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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:



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



Which reactant acts as an acid by contributing a $H ^{+}$ ion to create $H _3 O ^{+}$ in the chemical equation?


A$C _6 H _5 OH (a q)$
B$H _2 O (l)$
C$C _6 H _5 O ^{-}(a q)$
D$H _3 O ^{+}( aq )$

Verified Answer :

  • Identify the reaction: \(C_6H_5OH(aq) + H_2O(l) \longrightarrow C_6H_5O^{-}(aq) + H_3O^{+}(aq)\).
  • Recognize that \(C_6H_5OH\) loses an \(H^+\) ion to become \(C_6H_5O^-\).
  • Define the Brønsted-Lowry acid as a proton donor.
  • Conclude that since \(C_6H_5OH\) donates the proton, it is the acid, so the final answer is \(\boxed{C_6H_5OH(aq)}\).

Explanation

  1. Problem Analysis and Definitions

To determine which reactant acts as an acid in the given chemical equation, we first analyze the Bronsted-Lowry acid-base theory. An acid is defined as a proton donor—a substance that donates an \(H^+\) ion to another substance. Conversely, a base is a proton acceptor. The given chemical reaction is:
\[C_6H_5OH(aq) + H_2O(l) \longrightarrow C_6H_5O^{-}(aq) + H_3O^{+}(aq)\]
Here, the reactants are phenol (\(C_6H_5OH\)) and water (\(H_2O\)). We need to observe the transformation of each reactant as it converts into its corresponding product.

  1. Transformation Analysis

By comparing the reactants to the products, we can identify the transfer of protons.

  1. Phenol (\(C_6H_5OH\)) transforms into the phenoxide ion (\(C_6H_5O^-\)). Comparing their chemical formulas, we see that \(C_6H_5OH\) has lost one hydrogen ion (\(H^+\)).
  2. Water (\(H_2O\)) transforms into the hydronium ion (\(H_3O^+\)). Comparing their chemical formulas, we see that \(H_2O\) has gained one hydrogen ion (\(H^+\)).

According to the definition provided, the substance that loses (or donates) the \(H^+\) ion acts as the acid. Since \(C_6H_5OH\) donates the proton that \(H_2O\) accepts, \(C_6H_5OH\) is the reactant functioning as the acid.

  1. Conclusion

Based on our analysis, \(C_6H_5OH(aq)\) acts as the Bronsted-Lowry acid by donating a proton to water. Therefore, the correct reactant is \(C_6H_5OH(aq)\). The final answer is \(\boxed{C_6H_5OH(aq)}\).

Examples

Understanding acid-base reactions is fundamental in many real-world applications. For instance, in our own bodies, the pH balance is maintained by various buffer systems, which are essentially mixtures of weak acids and their conjugate bases that resist changes in pH. Another example is the use of vinegar (an acetic acid solution) in cooking, where its acidity helps in tenderizing proteins or creating specific chemical reactions in baking. By identifying which substance acts as an acid, we can predict how substances will interact in various chemical environments.

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