERMODYNAMICS MCQs

Total Questions : 30 | Page 2 of 3 pages

Question 11. In the conversion of limestone to lime:

C a C O_{3} (s) \to C a O (s) + C O_{2} (g)

The values of

△ H^{\circ}

and

△ S^{\circ}

are 179.1 kJ

m o l^{- 1}

and 160.2 J/K respectively at 298 K and 1 bar. Assuming

△ H^{\circ}

and

△ S^{\circ}

remains constant with temperature, at what temperature will the conversion of limestone to lime be spontaneous?

1118 K
1008 K
1200 K
845 K

Discuss Question

Answer: Option A. -> 1118 K
:
A
What is the criteria for spontaneity?
The

△ G^{\circ}

value.
We know that:

△ G^{\circ}

△ H^{\circ}

T △ S^{\circ}

For spontaneous reactions

△ G^{\circ}

= -ve

△ G^{\circ}

△ H^{\circ}

T △ S^{\circ}

∴ △ H^{\circ}

T △ S^{\circ}

Or T >

\frac{△ H^{\circ}}{△ S^{\circ}}

\frac{179.10 \times 10^{3}}{160.2}

> 1118 K

Question 12. A system undergoes a process in which system releases 200 J heat and work done by the surroundings is 300 J. Change in internal energy of the system is

−100 J
100 J
−500 J
500 J

Discuss Question

Answer: Option B. -> 100 J
:
B

Δ U = q + w

Since system releases heat, q will be negative. It is also given that work done by the surroundings is 300 J which is the same as the work done on the system. Hence, w has a +ve sign. Keeping this in mind, we get:
=

- 200 + 300 = 100 J

Question 13. State function among the following is

q
q−w
qw
q+w

Discuss Question

Answer: Option D. -> q+w
:
D
From the first law of thermodynamics, we know thatL:

q + w = Δ U

Δ U

- internal energy is a state function.

Question 14. Given the bond energies

N \equiv N

H - H

and

N - H

bonds are

945, 436

and

391 k J m o l e^{- 1}

respectively, the enthalpy of the following reaction

N_{2} (g) + 3 H_{2} (g) \to 2 N H_{3} (g)

-93 kJ
102 kJ
90 kJ
105 kJ

Discuss Question

Answer: Option A. -> -93 kJ
:
A

N \equiv N + 3 H - H \to 2 N | H | H - H

945 + 3 x 436

2 \times (3 \times 391) = 2346

E n e r g y a b s o r b e d     

E n e r g y r e l e a s e d     

Net. Energy released =

2346 - 2253 = 93 k J

i.e.,

Δ H = - 93 k J

Question 15. Enthalpy of neutralization of HCl with NaOH is x. The heat evolved when 500 ml of 2N HCl is mixed with 250 ml of 4N NaOH will be

500 x
100 x
x
10 x

Discuss Question

Answer: Option C. -> x
:
C

N o . o f g . e q . o f H C l = \frac{2 \times 500}{1000} = 1 N o . o f g . e q . o f N a O H = \frac{4 \times 500}{1000} = 1 H e a t e v o l v e d = x k J

Question 16. Which of the following acid will release maximum amount of heat when completely neutralized by strong base NaOH?

1M HCl
1M HNO3
1M HClO4
1M H2SO4

Discuss Question

Answer: Option D. -> 1M H2SO4
:
D
Ionisation of

H_{2} S O_{4}

gives double amount of

H^{+}

ions as compared to other acids

H_{2} S O_{4} ⟶ 2 H^{+} + S O_{4}^{- 2}

Therefore 1M

H_{2} S O_{4}

release more amount of heat compared to others.

Question 17. 2 mol of zinc is dissolved in

H C l

25^{\circ} C

. The work done in an open vessel is:

−2.447 kJ
−4.955 kJ
R2
3R

Discuss Question

Answer: Option B. -> −4.955 kJ
:
B
The following reaction takes place:

2 Z n + 4 H C l \to 2 Z n C l_{2} + 2 H_{2}

It is very impiortant to understand that this is an irreversible process since this reaction happens pretty quickly and hydrogengas escapes the system.
When

H_{2}

gas is liberated,it pushes back its surrounding and thus does some work against it.
Since it is an irreversible process, we can use the following equation: