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

12th Grade > Chemistry

CHEMICAL THERMODYNAMICS MCQs

Total Questions : 30 | Page 3 of 3 pages

Question 21. A well stopped thermos flask contains ice cubes. This is an example of

closed system
open system
isolated system
non-thermodynamic system

Discuss Question

Answer: Option C. -> isolated system
:
C
There is no exchange of matter or energy from the flask.

Question 22. Two moles of an ideal gas is expanded isothermally and reversibly from 1 litre to 10 litre at 300 K. The enthalpy change ( in kJ) for the process is

11.4 kJ
−11.4 kJ
0 kJ
4.8 kJ

Discuss Question

Answer: Option C. -> 0 kJ
:
C

△ H = n C_{p} △ T s o l u t i o n; s i n c e △ T = 0

so,

△ H = 0

Question 23. An ideal gas expands in volume from

1 \times 10^{- 3}

1 \times 10^{- 2} m^{3}

300 K

against a constant pressure of

1 \times 10^{5} N m^{- 2}

. The work done is

270 kJ
−270 kJ
−900 J
900 kJ

Discuss Question

Answer: Option C. -> −900 J
:
C

w = - P Δ V = - 10^{5} (1 \times 10^{- 2} - 1 \times 10^{- 3})

- 900 J

Question 24. The dissociation energy of

C H_{4}

is 400 Kcal

m o l^{- 1}

and that of ethane is 670 Kcal

m o l^{- 1}

. The C - C bond energy is.

270 kcal
70 kcal
200 kcal
240 kcal

Discuss Question

Answer: Option B. -> 70 kcal
:
B

C H_{4}

has 4 C - H bonds

∴

Energy needed to break a single C - H bond is

\frac{400}{4}

= 100 Kcal

C_{2} H_{6}

has 6 C - H bonds

∴

The C - C bond energy can be given by
670 Kcal - 6

\times

100 Kcal = 70 Kcal

Question 25. The standard enthalpies of formation of

C O_{2} (g)

H_{2} O (l)

and glucose(s) at

25^{\circ} C

are

- 400 k J / m o l

- 300 k J / m o l

and

- 1300 k J / m o l

, respectively. The standard enthalpy of combustion per gram of glucose at

25^{\circ} C

+ 2900 kJ
−2900 kJ
−16.11 kJ
+16.11 kJ

Discuss Question

Answer: Option C. -> −16.11 kJ
:
C
The standard enthalpy of the combustion of glucose can be calculated by the equ.

C_{6} H_{12} O_{6} (s) + 6 O_{2} (g) \to 6 C O_{2} (g) + 6 H_{2} O (l)

Δ H_{c} = 6 \times Δ H_{f} (C O_{2}) + 6 \times Δ H_{f} (H_{2} O) - Δ H_{f} [C_{6} H_{12} O_{6}]

Δ H^{0} = - \frac{2900}{180} = - 16.11 k J / g m

Question 26. In a process a system does 140 J of work on the surrondings and only 40 J of heat is added to the system, hence change in internal energy is

180 J
-180 J
-23.92 Cal
-180 J

Discuss Question

Answer: Option C. -> -23.92 Cal
:
C
Given

W = - 140 J; q = 405

Δ E = q + W = 40 - 140 = - 100 J (1 c a l = 4.18 J) = - 23.92 C a l

Question 27. Which of the following is /are true in the case of an adiabatic process

q = W
△U=q
q=0
None

Discuss Question

Answer: Option C. -> q=0
:
C
For adiabatic process that remain const

i . e . q - 0, △ U = W

Question 28. The enthalpies for the following reactions at 25°C are given.

\frac{1}{2} H_{2} (g) + \frac{1}{2} O_{2} (g) \to O H (g); △ H = 10.06 K c a l

H_{2} (g) \to 2 H (g); △ H = 104.18 K c a l

O_{2} (g) \to 2 O (g); △ H = 118.32 K c a l

Calculate the OH bond energy in the O - H group

100.19 K col
101.19 K col
-101.19 K col
-100.19 K col

Discuss Question

Answer: Option C. -> -101.19 K col
:
C
H(g) + O(g)

\to

O - H(g) Required equation
Given:

\frac{1}{2} H_{2} (g) + \frac{1}{2} O_{2} (g) \to O H (g); △ H = + 10.06 K c a l

H (g) \to \frac{1}{2} H_{2} (g); △ H = - 52.09 K c a l

O (g) \to \frac{1}{2} O_{2} (g); △ H = - 59.16 K c a l

Adding them we get

H (g) + O (g) + (O H) (g) △ H = - 101.19 K c a l

Question 29. The heat of combustion of yellow phosphorous & red phosphorous are - 9.91 KJ and - 8.78 KJ respectively. The heat of transition of yellow phosphorous to red phosphorous is: