1

### NEET 2016 Phase 2

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.
2

### AIPMT 2015 Cancelled Paper

When initial concentration of a reactant is doubled in a reaction, its half-life period is not affected. The order of the reaction is
A
second
B
more than zero but less than first
C
zero
D
first.

## Explanation

Half-life period of a first order reaction is independent of initial concentration,

${t_{{1 \over 2}}} = {{0.693} \over k}$
3

### AIPMT 2015 Cancelled Paper

The activation energy of a reaction can be determined from the slope of which of the following graphs?
A
ln k vs. ${1 \over T}$
B
${T \over {\ln \,k}}$ vs. ${1 \over T}$
C
ln k vs. $T$
D
${{\ln k} \over T}$ vs. $T$

## Explanation

According to Arrhenius equation,

$k = A{e^{ - {{{E_a}} \over {ET}}}}$

Taking natural log on both the sides we get,

ln k = ln A ${ - {{{E_a}} \over {ET}}}$ ...........(1)

Comparing (1) with standard form of equation of line

y = mx + C

We get Slope, m = ${ - {{{E_a}} \over R}}$

Hence, if ln k is plotted against 1/T, slope of the line will be ${ - {{{E_a}} \over R}}$.
4

### AIPMT 2015

The rate constant of the reaction A $\to$ B is 0.6 $\times$ 10$-$3 mol L$-$1 s$-$1. If the concentration of A is 5 M, then concentration of B after 20 minutes is
A
3.60 M
B
0.36 M
C
0.72 M
D
1.08 M

## Explanation

For zero order reaction unit of rate constant is mole per second.

$\therefore$ For zero order

x = kt

$\Rightarrow$ x = 0.6 × 10–3 × 20 × 60 = 0.72 M