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An acid is a substance that increases the concentration of $H_3 O^{+}$ ions in solution. Look at the reaction below of phenol $\left(C_6 H_5 OH\right)$ with water:



$C_6 H_5 OH(aq) + H_2 O(l) \longrightarrow C_6 H_5 O^{-}(aq) + H_3 O^{+}(aq)$



Which reactant acts as an acid by contributing an $H^{+}$ ion to create $H_3 O^{+}$ in the chemical equation?


A$C_6 H_5 OH(aq)$
B$H_2 O(l)$
C$C_6 H_5 O^{-}(aq)$
D$H_3 O^{+}(aq)$

Verified Answer :

  • The chemical reaction is \(C_6H_5OH(aq) + H_2O(l) \longrightarrow C_6H_5O^{-}(aq) + H_3O^{+}(aq)\).
  • An acid is defined as a substance that donates a proton (\(H^{+}\)) to form \(H_3O^{+}\).
  • Tracing the reaction shows \(C_6H_5OH\) loses an \(H^{+}\) ion, while \(H_2O\) gains one to become \(H_3O^{+}\).
  • Therefore, \(C_6H_5OH(aq)\) acts as the acid.
  • Final answer: \(\boxed{C_6H_5OH(aq)}\)

Explanation

  1. Analysis of the Chemical Reaction

An acid is defined as a substance that increases the concentration of hydronium ions (\(H_3O^{+}\)) in an aqueous solution. This process occurs through the donation of a proton, which is a hydrogen ion (\(H^{+}\)). To determine which reactant acts as an acid in the given equation, we need to trace the movement of the hydrogen ion from the reactants to the products:
\[C_6H_5OH(aq) + H_2O(l) \longrightarrow C_6H_5O^{-}(aq) + H_3O^{+}(aq)\]

  1. Tracing Proton Donation

By comparing the reactants on the left side to the products on the right side of the equation, we can observe the change in each chemical species:

  • The reactant \(C_6H_5OH\) (phenol) loses an \(H^{+}\) ion to become \(C_6H_5O^{-}\) (phenolate ion).
  • The reactant \(H_2O\) (water) gains an \(H^{+}\) ion to become \(H_3O^{+}\) (hydronium ion).

Since an acid is defined as a substance that donates a proton (\(H^{+}\)), the species that gives up the hydrogen ion is the acid. In this reaction, \(C_6H_5OH\) is the species donating the \(H^{+}\) ion to the water molecule.

  1. Conclusion

The definition of an acid states that it is the substance that contributes the \(H^{+}\) ion to form \(H_3O^{+}\). Since \(C_6H_5OH\) contributes an \(H^{+}\) ion to \(H_2O\) to form \(H_3O^{+}\), \(C_6H_5OH\) functions as the acid in this reaction. Thus, the reactant that acts as the acid is \(C_6H_5OH(aq)\).

Examples

In everyday life, acidity is important in many contexts, such as cooking and maintaining body health. For example, when you squeeze lemon juice (which contains citric acid) into tea, the acid donates protons to the molecules in the tea, changing the flavor and color. Understanding the mechanism of proton transfer, where an acid gives a proton to another molecule, allows chemists to predict how different substances will interact in various chemical environments, from industrial processes to biological systems.

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