What is a First-order Reaction

A first-order reaction is a chemical reaction where the reaction rate depends linearly on the reactant’s concentration. In other words, if the concentration is doubled, the reaction rate is also doubled. A first-order reaction can have one reactant, for example, a decomposition reaction, or two reactants ^[1-6].

First-order Reaction Rate Law

The kinetics of a chemical equation can be studied using rate law, which gives a relationship between the reaction rate and the concentration of the reactants. The rate law is expressed in differential and integral forms ^[1-7].

Differential Rate Law

The differential rate law gives the derivative of the reactant’s concentration with time. For a first-order reaction, it is given as,

R = – d[A]/dt = k [A]

Where,

R is the reaction rate

[A] is the concentration of the reactant A

k is the rate constant

The term d[A]/dt is the derivative of [A] with time.

Units

The unit for concentration is moles, the unit for the reaction rate is moles/second, and that for the rate constant is 1/second. Therefore, the numerical value of k for a first-order reaction is independent of the unit in which concentration is expressed.

Integrated Rate Law

The integrated rate law is beneficial in predicting how much of a reactant will remain at some time or how long it will take for the concentration to drop to some fixed amount (e.g., one-half). Integrating the above differential gives the expression for concentration as a function of time.

[A] = [A]_o exp (-kt)

Where,

[A]_o is the concentration at time t = 0

[A] is the concentration at time t

Taking the natural logarithm on both sides.

ln [A] = ln [A]_o – kt

The above equation is a straight line with k as the slope. It can be used to determine the rate constant.

How to Determine the First-order Reaction

To determine if the reaction is first-order, plot the natural logarithm of the concentration versus time and see whether the graph is linear. The reaction will be first-order if the graph is linear with a negative slope.

Half-life of a First-order Reaction ^[9]

The half-life of a reaction is defined as the time taken for the reactant’s concentration to reduce to one-half.

[A] = [A]_o / 2

Plugging in for [A] in the expression for [A], we get the half-life (t_1/2).

t_1/2 = 0.693 / k

Characteristics of First-order Reaction

Here are some facts and characteristics of the first-order reaction.

• The reaction rate is proportional to the reactant’s concentration
• Half-life is constant and independent of the reactant’s concentration

Examples of First-order Reaction

Here are some examples of first-order chemical reactions.

1. Trimethyl bromomethane ((CH₃)₃CBr) reacts with sodium hydroxide (NaOH) to give trimethyl methanol ((CH₃)₃COH) and sodium bromide (NaBr) ^[8].

(CH₃)₃CBr + NaOH → (CH₃)₃COH + NaBr

2. Decomposition of hydrogen peroxide (H₂O₂) into water (H₂O) and oxygen (O₂)

2 H₂O₂ (l) → H₂O (l) + O₂ (g)

3. Hydrolysis of methyl acetate (CH₃COOCH₃) in the presence of mineral acids

CH₃COOCH₃ (aq.) + H₂O (l) → CH₃COOH (aq.  + CH₃OH (aq.)

4. Decomposition of ammonium nitrite (NH₄NO₂) in aqueous solution

NH₄NO₂ (aq.) → N₂ (g) + 2 H₂O (l)

5. Hydrogenation of ethene (C₂H₄)

C₂H₄ (g) + H₂ (g) → C₂H₆ (g)

Pseudo First-order Reaction

In some chemical reactions, the concentration of one of the reactants is so high that it does not change with time. In other words, the concentration remains constant throughout the reaction.

Consider the following reaction,

A + B → Products

The rate is given by,

R = k [A] [B]

This equation is the rate law for a second-order reaction. Suppose the concentration of B remains constant throughout the reaction. Then, it can be incorporated into the rate constant term k, and the rate R is given by,

R = k’ [A]

Where,

k’ = k [B] is the new rate constant.

Therefore, the pseudo first-order reaction is a second-order reaction that behaves like a first-order reaction.

Example: Hydrolysis of sucrose (C₁₂H₂₂O₁₁) gives glucose (C₆H₁₂O₆) and fructose (C₆H₁₂O₆).

C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆

Zero, First, and Second-order Reactions

The following table lists the differences between the zero, first, and second-order reactions ^[10].

	Zero Order	First Order	Second Order
Definition	The reaction rate is constant over time and independent of the reactants’ concentration	The reaction rate is proportional to the first power of the reactant’s concentration	The reaction rate is proportional to the second power of the reactant’s concentration or product of two reactants’ concentration
Rate equation	R = constant	R = k [A]	R = k [A]2 or R = k [A] [B]
Concentration	[A] = kt	[A] = [A]o exp (-kt)	1 / [A] = 1 / [A]o + kt
Half-life	[A]o / 2k	0.693 / k	1 / k[A]o
Example	Decomposition of nitrous oxide	Decomposition of hydrogen peroxide	Decomposition of nitrogen dioxide