Question

The‌ ‌decomposition‌ ‌of‌ ‌$HI$‌ ‌on‌ ‌the‌ ‌surface‌ ‌of‌ ‌gold:‌ ‌
A)Pseudo‌ ‌first‌ ‌order‌ ‌
B)Zero‌ ‌order‌ ‌
C)First‌ ‌order‌ ‌
D)Second‌ ‌order‌

Answer 1

Hint: In a chemical kinetics reaction rate constant or reaction rate coefficient, tell us about the rate of the direction and tell us about the direction of a chemical reaction. By just looking at the rate and its S.I.unit, we can tell about the order and the quantity of products and leftover reactants.

Complete answer:
We can say that when a substance A reacts with B to form C then we can say that rate of the reaction:
$
  aA + bB \to cC \\
  r = k(T){[A]^m}{[B]^n} \\
 $
For a reaction as such, the rate of the reaction is directly proportional to molar concentrations of A and B given in the expression of rate. $k(T)$ is the reaction rate constant that depends completely on temperature. The exponents of concentrations A and B, constants are partial orders of reaction and not equal to the stoichiometric coefficient.
The reactions are further divided into different orders according to the number of molar coefficients of the elements on the left side of the reactions or the reactants of the product.
The orders in which they are divided are: -
-Pseudo first order
-Zero order
-First order
-Second order
And so on…
The orders can be pre-decided by seeing the units of the reaction.
Decomposition of $HI$ on the surface of gold is a zero-order reaction. Here, the concentration of the reactants on the metal surface remains constant. Because as the reactants react on the surface, more molecules get absorbed on the surface and thus the rate remains constant.
The order of the reaction of decomposition of $HI$ is zero order.

Note:
As we know that the half-life of a first order reaction or the time taken by the compound to reach the half concentration of what it was before starting the reaction. We get the half time by the expression ${t_{\dfrac{1}{2}}} = \dfrac{{0.693}}{K}$