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

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:

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

  4. can't be predicted
Reveal answer Fill a bubble to check yourself
B Correct answer
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,  $\Delta H <\Delta U$

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

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

  1. $\displaystyle =37.9kJ{ mol }^{ -1 }$
  2. $\displaystyle =27.5kJ{ mol }^{ -1 }$
  3. $\displaystyle =47.5kJ{ mol }^{ -1 }$
  4. $\displaystyle =17.5kJ{ mol }^{ -1 }$
Reveal answer Fill a bubble to check yourself
A Correct answer
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

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

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

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

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:

  1. 18.24 kJ

  2. 20.477 kJ

  3. 22.05 kJ

  4. 23.923 kJ
Reveal answer Fill a bubble to check yourself
B Correct answer
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

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 water: a natural resource where do we get water from? why conserve water uses of water

Why is water scarcity increasing day by day?

  1. Water cycle has stopped functioning.

  2. The total amount of water on earth has decreased.

  3. The amount of potable water is decreasing because of over usage.

  4. Due to increasing frequency of floods and droughts.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The amount of potable water is decreasing because of over usage. Water scarcity is the lack of sufficient available water resources to meet the demands of water usage within a region.

Multiple choice chemistry water: a natural resource where do we get water from? why conserve water uses of water

Which of the following does not show water shortage?

  1. Taps running dry

  2. Long queues for getting water

  3. Marches and protests for demand of water

  4. A family gets three buckets of water per person per day
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Water shortage is the lack of sufficient available water resources to meet the demands of water usage within a region.
There seems no water shortage as three buckets of water per person is enough for satisfying their daily needs.

Multiple choice chemistry water: a natural resource where do we get water from? why conserve water uses of water

The amount of water on earth today compared with the amount of water on earth at the time it was formed about 5 billion years ago is :

  1. The same amount

  2. About 80 per cent of the original amount

  3. About 50 per cent of what is was originally

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

The amount of water on earth today compared with the amount of water on earth at the time it was formed about $5$ billion years ago is about $50$ per cent of what it was originally.