Thermodynamics Test

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Thermodynamics Test - 28

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Question 1
1 / -0

The heats of combustion of rhombic and monoclinic sulphur are $$-70960$$ and $$-71030$$ calorie respectively. What will be the heat of conversion of rhombic sulphur to monoclinic sulphur?

A
$$-70960\ cal$$

B
$$-71030\ cal$$

C
$$+70\ cal$$

D
$$-70\ cal$$

Solution

The conversion reaction of rhombic sulphur to monoclinic sulphur is given by,

$$S_{rhombic} \rightarrow S_{monoclinic}$$

Hence, $$\Delta H_{conversion} = \Delta H_{\text{combustion of S(rhombic)}} -\Delta H_{\text{combustion of S(monoclinic)}}$$

$$\Delta H_{conversion} =[( -70960) - (-71030)] = 70cal.$$

$$\therefore$$ answer is option $$C$$.
Question 2
1 / -0

The heats of combustion of yellow phosphorus and red phosphorus are $$-9.91kJ$$ and $$-8.78kJ$$ respectively. The heat of transition of yellow phosphorus to red phosphorus is:

A
$$-18.69kJ$$

B
$$+1.13kJ$$

C
$$+18.69kJ$$

D
$$-1.13kJ$$

Solution

Since the conversion reaction can be written as

$$P_{(Yellow)} \rightarrow P_{(Red)}$$

Hence, the heat of reaction of above reaction ($$\Delta H $$) can be written as,

$$\Delta H = \text{heat of combustion of } P_{(Yellow)} - \text{heat of combustion of } P_{(Red)} = -9.91KJ - (-8.78KJ) = -1.13KJ.$$

Hence, the answer is option D.
Question 3
1 / -0

For an exothermic reaction to be spontaneous:

A
temperature must be high

B
temperature must be zero

C
temperature may have any magnitude

D
temperature must be low

Solution

Gibb's free energy equation : $$\Delta G=\Delta H-T\Delta S$$
For spontaneous reaction : $$\Delta G<0$$
For exothermic reaction : $$\Delta H<0$$
If $$\Delta H<0$$ & $$\Delta S>0$$, the temperature can have any value for reaction to be spontaneous. Because, $$\Delta H<0\quad \& \quad T\Delta S>0$$, So the $$\Delta G<0$$.
Question 4
1 / -0

For which reaction from the following, $$\Delta S$$ will be maximum?

A
$$Ca(s)+\cfrac{1}{2}{O}_{2}(g)\rightarrow CaO(s)$$

B
$$Ca{CO}_{3}(s)\rightarrow CaO(s)+{CO}_{2}(g)$$

C
$$C(s)+{O}_{2}(g)\rightarrow {CO}_{2}(g)$$

D
$${N}_{2}(g)+{O}_{2}(g)\rightarrow 2NO(g)$$

Solution

Explanation:
The entropy change depends upon the number of gaseous molecules.
For entropy change to be maximum, the difference between the moles of gaseous products and gasous reactants should be maximum.
$$\Delta {n_{g}}$$ $$=Sum\ of\ the\ number\ of\ moles\ of\ gaseous\ products - Sum\ of\ the\ number\ of\ moles\ of\ gaseous\ reactants$$
$$\Delta {n_g}$$ for the given reactions are given below,
A. $$\Delta {n_g}=0.5 $$
B. $$\Delta {n_g}=1 $$
C. $$\Delta {n_g}=0 $$
D. $$\Delta {n_g}=0 $$.
Final Answer: Since $$\Delta {n_g}$$ value is the maximum for reaction $$B$$, hence option $$B$$ is the correct answer.
Question 5
1 / -0

On combustion carbon forms two oxides $$CO$$ and $${CO}_{2}$$, heat of formation of $${CO}_{2}$$ is $$-94.3\ kcal$$ and that of $$CO$$ is $$-26.0 \ kcal$$. Heat of combustion of carbon is:

A
$$-26.0\ kcal$$

B
$$-68.3\ kcal$$

C
$$-94.3\ kcal$$

D
$$-120.3\ kcal$$

Solution
Question 6
1 / -0

The heat of combustion of ethanol determined in a bomb calorimeter is $$-670.48kcal$$ $${mol}^{-1}$$ at $$298K$$. What is the $$\Delta U$$ at $$298K$$ for the reaction?

A
$$-760\ kcal$$ $${mol}^{-1}$$

B
$$-670.48\ kcal$$ $${mol}^{-1}$$

C
$$+760\ kcal$$ $${mol}^{-1}$$

D
$$+670.48\ kcal$$ $${mol}^{-1}$$

Solution

Bomb calorimeter works on a basic principle that involves combustion of the organic compound at constant volume constraint.
Since, at constant volume the heat of combustion of compound = change in internal energy of that compound (as pressure-volume work will be zero).
Hence, $$\Delta U$$ $$= \text{heat of combustion in a bomb calorimeter} = -670.48\text{ Kcal mol}^{-1}.$$
Hence, answer is option B.
Question 7
1 / -0

$$\Delta { G }^{ o }$$ for the reaction $$x+y\rightleftharpoons z$$ is $$-4.606kcal$$. The value of equilibrium constant of the reaction at $${227}^{o}C$$ is:
($$R=2.0cal.{mol}^{-1}{K}^{-1}$$)

A
$$100$$

B
$$10$$

C
$$2$$

D
$$1$$
Question 8
1 / -0

Considering entropy(s) as a thermodynamic parameter, the criterion for the spontaneity of any process is:

A
$$\Delta {S}_{system}+\Delta {S}_{surroundings}> 0$$

B
$$\Delta {S}_{system}-\Delta {S}_{surroundings}> 0$$

C
$$\Delta {S}_{system}> 0$$ only

D
$$\Delta {S}_{surroundings}> 0 $$ only

Solution

Entropy (S) is the measure of randomness or disorder of the molecules. It is an extensive property and a state function.
A process is spontaneous if and only if the entropy of the universe increases.
Therefore, for spontaneous process,
$${ \Delta S }_{ universe }>0\quad or\quad { \Delta S }_{ system }+{ \Delta S }_{ surroundings }>0$$
Question 9
1 / -0

Calculate $$\triangle G^{\circ}$$ for the following cell reaction:

$$Zn(s) + Ag_{2}O(s) + H_{2}O(l) \rightleftharpoons Zn^{2+}(aq) + 2Ag(s) + 2OH^{-}(aq)$$

$$E^{\circ}_{Ag^{+}/Ag} = + 0.80V$$ and $$E^{\circ}_{Zn^{2+}/ ZN} = -0.76V$$.

A
$$-305\ kJ/mol$$

B
$$-301\ kJ/mol$$

C
$$+305\ kJ/mol$$

D
$$+301\ kJ/mol$$

Solution
Question 10
1 / -0

Which of the following is true for spontaneous process?

A
$$\Delta G> 0$$

B
$$\Delta G< 0$$

C
$$\Delta G= 0$$

D
$$\Delta G=T\Delta S$$

Solution

The process which proceeds in a particular direction by its own under a given set of conditions without outside help is called sponataneous process. All natural processes are spontaneous.

Gibb's free energy, $$\Delta G<0\quad \Rightarrow $$ for spontaneous process
$$\Delta G>0\quad \Rightarrow $$ for non-spontaneous process
$$\Delta G=\quad \Rightarrow $$ for equilibrium process

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Thermodynamics Test - 28

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Thermodynamics Test - 28

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Question 1

$$-70960\ cal$$

$$-71030\ cal$$

$$+70\ cal$$

$$-70\ cal$$

Solution

Question 2

$$-18.69kJ$$

$$+1.13kJ$$

$$+18.69kJ$$

$$-1.13kJ$$

Solution

Question 3

temperature must be high

temperature must be zero

temperature may have any magnitude

temperature must be low

Solution

Question 4

$$Ca(s)+\cfrac{1}{2}{O}_{2}(g)\rightarrow CaO(s)$$

$$Ca{CO}_{3}(s)\rightarrow CaO(s)+{CO}_{2}(g)$$

$$C(s)+{O}_{2}(g)\rightarrow {CO}_{2}(g)$$

$${N}_{2}(g)+{O}_{2}(g)\rightarrow 2NO(g)$$

Solution

Question 5

$$-26.0\ kcal$$

$$-68.3\ kcal$$

$$-94.3\ kcal$$

$$-120.3\ kcal$$

Solution

Question 6

$$-760\ kcal$$ $${mol}^{-1}$$

$$-670.48\ kcal$$ $${mol}^{-1}$$

$$+760\ kcal$$ $${mol}^{-1}$$

$$+670.48\ kcal$$ $${mol}^{-1}$$

Solution

Question 7

$$100$$

$$10$$

$$2$$

$$1$$

Question 8

$$\Delta {S}_{system}+\Delta {S}_{surroundings}> 0$$

$$\Delta {S}_{system}-\Delta {S}_{surroundings}> 0$$

$$\Delta {S}_{system}> 0$$ only

$$\Delta {S}_{surroundings}> 0 $$ only

Solution

Question 9

$$-305\ kJ/mol$$

$$-301\ kJ/mol$$

$$+305\ kJ/mol$$

$$+301\ kJ/mol$$

Solution

Question 10

$$\Delta G> 0$$

$$\Delta G< 0$$

$$\Delta G= 0$$

$$\Delta G=T\Delta S$$

Solution

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