NEET

For which of the following changes, ΔH ≠ ΔU

N_2(g) + O_2(g) → 2NO_(g)
H_2(g) + Cl_2(g) → 2HCl_(g)
C_(s) + O_2(g) → CO_2(g)
CaCO_3(s) → CaO_(s) + CO_2(g)

Solution

ΔH = ΔU + Δn_(g)RT;

Δn_(g) = 1 in (d);

Δn_(g) = 0 in other cases

Gibbs free energy,for the reaction at equilibrium, is

zero
– ve
+ ve
either (a) or (c)

Solution

The Gibb’s free energy is zero at equilibrium because the rate of backward reaction is equal to the rate of forward reaction.

Hence, option (a) is correct.

Which process takes place on chemical bond formation :

Decrease is energy
Increase in energy
No change in energy
Attractive and repulsive forces are equal

Solution

When a chemical bond is formed between the two free atoms in a gaseous state to form a molecular product in a gaseous state, some heat is always evolved which is known as bond energy.

Hence, there is increase in energy.

The following is an endothermic reaction :

decomposition of water
conversion of graphite to diamond
dehydrogenation of ethane to ethylene
All of these

Solution

Heat energy is absorbed during endothermic reaction.

In all the given reactions, heat is absorbed during reaction.

∴ All of them are endothermic reactions.

For the reaction

\(N_{2 \left ( g \right )} + 3H_{2 \left ( g \right )} \rightleftharpoons 2NH_{3 \left ( g \right )}, \triangle H = ?\)

ΔE + 2RT
ΔE – 2RT
ΔE – RT
None of these

Solution

Δn_g = 2 – 4 = – 2, ΔH = ΔE – 2RT.

Which of the following is correct order ?

1 erg > 1 Joule > 1 calorie
1 erg > 1 calorie > 1 joule
1 calorie > 1 Joule > 1 erg
1 Joule > 1 calorie > 1 erg

Solution

10⁷ erg = 1 Joule, 1 cal= 4.183 J

Hence calorie is the largest amount of energy.

If ΔH is the change in enthalpy and ΔE is change in internal energy accompanying a gaseous reaction, then

ΔH is always greater than ΔE
ΔH < ΔE only if the number of moles of the products is greater than the number of moles of the reactants
ΔH is always less than ΔE
ΔH < ΔE only if the number of moles of products is less than the number of moles of the reactants.

Solution

As ΔH = ΔE + Δn_gRT

if n_p < n_r; Δn_g = n_p – n_r = –ve

Hence ΔH < ΔE

In the isochoric process, ΔH for a system is equal to

P.ΔV
PV
E + P.ΔV
ΔE

Solution

ΔH = ΔE + PΔV

For isochoric process, ΔV = 0

∴ ΔH = ΔE

Calorie is equivalent to :

0.4184 Joule
4.184 Joule
41.84 Joule
418.4 Joule

Solution

1 calorie = 4.184 joule

For precipitation reaction of Ag⁺ ions with NaCl, which of the following statements is correct ?

ΔH for the reaction is zero
ΔG for the reaction is zero
ΔG for the reaction is negative
|ΔG| = |ΔH|

Solution

Ag⁺_(aq) + NaCl_(aq) → AgCl_(s) + Na⁺_(aq)

The Precipitation reaction is spontaneous and hence

ΔG_P, T <; 0.

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