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 chemical equilibrium equilibrium in chemical processes introduction to equilibrium chemical equilibrium and acids-bases

A reaction mixture containing $H _{2}, N _{2}$ and $NH _{3}$ has partial pressure $2\ atm, 1\ atm$ and $3\ atm$ respectively at $725\ K$. If the value of $K _{P}$ for reaction, $N _{2} + 3H _{2}\rightleftharpoons 2NH _{3}$ is $4.28\times 10^{-5} atm^{-2}$ at $725\ K$, in which direction the net reaction will go :

  1. Forward

  2. Backward

  3. No net reaction

  4. Direction of reaction cannot be predicted
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

                ${ N } _{ 2 }+{ 3H } _{ 2 }\rightleftharpoons 2{ NH } _{ 3 }$

at eqn,   1atm    2atm       3atm
given, ${ K } _{ p }=4.28\times { 10 }^{ -5 }{ atm }^{ -2 }$
${ Q } _{ p }=\dfrac { { \left( { P } _{ { NH } _{ 3 } } \right)  }^{ 2 } }{ \left( { P } _{ { N } _{ 2 } } \right) { \left( { P } _{ { H } _{ 2 } } \right)  }^{ 3 } } =\dfrac { { \left( 3 \right)  }^{ 2 } }{ 1\times { \left( 2 \right)  }^{ 3 } } =\dfrac { 3\times 3 }{ 2\times 2\times 2 } =\dfrac { 9 }{ 8 } =1.125{ atm }^{ -2 }$
Since, $\boxed { { Q } _{ p }>>{ K } _{ p } } $
Reaction will move in forward direction.

Multiple choice chemistry chemical equilibrium equilibrium in chemical processes introduction to equilibrium chemical equilibrium and acids-bases

Chemical equilibria is (I) _________ in nature. It occurs in (2) ________ reactions only.  At equilibria, all (3) __________ properties becomes constant. Chemical equilibrium gets affected by change in temperature, pressure, concentration, volume etc but is not altered by addition of (4)________. Equilibria are of two types (5) ________ and homogeneous equilibria.

  1. (1) dynamic (2) reversible (3) observable (4) catalyst (5) homogeneous

  2. (1) static (2) irreversible (3) observable (4) reactants (5) heterogeneous

  3. (1) dynamic (2) irreversible (3) observable (4) reactants (5) homogeneous

  4. (1) dynamic (2) reversible (3) observable (4) catalyst (5) heterogeneous

Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Chemical equilibria is dynamic in nature. It occurs in reversible reactions only. At equilibria all observable properties becomes constant. Chemical equilibrium is affected by a change in temperature, pressure, concentration, volume etc but is not altered by the addition of catalyst. Equilibria is of two types heterogeneous and homogeneous equilibria.

Multiple choice chemistry chemical equilibrium equilibrium in chemical processes introduction to equilibrium chemical equilibrium and acids-bases

In a dynamic equilibrium, the concentrations of reactants and products remains ___________.

  1. differs with substance

  2. changes

  3. constant

  4. equilibrium
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

In chemistry, a dynamic equilibrium exists once a reversible reaction ceases to change its ratio of reactants/products, but substances move between the chemicals at an equal rate, meaning there is no net change. It is a particular example of a system in a steady state. In thermodynamics a closed system is in thermodynamic equilibrium when reactions occur at such rates that the composition of the mixture does not change with time. Reactions do in fact occur, sometimes vigorously, but to such an extent that changes in composition cannot be observed.

Hence, in a dynamic equilibrium, the concentrations of reactants and products remains constant.

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

The relationship between enthalpy and internal energy change is

  1. $\,\Delta U = \Delta H + P\Delta V$
  2. $\,\Delta H = \Delta U + P\Delta V$
  3. $\,\Delta H = \Delta U - P\Delta V$
  4. $\,P\Delta V = \Delta U + \Delta H$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
Chemical reactions are generally carried out at constant pressure (atmospheric pressure) so it has been found useful to define a new state function Enthalpy (H) as :
$ H = U + PV$ ( By definition )
$ \Delta H = \Delta U + \Delta (PV)$
$ \Delta H = \Delta  U + P\Delta V $ (at constant pressure) combining with first law.
$ \Delta H = q _{p}$
Multiple choice chemistry enthalpy changes enthalpy changes and enthalpy profile diagrams enthalpy study of enthalpy

Under which of the following condition is the relation $\Delta H = \Delta U + P\Delta V$ valid for a closed system at

  1. constant pressure

  2. constant temperature

  3. constant temperature and pressure

  4. constant temperature, pressure and composition
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
Chemical reactions are generally carried out at constant pressure (atmospheric pressure) so it has been 
found useful to define a new state function Enthalpy(H) as:
$ H = U + PV$ (By definition)
$ \Delta H = \Delta U + \Delta (PV)$
$ \Delta H = \Delta U + P\Delta V$  (at constant pressure) combining with first law. 
$ \Delta H = q _p $
Multiple choice chemistry enthalpy changes enthalpy changes and enthalpy profile diagrams enthalpy study of enthalpy

Which of the following statements are correct?

  1. absolute value of enthalpy cannot be determined.

  2. absolute value of internal energy cannot be determined.

  3. absolute value of entropy can be determined.

  4. internal energy, enthalpy, and entropy are intensive properties.
Reveal answer Fill a bubble to check yourself
A,B Correct answer
Explanation

Properties which depend on the amount of the substance (or substances)

present in the system are called extensive propterties. e.g. Mass,

volume, heat capacity, internal energy, entropy, Gibb's free energy (G),

surface area etc. These properties will change with change in the

amount of matter present in the system.
The absolute value of

internal energy cannot be determined because it is not possible to

determine the exact values for the constituent energies such as

translational, vibrational, rotational energies, etc. However, we can

determine the change in internal energy (U) of the system when it

undergoes a change from initial state (Ul) to final state (Uf).
Enthalpy

is not a matter. It doesn't have mass and it doesn't occupy space.

Therefore, we cannot measure its absolute value (like energy).
We can only measure the changes because the energy of the universe is constant.

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

For which reaction will $\Delta H = \Delta U$?


 Assume each reaction is carried out in an open container.

  1. $H _2(g) + Br _2(g)\longrightarrow 2HBr(g)$
  2. $C(s) + 2H _2O(g)\longrightarrow 2H _2(g) + CO _2(g)$
  3. $4CO(g) + 2O _2(g)\longrightarrow 4CO _2(g)$
  4. $2PCl _5(g)\longrightarrow 2PCl _3(g) + 2Cl _2(g)$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$\Delta H = \Delta U + \Delta nRT$, where $\Delta n =$ Change in number of moles.


$H _2 + Br _2 \to 2HBr$


 $\Delta n = 0$.  

$\therefore$ $\Delta H = \Delta U$

Hence, option A is correct

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.