Chemical kinetics is concerned with the rate and mechanism of chemical reactions in terms of reactant and product molecules The Order of Reaction describes the link that exists between the rate of a chemical reaction and the concentration of the elements involved in it. In chemistry, zero order reaction kinetics characterize the rate of a chemical reaction in terms of reactant and product per unit time. In this article, we will look at Zero Order Reactions.
What is a Zero Order Reaction?
A zero order reaction is a chemical reaction in which the rate is independent of the concentration of the reactants, i.e. the rate does not change when the concentration of the reactants increases or decreases. As a result, the rate constant of the specified reactions is always equal to the rate constant of these reactions.
In reality, zero order reaction kinetics are rare. A zero order reaction is a human-made artefact of the conditions in which the reaction happens. As a result, reactions that follow zero order reactions are frequently referred to as pseudo-zero-order reactions.
Characteristics of Zero Order Reaction
- The reaction rate is determined by the square root of the reactant concentration, which is proportional to the former.
- Rate of the reaction is proportional to the concentration of the reactant.
- The rate of the reaction is not proportional to the concentration of the reactant.
- The rate of the reaction is proportional to the square of the concentration of the reactant.
- The reaction rate is determined by the natural logarithm of the reactant concentration, i.e. they are proportional in nature.
Differential and Integral Form of Zero Order Reaction
The differential rate equation is an equation that represents the rate of reaction’s dependence on the concentration of the reacting species. The instantaneous rate of response is represented by the slope of a tangent at any point in time on the concentration-time graph. The concentration-time graph complicates calculating the rate of reaction. As a result, we’d want to integrate the differential rate equation to obtain a relationship between the concentration at various points and, hence, the rate constant.
A zero-order reaction is one in which the rate of the reaction is proportional to the 0th power of the reactant concentration. Consider the reaction:
Rate = – d[A] / dt = k[A]0 = k
where, k = rate constant of the reaction
– d[A] = k dt
d[A] = – k dt
When we integrate on both sides, we get:
Where [A]0 denotes the reactant [A]’s initialconcentration at time t=0. When we solve for [A], we get:
[A] = – kt + [A]0
This equation is the required integral form.The integrated rate equation for zero-order reactions is the name given to this equation. This form allows us to calculate the population of the reactant at any point after the reaction has begun.
Graph of Zero Order Reaction
When we compare the above-derived equation with the equation of a straight line, y = mx + c, if we plot [A] against t, we get a straight line with slope = – k and intercept equal to [A]0
When the rate of reaction is plotted against concentration and time, the belowgraph isobtained.
Half-Life of a Zero Order Reaction
A chemical reaction’s half-life can be defined as the amount of time required for the concentration of an offered reactant to reach half of its underlying fixation (or the amount of time required for half of the reactants to be depleted). It is represented by the symbol ‘t1/2‘ and is measured in seconds.
The half-life of a zero-order reaction is given below:
[A] = – kt + [A]0
Replacing t with half-life t1/2we get:
½ [A] = – kt1/2 + [A]0
kt1/2= ½ [A]0
t1/2= 1/2k . [A]0
where,
t1/2= stands for the half-life of a reaction
[A]0 = stands for initial concentration (mol. L-1 or M)
k = stands for the zero-order rate constant
The above equation clearly shows that the half-life of the reaction is dependent on both the rate constant and the initial concentration of the reactant.
Examples of Zero Order Reaction
The reactions listed below are a few examples of zero order reactions that are not affected by the concentration of the reactants.
- Photochemical Reaction – The reaction of hydrogen with chlorine is known as the photochemical reaction.
H2+ Cl2→ 2HCl
Rate = k[H2]0[Cl2]0
Rate = k
- Decomposition of nitrous oxide over a hotplatinumsurface –
N2O → N2+ 1/2 O2
Rate [N2O]0= k[N2O]0= k
d[N2O] / dt = k
- Iodization of Acetone –
CH3COCH3 + I2 → ICH2COCH3 + HI
Unit of rate constant for a Zero Order Reaction
A unit of zero order reaction is equivalent to a unit of reaction speed. The rate constant of the reaction is denoted by k. The rate constant of a zero-order reaction is given as concentration/time or M/s, where ‘M’ is the molarity and ‘s’ is one second.
Rate (k) = dC/dt = concentration/time = mol L-1/s = mol L-1 s-1
Factors affecting rate of reaction
- The rate of the reactions is not affected by concentration.
- The half-life of a reactant is proportional to its original concentration.
- At all concentrations, the rate of zero order kinetics in chemistry or chemical science is always equal to the rate constant.
Enzyme catalysed reaction
Enzymes are complex proteins produced by our environment’s living organisms. These are in charge of catalysing an unlimited number of chemical changes that occur in our living system. Catalase, for example, derived from living plant cells, catalyses the breakdown of hydrogen peroxide. Yeast has a variety of enzymes, including maltase, which transforms maltose to glucose and zymase, which converts glucose to ethyl alcohol. For a given amount of enzyme, all reactions will occur in zero order with regard to the substrate.
Frequently Asked Questions on Zero Order Reaction
Q1. How can you determine if it is a Zero Order Reaction?
Answer. The reaction has zero-order kinetics as the reactant concentration increases. If it has no effect, it has first-order kinetics. If adding the reactant decreases the half-life, the reaction has second-order kinetics.
Q2. What exactly is the Zero Order Reaction?
Answer. A zero-order reaction is defined as “a chemical reaction in which the rate of reaction does not change when the concentration of the reactants grows or decreases.” The rate of these reactions is always equal to the rate constant of the specific reactions since the rate is proportional to the 0th power of the concentration of reactants.
Q3. Given below are the rate constants (k) of three reactions. Which of them represents a zero-order reaction?
- Reaction 1: k = 4.2 M-1 s-1
- Reaction 2: k = 1.1 M s-1
- Reaction 3: k = 0.37 s-1
Answer. Reaction 2 is a zero-order reaction since the units are in M/s. Rate constants in zero-order reactions are always indicated by molars per unit of time.
