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1

### NEET 2017

MCQ (Single Correct Answer)
A first order reaction has a specific reaction rate of 10$$-$$2 sec$$-$$1. How much time will it take for 20 g of the reactant to reduce to 5 g?
A
138.6 sec
B
346.5 sec
C
693.0 sec
D
238.6 sec

## Explanation

For a first order reaction,

k = $${{2.303} \over t}\log {{{{\left[ A \right]}_0}} \over {{{\left[ A \right]}_t}}}$$

$$\Rightarrow$$ 10-2 = $${{2.303} \over t}\log {{20} \over 5}$$

$$\Rightarrow$$ t = 138.6 sec
2

### NEET 2017

MCQ (Single Correct Answer)
Mechanism of a hypothetical reaction
X2 + Y2 $$\to$$ 2XY, is given below :
(i)   X2 $$\to$$ X + X (fast)
(ii)  X + Y2 $$\rightleftharpoons$$ XY + Y (slow)
(iii) X + Y $$\to$$ XY (fast)
The overall order of the reaction will be
A
2
B
0
C
1.5
D
1

## Explanation

Slow step is the rate determining step.

Rate = k[X][Y2] ......(i)

Equilibrium constant for fast step,

K = $${{{{\left[ X \right]}^2}} \over {\left[ {{X_2}} \right]}}$$

$$\Rightarrow$$ [X] = $$\sqrt {K\left[ {{X_2}} \right]}$$

By substituting [X] in equation (i), we get

Rate = k$$\sqrt {K\left[ {{X_2}} \right]}$$[Y2] = k' $${{{\left[ {{X_2}} \right]}^{{1 \over 2}}}}$$[Y2]

$$\therefore$$ Order of reaction = $${1 \over 2} + 1$$ = 1.5
3

### NEET 2016 Phase 1

MCQ (Single Correct Answer)
The addition of a catalyst during a chemical reaction alters which of the following quantities?
A
Enthalpy
B
Activation energy
C
Entropy
D
Internal energy

## Explanation

A catalyst provides an alternate path to the reaction which has lower activation energy.
4

### NEET 2016 Phase 2

MCQ (Single Correct Answer)
The decomposition of phosphine (PH3) on tungsten at low pressure is a first-order reaction. It is because the
A
rate is proportional to the surface coverage
B
rate is inversely proportional to the surface coverage
C
rate is independent of the surface coverage
D
rate of decomposition is very slow.

## Explanation

At low pressure, rate is proportional to the surface coverage and is of first order while at high pressure it follows zero order kinetic due to complete coverage of surface area.

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NEET

Class 12