Tag: enthalpy changes

Questions Related to enthalpy changes

A mixture of 2 mole of carbon monoxide and one mole of oxygen in a closed vessel is ignited to get carbon dioxide. If $\Delta H$ is the enthalpy change and $\Delta U$ is the change in internal energy, then:

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. can't be predicted

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Correct Option: B
Explanation:

A mixture of 2 moles of carbon monoxide and one mole of oxygen in a closed vessel is ignited to get carbon dioxide. 


 $2CO(g)+ O _2(g) \longrightarrow 2CO _2(g)$

 $\Delta n _g =2-3=-1$

If $\Delta H$ is the enthalpy change and $\Delta U$ is the change in internal energy, then:

 $\Delta H=\Delta U+\Delta n _gRT=\Delta U-RT$

Hence,  $\Delta H <\Delta U$

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

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

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

  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

Show answer
Correct Option: A
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 

Water is boiled under a pressure of 1.0atm. When an electric current of 0.50A from a 12V supply is passed for 300 second through a resistance in thermal contact with it, it is found that 0.789g of water is vapourized. The molar internal energy change at boiling point (373.15K) is

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

  1. $\displaystyle =37.9kJ{ mol }^{ -1 }$

  2. $\displaystyle =27.5kJ{ mol }^{ -1 }$

  3. $\displaystyle =47.5kJ{ mol }^{ -1 }$

  4. $\displaystyle =17.5kJ{ mol }^{ -1 }$

Show answer
Correct Option: A
Explanation:

The vaporization occurs at constant pressure therefore the enthalpy change is equal to the work done by the heater:

$\displaystyle \Delta { H }^{ \prime  }=0.50\times 12\times 300\quad (\Delta H=i\times V\times t)$
$\displaystyle =1800J$
$\displaystyle =+1.8kJ$

$\displaystyle \therefore $Molar enthalpy of vaporization, $\displaystyle \Delta H=\frac { \Delta { H }^{ \prime  } }{ mole\ of\ { H } _{ 2 }{ O } } $
$\displaystyle =\frac { 1.8 }{ \left( \frac { 0.789 }{ 18 }  \right)  } $

$\displaystyle =41.06\ kJ{ mol }^{ -1 }$

Also, $\displaystyle \Delta H=\Delta U+P\Delta V$

$\displaystyle =\Delta U+\Delta { n } _{ g }RT$

$\displaystyle =\Delta U+RT$ $\displaystyle ((\because { H } _{ 2 }O\left( l \right) \rightleftharpoons { H } _{ 2 }O\left( g \right) ,\Delta { n } _{ g }=1)$

$\displaystyle \therefore \Delta U=$ molar internal energy change

$\displaystyle\Delta U= \Delta H-RT$

$\displaystyle =41.06-(8.314\times { 10 }^{ -3 }\times 373.15)$
$\displaystyle =37.96\ kJ{ mol }^{ -1 }$

A piston exerting a pressure of 1.0 atmosphere rests on the surface of water at $100^{\circ}C$. The pressure is reduced to smaller extent and as a result 10 g of water evaporates and absorbs 22.2 kJ of heat. The change in internal energy is:

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

  1. 18.24 kJ

  2. 20.477 kJ

  3. 22.05 kJ

  4. 23.923 kJ

Show answer
Correct Option: B
Explanation:

The relationship between the enthalpy change and the change in the internal energy is
 $\Delta H=\Delta U+P\Delta V=\Delta U+\Delta n _gRT$
$\Delta U=\Delta H-\Delta n _gRT$
Substitute values in the above expression.
$\Delta U=\displaystyle 22.2-\frac{10}{18}\times 8.314\times 10^{-3}\times 373$
                       $=20.477 :kJ$
The change in internal energy is 20.477 kJ

Enthalpy is_________.

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

  1. Energy of reaction

  2. Tendency for loss of energy

  3. Tendency for randomness

  4. Use of energy in overcoming entropy

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Correct Option: A
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.

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

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

  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.

Show answer
Correct Option: E
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.