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Thermodynamics

Multiple Choice Questions

171.

Standard enthalpies of formation of O₃, CO₂, NH₃ and HI are 142.2, -393.2,- 46.2 and 25.9 kJ mol^-1 respectively. Decreasing order of their stability is

HI > NH₃ > CO₂ > O₃
NH₃ > CO₂ > HI > O₃
CO₂ > NH₃ > HI > O₃
O₃ > HI > NH₃ > CO₂

172.

First and second ionisation enthalpies of Mg are 737. 76 and 1450.73 J mol^-1 respectively. The energy required to convert all the atoms of magnesium to magnesium ions present in 24g of magnesium vapours is

24 kJ
2.188 kJ
12 kJ
4.253 kJ

173.

The standard enthalpy of vapourisation Δ_vap H° for water at 100°C is 40.66 kJ mol^-1. The internal energy of vapourisation of water 100°C (in kJ mol^-1) is (assume water vapour to behave like an ideal gas).

+ 4376
+40.66
+37.56
None of these

174.

Which one of the following statements is true?

$∆$ E ia always greater than $∆$ H
$∆$ E is always less than $∆$ H
$∆$ E may be lesser or greater or equal to $∆$ H
$∆$ E is always proportional to $∆$ H

175.

The intensive property among these quantities is

enthalpy
mass/ volume
mass
volume

176.

If the adsorption of a gas on solid metal surface is spontaneous and exothermic, then

H increases
S increases
G increases
S decreases

177.
30g of Mg and 30g of oxygen are reacted and the residual mixture contains

45 g of MgO and 15 g of O₂

50 g of MgO and 10 g of O₂

60 g of MgO only

40 g of MgO and 20 g of O₂

50 g of MgO and 10 g of O₂

Mg and oxygen reacts as

$\underset{(48)}{\underset{2 \times 24}{2 Mg}} + \underset{(32)}{\underset{32}{O_{2}}} \to \underset{(80)}{\underset{2 \times 40}{2 MgO}}$

48 gm of Mg produce 80 gm of MgO

$∴$ 30 gm of Mg produce

$\frac{80 \times 30}{48} = 50 gm$

$∵$ 48 gm of Mg reacts with 32 gm of oxygen.

$∴$ 30 gm of Mg reacts with

$\frac{32 \times 30}{48}$ = 10 gm

178.

Compounds with high heat of formation are less stable because

high temperature is required to synthesise them
molecules of such compounds are distorted
it is difficult to synthesise them
energy rich state leads to instability

179.

Enthalpy of formation of HF and HCl are -161 kJ and -92 kJ respectively. Which of the following statements is incorrect?

HCl is more stable than HF
HF and HCl are exothermic compounds
The affinity of fluorine to hydrogen is greaterthan the affinity of chlorine to hydrogen
HF is more stable than HCl