The enthalpy of reaction for the reaction: 2 H 2 ( g ) + O 2 ( g ) → 2 H 2 O ( l ) is ∆ r H ∘ = – 572 kJ mol - 1 . What will be standard enthalpy of formation of H 2 O ( l ) ?

The standard enthalpy of formation of H 2 O ( l ) can be explained on the basis of the following reaction: H 2 g + 1 2 O 2 g → H 2 O l So, the standard enthalpy of formation of H 2 O ( l ) will be half of the enthalpy of the reaction △ r H ∘ . ∴ △ f H H 2 O ∘ l = 1 2 × △ r H ∘ = - 572 kJ mol - 1 2 = - 286 kJ / mol Note: The enthalpy is a state function.

Represent the potential energy/enthalpy change in the following processes graphically. (a) Throwing a stone from the ground to roof. (b) 1 2 H 2 g + 1 2 Cl 2 g ⇌ HCl g ∆ r H ∘ = – 92 . 32 kJ mol - 1 In which of the processes potential energy/enthalpy change is contributing factor to the spontaneity?

For the process involved in (a), throwing a stone from the ground to the roof, the potential energy will be increasing as the stone would cover a greater distance from the ground. Graphically it can be represented as: Here, some extra energy is required to perform the work to start the process. Thus, energy remains conserved at all the points following the first law of thermodynamics. From the reaction, 1 2 H 2 g + 1 2 Cl 2 g ⇌ HCl g ∆ r H ∘ = – 92 . 32 kJ mol - 1 , we get to know that, it is an exothermic reaction. The enthalpy of reaction decreases throughout the process. The graphical representation will be: Energy increases in (a) part and decreases in (b). Hence, in the process (b), the enthalpy change is the contributing factor to the spontaneity.

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Heat capacity $(C_{p})$ is an extensive property but specific heat $(c)$ is an intensive property. What will be the relation between $C_{p}$ and $c$ for $1$ mol of water?

Important Questions on Thermodynamics

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NCERT Exemplar Chemistry - Class 11>Chapter 6 - Thermodynamics>Short Answer Type>Q 40

The difference between

C_{p}

and

C_{v}

can be derived using the empirical relation

H = U + pV

. Calculate the difference between

C_{p}

and

C_{v}

for

10

moles of an ideal gas.

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NCERT Exemplar Chemistry - Class 11>Chapter 6 - Thermodynamics>Short Answer Type>Q 41

If the combustion of

1 g

of graphite produces

20.7 kJ

of heat, what will be molar enthalpy change? Give the significance of sign also.

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NCERT Exemplar Chemistry - Class 11>Chapter 6 - Thermodynamics>Short Answer Type>Q 42

The net enthalpy change of a reaction is the amount of energy required to break all the bonds in reactant molecules minus amount of energy required to form all the bonds in the product molecules. What will be the enthalpy change for the following reaction.

$H_{2} (g) + {Br}_{2} (g) \to 2 HBr (g)$

Given that Bond energy of $H_{2}, {Br}_{2}$ and $HBr$ is $435 kJ {mol}^{- 1}, 192 kJ {mol}^{- 1}$ and $368 kJ {mol}^{- 1}$ respectively.

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NCERT Exemplar Chemistry - Class 11>Chapter 6 - Thermodynamics>Short Answer Type>Q 43

The enthalpy of vapourisation of

{CCl}_{4}

30.5 kJ {mol}^{- 1}

. Calculate the heat required for the vapourisation of

284 g

{CCl}_{4}

at constant pressure. (Molar mass of

{CCl}_{4} = 154 g {mol}^{- 1}

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NCERT Exemplar Chemistry - Class 11>Chapter 6 - Thermodynamics>Short Answer Type>Q 44

The enthalpy of reaction for the reaction:

$2 H_{2} (g) + O_{2} (g) \to 2 H_{2} O (l)$ is $∆_{r} H^{\circ} = - 572 kJ {mol}^{- 1}$ . What will be standard enthalpy of formation of $H_{2} O (l)$ ?

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NCERT Exemplar Chemistry - Class 11>Chapter 6 - Thermodynamics>Short Answer Type>Q 45

What will be the work done on an ideal gas enclosed in a cylinder, when it is compressed by a constant external pressure, $p_{ext}$ in a single step as shown in Figure? Explain graphically.

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NCERT Exemplar Chemistry - Class 11>Chapter 6 - Thermodynamics>Short Answer Type>Q 46

How will you calculate work done on an ideal gas in a compression, when change in pressure is carried out in infinite steps?

HARD

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IMPORTANT

NCERT Exemplar Chemistry - Class 11>Chapter 6 - Thermodynamics>Short Answer Type>Q 47

Represent the potential energy/enthalpy change in the following processes graphically.

(a) Throwing a stone from the ground to roof.

(b) $\frac{1}{2} H_{2} (g) + \frac{1}{2} {Cl}_{2} (g) ⇌ HCl (g) ∆_{r} H^{\circ} = - 92.32 kJ {mol}^{- 1}$

In which of the processes potential energy/enthalpy change is contributing factor to the spontaneity?

Heat capacity Cp is an extensive property but specific heat c is an intensive property. What will be the relation between Cp and c for 1 mol of water?

Important Questions on Thermodynamics

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Heat capacity $(C_{p})$ is an extensive property but specific heat $(c)$ is an intensive property. What will be the relation between $C_{p}$ and $c$ for $1$ mol of water?