Tag: enthalpy changes

Questions Related to enthalpy changes

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

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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

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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

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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

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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

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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 $\Delta E\neq 0$

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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.

Two reactions are given below:

(i) $CO _{(g)} + \frac{1}{2}O _2{(g)} \rightarrow CO _2{(g)}$

(ii) $ Ag _2O _{(s)} \rightarrow 2Ag _{(s)} + \frac{1}{2}O _2{(g)}$

Which of the following statements is true?

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

  1. For (i) $\vartriangle$H < $\vartriangle$E and for (ii) $\vartriangle$H > $\vartriangle$E

  2. For (i) $\vartriangle$H > $\vartriangle$E and for (ii) $\vartriangle$H < $\vartriangle$E

  3. For both (i) and (ii) $\vartriangle$H > $\vartriangle$E

  4. For both (i) and (ii) $\vartriangle$H < $\vartriangle$E

Show answer
Correct Option: A
Explanation:

$\Delta H=\Delta E+\Delta { n } _{ g }RT$ 


(i) $CO(g) + \cfrac { 1 }{ 2 } { O } _{ 2 }(g) \longrightarrow  C{ O } _{ 2 }(g)$

$ \quad   \Delta { n } _{ g }=1-\cfrac { 1 }{ 2 } -1=-\cfrac { 1 }{ 2 } <0$

$\therefore \Delta H<\Delta E$

(ii) ${ Ag } _{ 2 }O(s) \longrightarrow  2Ag(s) + \cfrac { 1 }{ 2 } { O } _{ 2 }(g)$

$\quad  \Delta { n } _{ g }=\cfrac { 1 }{ 2 } +0-0=\cfrac { 1 }{ 2 } >0$ 

$\therefore \Delta H>\Delta E$

So, the correct option is A.

Which of the following is abbreviated as $H$?

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

  1. Standard voltaic potential

  2. Entropy

  3. Enthalpy

  4. Reaction rate

  5. Gibbs free energy

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Correct Option: C
Explanation:

Enthalpy is abbreviated as $H$.
The abbreviations for other terms are:
Standard voltaic potential: $V$
Entropy: $S$ 
Gibbs free energy: $G$

Total enthalpy of a system H is given by:
chemistry enthalpy changes enthalpy changes and enthalpy profile diagrams enthalpy study of enthalpy

  1. $H=h/m$

  2. $H=m/h$

  3. $H=m\times h$

  4. None of the mentioned

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Correct Option: C