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1

NEET 2013 (Karnataka)

Three thermochemical equations are given below
(i)  C(graphite) + O2(g) $$\to$$ CO2(g); $$\Delta$$rHo = x kJ mol$$-$$1
(ii)  C(graphite) + $${1 \over 2}$$O2(g) $$\to$$ CO(g); $$\Delta$$rHo = y kJ mol$$-$$1
(iii)  CO(g) + $${1 \over 2}$$O2(g) $$\to$$ CO2(g); $$\Delta$$rHo = z kJ mol$$-$$1
Based on the above equations, find out which of the relationship given below is correct.
A
z = x + y
B
x = y + z
C
y = 2z $$-$$ x
D
x = y $$-$$ z

Explanation

According to Hess's law, equation (i) is equal to equations (ii) + (iii)
2

NEET 2013 (Karnataka)

When 5 litres of a gas mixture of methane and propane is perfectly combusted at 0oC and 1 atmosphere, 16 litres of oxygen at the same temperature and pressure is consumed, The amount of heat released from this combustion in kJ ($$\Delta$$Hcomb. (CH4) = 890 kJ mol$$-$$1, $$\Delta$$Hcomb. (C3H8) = 2220 kJ mol$$-$$1) is
A
38
B
317
C
477
D
32

Explanation

CH4 + 2O2 $$\to$$ CO2 + 2H2O

C3H8 + 5O2 $$\to$$ 3CO2 + 4H2O

CH4 + C3H8 = $${5 \over {22.4}} = 0.22$$ moles.

$${O_2} = {{16} \over {22.4}} = 0.71$$ moles

$$2x + (0.22 \times x)5 = 0.71$$

x = 0.13

Heat liberated = 0.13 $$\times$$ 890 + 0.09 $$\times$$ 2220 = 316 kJ
3

NEET 2013

A reaction having equal energies of activation for forward and reverse reactions has
A
$$\Delta$$H = 0
B
$$\Delta$$H = $$\Delta$$G = $$\Delta$$S = 0
C
$$\Delta$$S = 0
D
$$\Delta$$G = 0

Explanation

For a general reaction,

ΔH = Activation energy of forward reaction – Activation energy of backward reaction.

As, both the energies of activation have same value thus, ΔH = 0.

$$\Delta$$G is not equal to zero because if it is so the reaction must be in equilibrium which is not in this case
4

AIPMT 2012 Prelims

The enthalpy of fusion of water is 1.435 kcal/mol. The molar entropy change for the melting of ice at 0oC is
A
10.52 cal/(mol K)
B
21.04 cal/(mol K)
C
5.260 cal/(mol K)
D
0.526 cal/(mol K)

Explanation

H2O($$l$$) → H2O(s)

∆H = 1.435 Kcal/mol

T = 0 + 273K = 273K

$$\Delta S = {{\Delta H} \over T}$$

$$\Rightarrow$$ $$\Delta S = {{1.435} \over {273}}$$ = 5.26 $$\times$$ 10-3 kcal/mol K

$$\Rightarrow$$ $$\Delta S$$ = 5.260 cal/mol K

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