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Chemistry MCQs for NEET — Practice Questions with Answers

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The heat liberated when 1.89 g of benzoic acid is burnt in a bomb calorimeter at 25°C increases the temperature of 18.94 kg of water by 0.632°C. If the specific heat of water at 25°C is 0.998 cal/g-deg, the value of the heat combustion of benzoic acid is [CPMT 1999; BHU 2000]

771.1 kcal

871.2 kcal

881.1 kcal

981.1 kcal

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Explanation

$Q = \frac{18 .94 \times 0 .632 \times 0 .998 \times 122}{1 .89}$ = $771 .1 K c a l$

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In thermodynamics, a process is called reversible when [AIIMS 2001]

Surroundings and system change into each other

There is no boundary between system and surroundings

The surroundings are always in equilibrium with the system

The system changes into the surroundings spontaneously

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Explanation

Thermodynamics is a reversible process in which surroundings are always equilibrium with system.

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$C_{(graphite)} + O_{2} (g) \to C O_{2} (g)$ ; $Δ H = - 94 .05 k c a l m o l^{- 1}$

$C_{(diamond)} + O_{2} (g) \to C O_{2} (g);$ $Δ H = - 94 .50 k c a l m o l^{- 1}$

which of the following is correct?

$C_{(graphite)} \to C_{(diamond)}$ ; $Δ H_{298 K}^{o} = - 450 c a l m o l^{- 1}$

$C_{(diamond)} \to C_{(graphite)};$ $Δ H_{298 K}^{o} = + 450 c a l m o l^{- 1}$

Graphite is the stabler allotrope

Diamond is harder than graphite

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Explanation

Heat energy is also involved when one allotropic form of an element is converted in to another. graphite is the stabler allotrope because the heat of transformation of $C_{(d i a m o n d)} \to C_{(g r a p h i t e)}$ .

(i) $C_{(d i a)} + O_{2 (g)} = C O_{2 (g)} Δ H = - 94 .5 k c a l$

(ii) $C_{(g r a p h i t e)} + O_{2 (g)} = C O_{2 (g)} Δ H = - 94 .0 k c a l$

$Δ H_{t r a n s f o r m a t i o n} = - 94 .5 - (- 94 .0)$

= –0.5 k cal.

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Which of the following is not a correct statement? [ (Engg.) 2002]

When ΔG is negative, the process is spontaneous

When ΔG is zero, the process is in a state of equilibrium

When ΔG is positive, the process is non-spontaneous

None of these

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Explanation

All are the correct statements.

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Among mass, volume, density and specific volume of a gas, the intensive properties are

Density and specific volume

volume and density

specific volume and mass

density only

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Explanation

Density is mass per unit volume and specific volume is volume per unit mass.

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The heat of formation of HCl(g) from the reaction

H₂(g) + Cl₂(g) ; $∆ H = - 44 k c a l$ is

+44 kcal

-44 kcal

+22 kcal

-22 kcal

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Explanation

The heat of formation of HCl is to be calculated on per mole of HCl produced

$∴$ heat of formation of HCl = $- \frac{44}{2}$ = -22 kcal.

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If a chemical change is brought about by one or more methods in one or more steps, then the amount of heat absorbed or evolved during the complete course of reaction is same, which ever method was followed. This law is known as-

Le Chatelier's principle

Hess's law

Joule Thomson effect

Trouton's law

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Explanation

The statement is definition of Hess's law

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If 1.00 kcal of heat is added to 1.2 L of oxygen in a cylinder at constant pressure of 1.000 atm, the volume increases to 1.5 L, Hence $∆ E$ for this process is:

0.993 kcal

1.0073 kcal

0.0993 kcal

1.00073 kcal

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Explanation

$∆ H = ∆ E + P ∆ V$

1.00 = $∆ E + \frac{1 (1.5 - 1.2) L a t m}{0.082 L a t m} \times 2 \times 10^{- 3} k c a l ∆ E = 0.993 k c a l$

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Heats of combustion of $C H_{4}, C_{2} H_{4}, C_{2} H_{6}$ are -890, -1411 and -1560 kJ/mole respectively. Which has the lowest fuel value in kJ/gm ?

$C H_{4}$

$C_{2} H_{4}$

$C_{2} H_{6}$

All same

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Explanation

The heat of combustion of per gm of $C H_{4}$

= $- \frac{890}{16} = - 55.625 k J$

The heat of combustion of per gm of $C_{2} H_{4}$

= $\frac{1411}{28} = - 50.393 k J$

The heat of combustion of per gm of $C_{2} H_{6}$

= $- \frac{1560}{30} = - 52 k J$

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The standard heat of combustion of a hydrocarbon compound is an/a-

Extensive property

Colligative property

Intensive property

Constitutive property

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Explanation

The standard heat of combustion is defined as the heat evolved on burning one mole of the compound in presence of O₂ to give CO₂ and water. Since the amount of the substance is specified, it becomes an intensive property.

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