Question

What does first order mean?

Answer 1

Hint: Let us know about first order reaction. A reaction that solely depends on the concentration of one ingredient is referred to as a first-order reaction (a unimolecular reaction). Other reactants are allowed, but they must all be zero-order.

Complete answer:
Only one reactant's concentration determines the outcome of a first-order reaction. As a result, a first-order reaction is often known as a unimolecular reaction. Other reactants may be present, but they will all be zero-order because their concentrations have no effect on the rate. As a result, the rate law for a first-order elementary reaction with regard to a reactant A is:
\[r = - \dfrac{{d[A]}}{{dt}} = k[A]\]
The rate constant, $k$, must have units of\[\dfrac{{Concentration}}{{time}}\], which it does in this case with units of\[{s^{ - 1}}\].
Another way to describe reaction rates is to use the time it takes for a reactant's concentration to drop to half its initial value. The reaction's half-life, abbreviated as\[{t_{\dfrac{1}{2}}}\], is the amount of time it takes for the reaction to complete. As a result, a reaction's half-life is the time it takes for the reactant concentration to drop from ${[A]_0}$to\[{\left[ A \right]_{\dfrac{0}{2}}}\]. When two reactions occur in the same order, the faster reaction has a shorter half-life whereas the slower reaction has a longer half-life.
Under a given set of reaction circumstances, the half-life of a first-order reaction is constant. For zeroth- and second-order reactions, this is not the case. A first-order reaction's half-life is independent of the reactant concentration.

Note:
A first-order reaction's half-life is a constant that is proportional to the reaction's rate constant:\[{t_{\dfrac{1}{2}}}{\text{ }} = {\text{ }}0.693/k\]. First-order reactions are radioactive decay reactions. The decrease in the number of radioactive nuclei per unit time is the rate of decay, or activity, of a sample of a radioactive substance.