Tag: enthalpy changes

Questions Related to enthalpy changes

Heat of reaction at constant pressure is called

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

  1. Enthalpy.

  2. Entropy

  3. Free energy

  4. None of these

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Correct Option: A
Explanation:

Enthalpy : Chemical reactions are generally carried out at constant pressure (atmospheric pressure) and heat of reaction at constant pressure is called Enthalpy (H) as:
$H = U + PV$ (By definition).

The relationship between enthalpy and internal energy change is

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

  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. $\,P\Delta V = \Delta U + \Delta H$

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Correct Option: B
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}$

An athelete is given 100 g of glucose $(C _6H _{12}O _6)$ of energy equivalent to 1560 kJ. He utilises 50 percent of this gained energy in the event. In order to avoid storage of energy in the body, Determine the weight of water he would need to perspire. (The enthalpy of evaporation of water is 44 kJ/mole.)

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

  1. 319 gm

  2. 323 gm

  3. 342 gm

  4. 312

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Correct Option: A
Explanation:

 Net amount of energy given to athlete $= 1560 kJ$

$1560 × 50/100$ Energy lost in an event $= 780 kJ$
 Energy left out $= 1560 – 780 kJ = 780 kJ$
 Now, consider the evaporation of water $H _2O(l) \rightarrow H _2O(g)$; $\Delta H = 44 kJ mole^{–1}$
 Thus, for consumption of $44 kJ$ of energy the amount of water evaporated $=1mole= 18 g$ 
For consumption of $780 kJ$ of energy the amount of water to be evaporated $ \frac {18 \times  780}{44} = 319·09 g$

Enthalpy of the system is given as :

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

  1. $\displaystyle H+PV$

  2. $\displaystyle U+PV$

  3. $\displaystyle U-PV$

  4. $\displaystyle H-PV$

Show answer
Correct Option: B
Explanation:

Enthalpy: 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 :
$H = U + PV$ 

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

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

  1. constant pressure

  2. constant temperature

  3. constant temperature and pressure

  4. constant temperature, pressure and composition

Show answer
Correct Option: A
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 $

In the hydrogenation of ethylene,$\Delta n$ is equal to:

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

  1. 1

  2. -2

  3. -1

  4. 2

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Correct Option: C
Explanation:

$C _{2}H _{4}(g)+H _{2}(g)\rightarrow C _{2}H _{6}(g)$

$\Delta n=1-2$

$=$ - 1


Option C is correct.

For the reaction,

$C(s)+O {2}(g)\rightarrow CO _{2}(g);$

$\Delta n$ value is _________.

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

  1. Zero

  2. +1

  3. -1

  4. unpredictable

Show answer
Correct Option: A
Explanation:

$C(s)+O _{2}(g)\rightarrow CO _{2}(g)$


$\Delta n _{g}=1-1$

$=0$ 

$=$ difference of moles of gaseous substances

Option A is correct.

the $\triangle { H } _{ 1 }\quad $ and $\triangle { H } _{ 2 }\quad $ $(in kJ mol^-1)$ and the $\triangle {egH } $  $(in kJ mol^-1)$ of a few 3 elements are given below:
Which of the element is likel;y to be:

Elements $\triangle { H } _{ 1 }\quad $ $\triangle { H } _{ 2 }\quad $  $\triangle {egH } $
I 520 7300 -60
II 419 3051 -48
III 1681 3374 -328
IV 1008 1846 -295
V 2372 5251 +48
VI 738 1451 -40
chemistry enthalpy changes enthalpy changes and enthalpy profile diagrams enthalpy study of enthalpy

  1. the least reactive element

  2. the most reactive mental

  3. the most reactive non- mental

  4. the least reactive non-mental

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Correct Option: A

What 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

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Correct Option: C
Explanation:

$H$ is enthalpy.

Enthalpy is equivalent to the total heat content of a system.
$\Delta H=\Delta U=\Delta (PV)$

Which of the following statements are correct?

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

  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.

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Correct Option: A,B
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.