Question

If the reaction $A+B\to C+D$ is known to be zero order, what is the expression for its rate law?
A.Rate = $k\left[ A \right]{{\left[ B \right]}^{2}}$
B.Rate = $k{{\left[ A \right]}^{x}}{{\left[ B \right]}^{y}}$
C.Rate = $k\left[ A \right]\left[ B \right]$
D.Rate = $k$

Answer 1

Hint: A zero order reaction is the one that has a rate that is independent of the concentration of the reactant(s) i.e. increasing or decreasing the concentration of the reacting species will not speed up or slow down the reaction rate.

Complete answer:
A reaction is zero-order if concentration data is plotted versus time and the result is a straight line. In the above given options, the one in which rate law is independent of reactant concentrations is option- (D). The square bracket term represents the concentration of the term written inside it. So, in all the other options, the rate is dependent on concentrations of A and B which are the reactants according to the given equation- $A+B\to C+D$. Therefore, the rate law for a zero order reaction is Rate=$k$. The rate constant k will have units of concentration/time, such as M/s.

So, option D is correct.

Additional Information:
 Important points regarding zero-order reactions:
For a zero-order reaction, increasing the concentration of the reacting species will not speed up the rate of the reaction.
Zero-order reactions are found when a material that is required for the reaction to proceed, such as a surface is saturated by the reactants.
A reaction is zero-order if concentration data is plotted versus time and the result is a straight line.
The integral form of zero order reaction is written as- \[\left [A \right]=-k t +\left [{{A} {0}} \right]\]
So when we compare this equation to straight line equation, $y=mx+ c$ so we get a graph of \[\left [A \right]\]against t as straight line with slope equal to (-k) and intercept equal to\ [\ left [ {{A}_{0}} \right]\].
The rate constant is the proportionality constant in the equation that expresses the relationship between the rate of a chemical reaction and the concentrations of the reacting substances.
Half Life Time: The half- life of a reaction is the time required for the reactant concentration to decrease to one-half of its initial value. The half -life of a zero order reaction decreases as the initial concentration of the reactant in the reaction decreases. It is given by- \[{{t}_{{}^{1}/{}_{2}}}=\dfrac {\left [{{A}_ {0}} \right]}{2k}\]
Where the expression \[\left [{{A}_ {0}} \right]\]is initial concentration.

Note:
Rate law for First order reaction is: A first-order reaction is a reaction—that precedes at a rate that depends linearly on only one reactant concentration.
Rate law for Second order reaction is: Second order reactions can be defined as chemical reactions wherein the sum of the exponents in the corresponding rate law of the chemical reaction is equal to two.