Tag: study of enthalpy

Questions Related to study of enthalpy

Heat of reaction at constant pressure and heat of reaction at constant volume for the gaseous reaction $N _2 + 3H _2 \longrightarrow  2NH _3$ differ $(\Delta H- \Delta U)$ by the amount:

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

  1. $2RT$

  2. $-2RT$

  3. $3RT$

  4. $RT$

Show answer
Correct Option: B
Explanation:

The heat of reaction at constant pressure and heat of reaction at constant volume for the gaseous reaction 


$N _2 + 3H _2 \longrightarrow  2NH _3$

$\Delta H = \Delta U + (\Delta n _g)\times RT$

$\Delta H = \Delta U + (-2)\times RT$

This is because the change in the number of moles of gaseous products and the gaseous reactants in the above reaction is -2.

$\Delta H - \Delta U = (-2)\times RT$

Hence, option B is correct.

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:

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

  1. $\Delta H = \Delta E + RT$

  2. $\Delta H = \Delta E - RT$

  3. $\Delta H = \Delta E - 2RT$

  4. $\Delta H = \Delta E + 2RT$

Show answer
Correct Option: B
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, $\Delta H = \Delta E - RT$

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

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

  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.

Show answer
Correct Option: C,D
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.

At constant pressure:

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

  1. pdv=d(pv)

  2. dQ=du+d(pv)

  3. dQ=d(u+pv)

  4. all of the mentioned

Show answer
Correct Option: D

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

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

  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

Show answer
Correct Option: C
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 $\therefore\Delta U > \Delta H$

Hess's law is based on:

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

  1. Law of conservation of mass

  2. Law of conservation of energy

  3. Second law of thermodynamics

  4. None of the above

Show answer
Correct Option: B
Explanation:

The Hess's law states that the total enthalpy change during the complete course of a chemical reaction is the same whether the reaction is made in one step or in several steps. Hess's law is now understood as an expression of the principle of conservation of energy, also expressed in the first law of thermodynamics, and the fact that the enthalpy of a chemical process is independent of the path taken from the initial to the final state (i.e. enthalpy is a state function).

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

  1. True

  2. False

Show answer
Correct Option: B
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.

Select the correct option(s):

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

  1. $q=nC _{v}\mathrm{d} T$ is applicable to all substances during heating/cooling at constant 'v'.

  2. $q=nC _{v}\mathrm{d} T$ is applicable to ideal gas during heating/cooling at constant 'v'.

  3. $\mathrm{d} U=nC _{v}\mathrm{d} T$ is applicable for real gas at constant 'v'

  4. $\mathrm{d} U=nC _{v}\mathrm{d} T$ is applicable for ideal gas at constant 'v' only

Show answer
Correct Option: A,B
Explanation:

An isochoric process is a thermodynamic process during which the volume of the closed system undergoing such a process remains constant. 


Work done by system is given as $W=P\Delta V$

So workdone by the ideal and real gases will be zero in all cases.

From first law of thermodynamics, $Q=W+\Delta U$
$\Longrightarrow Q=\Delta U$

$\Delta U=n{ C } _{ v }\Delta T$ is applicable for all conditions whether the volume is constant or not.

Enthalpy change of a reaction will be equal to:

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

  1. $\Delta U + P\Delta V$

  2. $\Delta U + V\Delta P$

  3. $\Delta U + \Delta (PV)$

  4. $\Delta U + (\Delta n _{ g } )\Delta (PV)$

Show answer
Correct Option: C
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 $\Delta U + \Delta (PV)$.

In an isothermal process:

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

  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 $\Delta E\neq 0$

Show answer
Correct Option: B
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.