Question

Why don't we use double arrows in the equation for the reaction of a strong acid with water?

Short answer

Double arrows are not used for the reaction of a strong acid with water because strong acids completely dissociate in water, leaving no significant reverse reaction. A single arrow properly indicates the one-directional reaction.

Step-by-step solution

Understanding Strong Acids

Recognize that strong acids dissociate completely in water. This means that when a strong acid reacts with water, it donates all its hydrogen ions (H+) to the water, forming hydronium ions (H3O+).

Writing the Chemical Equation

Write the chemical equation for the reaction. Since strong acids fully dissociate, the products of the reaction do not revert back to reactants. Therefore, a single arrow is used to indicate the direction from reactants to products.

Clarifying the Purpose of Double Arrows

Understand that double arrows (↔) are used in chemical reactions to depict equilibrium when both the forward and reverse reactions occur. In the case of strong acids, there is no significant reverse reaction because the dissociation is complete.

Key concepts

Acid Dissociation

When we talk about strong acids in chemistry, we refer to substances that have the ability to almost completely dissociate into ions when they are dissolved in water. Dissociation is a process where acid molecules split into smaller parts, usually hydrogen ions (\text{H}^+) and the corresponding anions. An example of such a reaction with hydrochloric acid (\text{HCl}) can be written as:
\[ \text{HCl} (aq) \rightarrow \text{H}^+ (aq) + \text{Cl}^- (aq) \]
In this reaction, \text{HCl} splits into \text{H}^+ and \text{Cl}^- ions. It is important to remember that in aqueous solutions, the \text{H}^+ ions associate immediately with water molecules to form \text{H}_3\text{O}^+ ions, which are referred to as hydronium ions. This complete dissociation property of strong acids means there is a very high concentration of these ions in solution, and this is what makes these acids 'strong.'

Chemical Equilibrium

Chemical equilibrium is a state in which the rates of the forward and reverse reactions in a chemical system are equal, resulting in no observable change in the amounts of reactants and products over time. In reactions where chemical equilibrium plays a role, double arrows \[\leftrightarrow\] are used to indicate that both the forward and backward reactions are happening at equal rates. However, in the case of strong acids reacting with water, strong acids completely dissociate into their ions, and so there is effectively no reverse reaction taking place. Equilibrium concepts do not apply here because we don't have a scenario where the formation and recombination of reactants and products achieve a balance. Instead, the completion of dissociation of strong acids justifies the use of a single arrow, pointing from left to right, representing the irreversible reaction.

Writing Chemical Equations

Writing chemical equations is a fundamental task in chemistry, which allows one to represent the transformations of reactants into products using symbols and formulas. For the reaction of a strong acid with water, the equation should clearly illustrate that the acid completely dissociates, leaving no undissociated molecules behind. A single arrow (\text{\rightarrow}) is used to show the complete and irreversible conversion of acid into its ions. A good example is the dissociation of sulfuric acid:
\[ \text{H}_2\text{SO}_4 (aq) \rightarrow 2\text{H}^+ (aq) + \text{SO}_4^{2-} (aq) \]
The use of the single arrow indicates that, upon dissolution in water, the sulfuric acid molecules fully separate into two \text{H}^+ ions and one \text{SO}_4^{2-} ion, and there is no significant reformation of \text{H}_2\text{SO}_4.

Hydronium Ion Formation

The formation of hydronium ions (\text{H}_3\text{O}^+) is central to our understanding of acid-base chemistry. When a strong acid dissociates in water, the free \text{H}^+ ions that are released do not exist in isolation. Instead, they quickly react with water molecules to create hydronium ions through the following reaction:
\[ \text{H}^+ (aq) + \text{H}_2\text{O} (l) \rightarrow \text{H}_3\text{O}^+ (aq) \]
The rapid formation of hydronium ions is a distinctive characteristic of strong acids in aqueous solutions. Since a strong acid reacts with water to give \text{H}_3\text{O}^+ ions directly and completely, the concept of hydronium ion formation reinforces the use of a single arrow to indicate the one-way direction of strong acid dissolution and dissociation in water.