Thermodynamics Test

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Thermodynamics Test - 56

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Question 1
1 / -0

What can be used in combination with a calorimeter to compare the specific heats of two substances?

A
Thermometer

B
Conductivity tester

C
Graduated cylinder

D
Buret

E
Salt bridge

Solution

Thermometer can be used in combination with a calorimeter to compare the specific heats of two substances. Thermometer will measure the temperature.
Question 2
1 / -0

Statement 1: The $$\Delta H_{reaction}$$ of a particular reaction can be arrived at by the summation of the $$\Delta H_{reaction}$$ values of two or more reactions that, added together, will give the $$\Delta H_{reaction}$$ of the particular reaction.
Statement 2: Hess's Law conforms to the First Law of Thermodynamics, which states that the total energy of the universe is a constant.

A
Both Statement 1 and Statement 2 are correct and Statement 2 is the correct explanation of Statement 1.

B
Both Statement 1 and Statement 2 are correct and Statement 2 is not the correct explanation of Statement 1.

C
Statement 1 is correct but Statement 2 is not correct.

D
Statement 1 is not correct but Statement 2 is correct.

E
Both the Statement 1 and Statement 2 are not correct.

Solution

Statement 1: The $$\displaystyle \Delta H_{reaction}$$ of a particular reaction can be arrived at by the summation of the $$\displaystyle \Delta H_{reaction}$$ values of two or more reactions that, added together which give the $$\displaystyle \Delta H_{reaction}$$ of the particular reaction.
Statement 2: Hess's Law conforms to the First Law of Thermodynamics, which states that the total energy of the universe is a constant.
Both Statement 1 and Statement 2 are correct and Statement 2 is the correct explanation of Statement 1.
Consider following example
$$\displaystyle C(s) + \dfrac {1}{2} O_2 (g) \rightarrow CO (g) \: \: \: \Delta H_{1, reaction}$$
$$\displaystyle CO(g) + \dfrac {1}{2} O_2 (g) \rightarrow CO_2 (g) \: \: \: \Delta H_{2, reaction}$$
$$\displaystyle C(s) + O_2 (g) \rightarrow CO_2 (g) \\ \: \: \: \Delta H_{3, reaction} = \Delta H_{1, reaction} + \Delta H_{2, reaction}$$
Question 3
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The heat of formation of $$CO(g)$$ and $$CO_2(g)$$ are $$\Delta H=-110$$ and $$\Delta H=-393kJmmol^{-1}$$ respectively. What is the heat of reaction$$(\Delta H)$$ (in kJ $$mol^{-1}$$) for the following reaction?
$$CO(g)+\displaystyle\frac{1}{2}O_2(g)\rightarrow CO_2(g)$$

A
$$-504$$

B
$$-142.5$$

C
$$-283$$

D
$$504$$

Solution

For $$CO$$,
$$C+\frac { 1 }{ 2 } { O }_{ 2 }\longrightarrow CO$$ $$\Delta { H }_{ CO }=-110$$

For $$CO_2$$,
$$C+ { O }_{ 2 }\longrightarrow CO_2$$ $$\Delta { H }_{ CO_2 }=-393$$

For the reaction
$$CO+\frac { 1 }{ 2 } { O }_{ 2 }\longrightarrow CO_2$$
We will substract equation 1 from equation 2, we get
$$CO+\frac { 1 }{ 2 } { O }_{ 2 }\longrightarrow CO_2$$ $$\Delta { H }_{ CO_2 }-\Delta { H }_{ CO }=-393-(-110)=-283KJ{mol}^{-1}$$
Question 4
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Calculate the enthalpy , $$\Delta H$$, for the given reaction using the the given bond energies ?
$$C_{2}H_{4} + Cl_{2}\rightarrow ClH_{2}C-CH_{2}Cl$$
Bond energies kJ/ mol
$$C-C$$ $$347$$
$$C = C$$ $$612$$
$$C-Cl$$ $$341$$
$$C-H$$ $$414$$
$$Cl-Cl$$ $$243$$

A
$$\Delta H = -800 kJ$$

B
$$\Delta H = -680 kJ$$

C
$$\Delta H = -174 kJ$$

D
$$\Delta H = +174 kJ$$

E
$$\Delta H = +200 kJ$$

Solution

$$\displaystyle \Delta H^o _{rxn} = \Sigma \Delta H^o _{\text {reactant bonds}} - \Sigma \Delta H^o _{\text {product bonds}} \\ \Delta H^o _{rxn}= [4\Delta H^o _{C-H}+ \Delta H^o _{C=C}+Cl-Cl] - [\Delta H^o _{C-C}+4\Delta H^o _{C-H}+2\Delta H^o _{C-Cl}]$$
$$\displaystyle \Delta H^o _{rxn} = [ \Delta H^o _{C=C}+Cl-Cl] - [\Delta H^o _{C-C}+2\Delta H^o _{C-Cl}]$$
$$\displaystyle \Delta H^o _{rxn} = [612 + 243] - [347 + 2(341)] $$
$$\displaystyle \Delta H^o _{rxn} = 855-1029$$
$$\displaystyle \Delta H^o _{rxn} = -174kJ$$
Question 5
1 / -0

The spontaneity of a reaction is indicated by:

A
enthalpy change

B
entropy change

C
gibbs free energy change

D
activation energy

E
specific heat capacity

Solution

The spontaneity of a reaction is indicated by gibbs free energy change.
A negative value of gibbs free energy change indicates the reaction is spontaneous.
A positive value of gibbs free energy change indicates the reaction is non spontaneous.
Zero value of gibbs free energy change indicates the reaction is at equilibrium.
Question 6
1 / -0

The reverse of a spontaneous reaction is ......... .

A
always spontaneous

B
always non spontaneous

C
sometimes spontaneous

D
sometimes non spontaneous

E
There is no way of telling

Solution

The revere of a spontaneous reaction is always non-spontaneous.
According to the second law of thermodynamics, a reaction is spontaneous if the overall entropy or disorder increases. If we go back to our cleaning the house analogy, when our home becomes messy, then the entropy increases. The mess that accumulates is a spontaneous reaction, and as a result, the entropy increases.
Question 7
1 / -0

What is the necessary condition for the sponataneity of a process?

A
$$\Delta S>0$$

B
$$\Delta E<0$$

C
$$\Delta H<0$$

D
$$\Delta G<0$$

Solution

For spontaneity of any process $$\Delta G<0$$. $$\Delta S$$ need not be necessarily greater than 0.
Question 8
1 / -0

The important considerations in deciding if a reaction will be spontaneous are :

A
stability & state of reactants

B
energy gained & heat evolved

C
exothermic energy & randomness of the products

D
endothermic energy & randomness of the products

E
endothermic energy & structure of the products

Solution

ΔG=ΔH-TΔS, for a reaction to be spontaneous ΔG, should be negative
it is guaranteed only if both terms in RHS is negative. therefore ΔH should be negative and ΔS should be positive. Negative ΔH implies exothermic energy and positive ΔS implies randomness of the product.
Question 9
1 / -0

What is the value of $$\Delta H$$ for the reaction $$X + 2Y \rightarrow 2Z$$?
$$ W+X\rightarrow 2Y$$ ; $$\Delta H=- 400 $$ kcal/mol
$$ 2W + 3X \rightarrow 2Z + 2Y$$ ; $$\Delta H=- 150 $$ kcal/mol

A
$$- 550$$ kcal/mol

B
$$+50$$ kcal/mol

C
$$- 50$$ kcal/mol

D
$$+ 650$$ kcal/mol

E
$$+250$$ kcal/mol

Solution

Multiply equation $$\displaystyle W+X\rightarrow 2Y$$ with $$2$$.

$$\displaystyle 2W+2X\rightarrow 4Y$$ ; $$\displaystyle \Delta H= -400 \times 2 = -800$$ kcal .....(1)

$$\displaystyle 2W + 3X \rightarrow 2Z + 2Y$$ $$\displaystyle \Delta H= -150$$ kcal .....(2)

Subtracting equation (1) from equation (2) we get,

$$\displaystyle X + 2Y \rightarrow 2Z$$

$$\displaystyle \Delta H= -150 -(-800)=+650$$ kcal
Question 10
1 / -0

Which of the following drives spontaneous reactions?

A
Low enthalpy values and high entropy values.

B
Low enthalpy values and low entropy values.

C
High enthalpy values and low entropy values.

D
High enthalpy values and high entropy values.

E
High temperatures and low pressures.

Solution

Low enthalpy and high entropy drives spontaneous reactions.
These leads to negative value of Gibbs free energy change.
$$\displaystyle \Delta G = \Delta H - T \Delta S$$

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Bond energies	kJ/ mol
$$C-C$$	$$347$$
$$C = C$$	$$612$$
$$C-Cl$$	$$341$$
$$C-H$$	$$414$$
$$Cl-Cl$$	$$243$$

Thermodynamics Test - 56

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Thermodynamics Test - 56

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SHARING IS CARING

Question 1

Thermometer

Conductivity tester

Graduated cylinder

Buret

Salt bridge

Solution

Question 2

Both Statement 1 and Statement 2 are correct and Statement 2 is the correct explanation of Statement 1.

Both Statement 1 and Statement 2 are correct and Statement 2 is not the correct explanation of Statement 1.

Statement 1 is correct but Statement 2 is not correct.

Statement 1 is not correct but Statement 2 is correct.

Both the Statement 1 and Statement 2 are not correct.

Solution

Question 3

$$-504$$

$$-142.5$$

$$-283$$

$$504$$

Solution

Question 4

$$\Delta H = -800 kJ$$

$$\Delta H = -680 kJ$$

$$\Delta H = -174 kJ$$

$$\Delta H = +174 kJ$$

$$\Delta H = +200 kJ$$

Solution

Question 5

enthalpy change

entropy change

gibbs free energy change

activation energy

specific heat capacity

Solution

Question 6

always spontaneous

always non spontaneous

sometimes spontaneous

sometimes non spontaneous

There is no way of telling

Solution

Question 7

$$\Delta S>0$$

$$\Delta E<0$$

$$\Delta H<0$$

$$\Delta G<0$$

Solution

Question 8

stability & state of reactants

energy gained & heat evolved

exothermic energy & randomness of the products

endothermic energy & randomness of the products

endothermic energy & structure of the products

Solution

Question 9

$$- 550$$ kcal/mol

$$+50$$ kcal/mol

$$- 50$$ kcal/mol

$$+ 650$$ kcal/mol

$$+250$$ kcal/mol

Solution

Question 10

Low enthalpy values and high entropy values.

Low enthalpy values and low entropy values.

High enthalpy values and low entropy values.

High enthalpy values and high entropy values.

High temperatures and low pressures.

Solution

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