Chemistry · Physics

Thermodynamics and Chemical Kinetics

1,092 Questions

Thermodynamics and chemical kinetics are crucial branches of physical chemistry. This area focuses on energy transformations, reaction rates, equilibrium constants, and catalysts. These topics carry significant weight in competitive science and engineering examinations.

Enthalpy and energyChemical equilibriumEntropy conceptsReaction kinetics and catalysts

Thermodynamics and Chemical Kinetics Questions

Multiple choice chemistry enthalpy changes enthalpy changes and enthalpy profile diagrams enthalpy study of enthalpy

For the combustion reaction at $298\,K$
$2Ag(s) +1/2O _2(g)\longrightarrow 2Ag _2O(s)$ Which of the following alternative is correct?

  1. $\Delta H = \Delta U$
  2. $\Delta H > \Delta U$
  3. $\Delta H < \Delta U$
  4. $\Delta H and \Delta U$ bear no relation with each other
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

As we know,
$\,\Delta H = \Delta U + \Delta nRT$
$\Delta n = n _{ P } - n _{ R }$
$\therefore\Delta n = 2 \displaystyle - \frac { 5 }{ 2 } =\displaystyle -\frac{ 1 }{ 2 }$
$\therefore\Delta H = \Delta U \displaystyle - \frac { 1 }{ 2 }RT$
$\therefore\Delta U = \Delta H +\displaystyle \frac{ 1 }{ 2 }RT$
or $\therefore\Delta U > \Delta H$

Multiple choice chemistry enthalpy changes enthalpy changes and enthalpy profile diagrams enthalpy study of enthalpy
For a particular reaction ; $\Delta E$ $=\Delta H + P\Delta $V.

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The given statement is false.
For a particular reaction $\displaystyle \Delta H = \Delta  E + P \Delta  V$
The change in enthalpy is equal to the sum of the change in the internal energy and the pressure volume work. This expression is applicable at constant pressure.

Multiple choice chemistry enthalpy changes enthalpy changes and enthalpy profile diagrams enthalpy study of enthalpy

Enthalpy change of a reaction will be equal to:

  1. $\Delta U + P\Delta V$
  2. $\Delta U + V\Delta P$
  3. $\Delta U + \Delta (PV)$
  4. $\Delta U + (\Delta n _{ g } )\Delta (PV)$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The relationship between the enthalpy $H$, internal energy $E$, pressure $P$ and volume $V$ is $H=E+PV$
Hence, the expression for the enthalpy change becomes $\Delta H = \Delta U + \Delta (PV)$
Hence, the enthalpy change of a reaction will be equal to $\Delta U + \Delta (PV)$.

Multiple choice chemistry enthalpy changes enthalpy changes and enthalpy profile diagrams enthalpy study of enthalpy

In an isothermal process:

  1. $q=0$ and $\Delta E=0$
  2. $q\neq 0$ and $\Delta E=0$
  3. $q=0$ and $\Delta E\neq 0$
  4. $q\neq 0$ and $\Delta E\neq 0$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

An isothermal process is a change of a system, in which the temperature remains constant: ΔT = 0. In other words, in an isothermal process, the value ΔT = 0 and therefore the change in internal energy ΔU = 0 (only for an ideal gas) but Q ≠ 0, while in an adiabatic process, ΔT ≠ 0 but Q = 0.

Multiple choice chemistry enthalpy changes enthalpy changes and enthalpy profile diagrams enthalpy study of enthalpy

Predict $\Delta H>\Delta U$ or $\Delta H<\Delta U$.

a. $C(graphite) + O _2(g)\longrightarrow CO _2(g)$
b. $PCl _5(g)\longrightarrow PCl _3(g) + Cl _2(g)$

  1. a. $\Delta H = \Delta U$; b. $\Delta H>\Delta U$
  2. a. $\Delta H > \Delta U$; b. $\Delta H=\Delta U$
  3. a. $\Delta H >\Delta U$; b. $\Delta H<\Delta U$

  4. None of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
We know,

$\Delta H= \Delta U + \Delta n _gRT$
a. $\Delta H = \Delta U$ because $\Delta n _g = 0$.   

b. $\Delta H>\Delta U$ because $\Delta n _g = 1$ and decomposition of $PCl _5$ is endothermic reaction.

Hence, option A is correct 
Multiple choice chemistry enthalpy changes enthalpy changes and enthalpy profile diagrams enthalpy study of enthalpy

Enthalpy is_________.

  1. Energy of reaction

  2. Tendency for loss of energy

  3. Tendency for randomness

  4. Use of energy in overcoming entropy
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Enthalpy is the measurement of energy in a thermodynamic system. It is equal to the internal energy of the system plus the product of pressure and volume. The thermal change that occurs in a chemical reaction is only due to the difference in the sum of internal energy of the products and the sum of the internal energy of reactants. At constant pressure, the heat of the reaction is exactly equal to the enthalpy change, of the reacting system.

So, the correct answer is option A.

Multiple choice chemistry enthalpy changes enthalpy changes and enthalpy profile diagrams enthalpy study of enthalpy

Statement I : An exothermic reaction has a positive $\displaystyle \Delta H$ value
Because
Statement II : Heat must be added to the reaction for the reaction to occur

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

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

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

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

  5. Both the Statement 1 and Statement 2 are not correct.
Reveal answer Fill a bubble to check yourself
E Correct answer
Explanation
Answer (E)

An exothermic reaction has a negative ΔHΔH value,so statement 1 is not correct.
It is not necessary that heat must be added to the reaction for the reaction to occur, only endothermic reactions requires heat.
so both statement 1 and 2 are not correct.
Multiple choice physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

That 'Entropy of a system increases in all spontaneous processes' is  known as

  1. 1 st law of Thermodynamics

  2. 2nd law of Thermodynamics

  3. Zeroth law of Thermodynamics

  4. 3rd law of Thermodynamics
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The second law of thermodynamics states that the entropy of an isolated system never decreases, because isolated systems spontaneously evolve toward thermodynamic equilibriumthe state of maximum entropy

Multiple choice physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

Which of the following option are related with the second law of thermodynamics (law of entropy)?

  1. The heat lost by one object must be gained by another object

  2. Heat flow naturally from a hotter body to a cooler body

  3. Celsius degrees and Kelvin degrees are equivalent

  4. Heat can be transformed into work

  5. The average kinetic energy of molecules is proportional to temperature
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The second law of thermodynamics says that heat flows 'naturally' from an object at a higher temperature to an object at a lower temperature, and heat doesn’t flow in the opposite direction of its own accord, since the entropy in such a process increases. 

The reverse is not possible due to decrease in entropy in such a process.

Multiple choice physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

Which of the following statement is true as per the second law of thermodynamics for an isolated, ordered system?

  1. Heat will flow into the system

  2. Heat will flow out of the system

  3. Work will be done by the system

  4. Work will be done on the system

  5. The entropy withing the system will increase
Reveal answer Fill a bubble to check yourself
E Correct answer
Explanation

The second law of thermodynamic states that the entropy of an isolated system increases over time and remain constant for reversible process. 

Multiple choice physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

The second law of thermodynamics says that in a cyclic process

  1. Work cannot be converted into heat

  2. Heat cannot be converted into work

  3. Work cannot be completely converted into heat

  4. Heat cannot be completely converted into work
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Second law of thermodynamics states that in a cyclic process heat cannot be converted completely into work because attainment of $0K$ temperature is impossible.

Multiple choice physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

Which statement is true for second law of thermodynamics ?

  1. Heat can flow spontaneously from a cold object to a hot object.

  2. You can not create a heat engine which extracts heat and converts it all to useful work.

  3. According to Plank-Kelvin's law under some circumstances a perfect heat engine is possible.

  4. None of these.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The second law of thermodynamics states , that it is impossible to construct a heat engine , operating in cycle , which extracts heat and can convert it all to useful work .In other words , it is impossible for a heat engine to convert heat completely in work .

The efficiency of a heat engine is ,
            $\eta=1-\frac{Q _{2}}{Q _{1}}$
if $Q _{2}=0$ i.e. heat is not given to sink , it means total heat absorbed is converted into work , then
            $\eta=1$ , which is practically not possible .

Multiple choice physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

Which of the following laws of thermodynamics leads to the inference that it is difficult to convert whole of heat into work :

  1. zeroth

  2. second

  3. first

  4. third
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The second law of thermodynamics states that when energy change from one form to another form, entropy in a closed system increases.

In other words it can be stated as,
It is impossible to construct a device which produces no other effect than transfer of heat from lower temperature body to higher temperature body.
Hence it is clear that second law of thermodynamics leads to the interference that is difficult to convert whole of heat into work.

Multiple choice physics heat engine: second law of thermodynamics second law of thermodynamics the second law of thermodynamics second law of thermodynamic

The second law of thermodynamics implies : 

  1. whole of heat can be converted into mechanical energy

  2. no heat engine can be 100% efficient

  3. every heat engine has an efficiency of 100%

  4. a refrigerator can reduce the temperature to absolute zero
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

second law of thermodynamics states that ,

"It is impossible to construct a device which produces no other effect than transfer of heat from lower temperature body to higher temperature body."
$\therefore$ The second law of thermodynamics implies that no heat engine can be 100% efficient.