Chemistry · Physics

Thermodynamics and Chemical Kinetics

1,132 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

In which of the following reactions, $\Delta H > \Delta U$?

  1. $H _2(g) + I _2(g)\rightarrow 2HI(g)$
  2. $PCl _5(g)\rightarrow PCl _3(g) + Cl _2(g)$
  3. $2H _2O _2(l)\rightarrow 2H _2O(l) + O _2(g)$
  4. $C(s) + O _2(g)\rightarrow CO _2(g)$
Reveal answer Fill a bubble to check yourself
B,C Correct answer
Explanation

As we know,
$\Delta H = \Delta U + \Delta nRT$, where $\Delta n = n _P - n _R$ (n = number of moles)
$H _2(g) + I _2(g)\rightarrow 2HI(g)$      $\Delta n = 0$
$PCl _5(g)\rightarrow PCl _3(g) + Cl _2(g)$                     $\Delta n = 1$
$2H _2O _2(l)\rightarrow 2H _2O(l) + O _2(g)$                 $\Delta n = 1$
$C(s) + O _2(g)\rightarrow CO _2(g)$                  $\Delta n = 0$

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

$H _2(g) + I _2(g)\longrightarrow 2HI(g)$ 


For this reaction, relate $\Delta H$ and $\Delta U$.

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

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

As we know,
$\Delta H$ =$\Delta U+ \Delta n _g RT$
here, 
$H _2(g) + I _2(g)\longrightarrow 2HI(g)$ 
$\Delta n _g = 0$
so,
$\Delta H$ =$\Delta U$

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

If $\Delta E$ is the heat of reaction for
${C} _{2}{H} _{5}OH\left(l\right) + 3{O} _{2}\left(g\right) \longrightarrow 2C{O} _{2}\left(g\right) + 3{H} _{2}O\left(l\right)$
at constant volume, the $\Delta H$ (heat of reaction at constant pressure), at constant temperature is:

  1. $\Delta H = \Delta E + RT$
  2. $\Delta H = \Delta E - RT$
  3. $\Delta H = \Delta E - 2RT$
  4. $\Delta H = \Delta E + 2RT$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

We know that,
$\Delta H = \Delta E + \Delta nRT$
where,
$\Delta n =$ number of moles of gaseous products - number of moles of gaseous reactants
$= 2-3=-1$
So, $\Delta H = \Delta E - RT$

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

For gaseous reactions, if $\Delta H$ is the change in enthalpy and $\Delta U$ that in internal energy, then

  1. $\Delta H$ is always greater than $\Delta U$
  2. $\Delta H$ is always less than $\Delta U$
  3. $\Delta H > \Delta U$ only if the number of mole of the products is less than that of the reactants.
  4. $\Delta U < \Delta H$ only if the number of mole of the reactants is less than that of the products.
Reveal answer Fill a bubble to check yourself
C,D Correct answer
Explanation

As we know,
$\,\Delta H = \Delta U + \Delta nRT$
$\Delta n = n _{ P } - n _{ R }$
so
$\Delta U < \Delta H$ only if the number of mole of the reactants is less than that of the products.
$\Delta H > \Delta U$ only if the number of mole of the products is less than that of the reactants.

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 chemistry ionic equilibrium introduction to ionic equilibria in solution ionic equilibrium in solution ionisation of weak acids and weak bases

Which of the following statements are correct about dissociation?

  1. Dissociation generally refers to breaking a compound into smaller pieces, usually reversibly.

  2. Dissociation can happen without the formation of charged species
  3. Both $A$ and $B$

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

Dissociation in chemistry and biochemistry is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into smaller particles such as atoms, ions or radicals, usually in a reversible manner. It may or may not form charged species as shown below. 

i). $NaCl \leftrightharpoons Na^++Cl^-$

ii). $N _2O _4 \leftrightharpoons 2NO _2$

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.