Question

What is the order of thermal decomposition of gaseous ammonia on platinum?

Answer 1

Order of a reaction refers to the number of reacting particles whose concentration terms determine the rate of the reaction. It may be defined as the sum of powers or exponents to which the concentration terms are raised in the rate law expression.

Complete step by step answer:
For a hypothetical reaction,
${{aA + bB}} \to {{P}}$, where ${{A,B}}$ are reactants and ${{P}}$ is the product
The rate law expression for this reaction will be;
${{Rate = k[A}}{{{]}}^{{m}}}{{{[B]}}^{{n}}}$
The sum, that is, ${{m}} + {{n}}$ will be the order of the reaction. It will be a first order reaction if the sum is one and a second order reaction if it is two. There are some special cases when the order of the reaction is zero. The decomposition of ammonia on platinum is such a one.
The decomposition of ammonia can also be represented by the chemical equation as follows;
${{2N}}{{{H}}_{{3}}}\xrightarrow{{{{Pt}}}}{{{N}}_{{2}}}{{ + 3}}{{{H}}_{{2}}}$
As we know that the order of a reaction cannot be derived from the balanced chemical equation, the experimentally proved order of this reaction is zero and it can be represented as follows;
${{r = k[N}}{{{H}}_{{3}}}{{{]}}^{{0}}}$
This is because of the fact that in this reaction, the metal platinum acts as a catalyst. At sufficiently high pressure the surface of metal is completely covered by the ammonia molecules. Therefore, further increase in pressure (concentration) of ${{N}}{{{H}}_{{3}}}$does not change the reaction rate. It is thus independent of concentration and is a zero-order reaction.
Therefore, the decomposition of gaseous ammonia on platinum is a zero-order reaction.

Note: The order of a reaction can be fractional or zero. The simple way to calculate the order of a reaction is from the rate law expression. But it will be more accurate when we calculate it experimentally. Catalysts are substances that are used to increase or decrease the rate of reaction. The affect the rate of the reaction without actually undergoing the reaction.