When the initial concentration of the reactant is doubled,
the half-life period of a zero-order reaction:

Mechanism of a hypothetical reaction

${\mathrm{X}}_2<mspace\; width="1em"></mspace>+<mspace\; width="1em"></mspace>{\mathrm{Y}}_2<mspace\; width="1em"></mspace>\stackrel{<mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace>}{\to }<mspace\; width="1em"></mspace>2\mathrm{XY}$ is given below

(i) ${\mathrm{X}}_2<mspace\; width="1em"></mspace>\stackrel{<mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace>}{\rightleftharpoons }<mspace\; width="1em"></mspace>\mathrm{X}<mspace\; width="1em"></mspace>+<mspace\; width="1em"></mspace>\mathrm{X}<mspace\; width="1em"></mspace>\left(\mathrm{fast}\right)$

(ii) $\mathrm{X}<mspace\; width="1em"></mspace>+<mspace\; width="1em"></mspace>{\mathrm{Y}}_2<mspace\; width="1em"></mspace>\stackrel{<mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace>}{\to }<mspace\; width="1em"></mspace>\mathrm{XY}<mspace\; width="1em"></mspace>+<mspace\; width="1em"></mspace>\mathrm{Y}<mspace\; width="1em"></mspace>\left(\mathrm{slowest}\right)$

(iii) $\mathrm{X}<mspace\; width="1em"></mspace>+<mspace\; width="1em"></mspace>\mathrm{Y}<mspace\; width="1em"></mspace>\stackrel{<mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace>}{\to }\mathrm{XY}<mspace\; width="1em"></mspace>\left(\mathrm{fast}\right)$

The overall order of the reaction will be

1. 1

2.  2

3. 0

4. 1.5

The mechanism of a hypothetical reaction

X₂ + Y₂ → 2XY  is given below: 
(i) X₂ → X + X (Fast)
(ii) X + Y₂  ⇄  XY + Y (slow)
(iii) X + Y → XY (Fast)

The overall order of the reaction will be:

1. 2

2. 0

3. 1.5

4. 1