Ethyne (HC≡CH) has a pK_a value of 25, water has a pK_a value of 15.7, and ammonia (NH3) has a pKa value of 36. Draw the equation, showing equilibrium arrows that indicate whether reactants or products are favored, for the acid–base reaction of ethyne with
a. HO^-.

Verified step by step guidance

1. Step 1: Identify the acid and base in the reaction. Ethyne (HC≡CH) acts as the acid because it donates a proton, and hydroxide ion (HO⁻) acts as the base because it accepts a proton.
2. Step 2: Write the acid-base reaction. Ethyne reacts with hydroxide ion to form its conjugate base, acetylide ion (HC≡C⁻), and water (H₂O), which is the conjugate acid of hydroxide. The reaction can be written as: HC≡CH + HO⁻ ⇌ HC≡C⁻ + H₂O.
3. Step 3: Compare the pKa values of the acid and conjugate acid. Ethyne has a pKa of 25, and water has a pKa of 15.7. Since water has a lower pKa, it is a stronger acid than ethyne.
4. Step 4: Determine the direction of equilibrium. The equilibrium will favor the formation of the weaker acid and weaker base. Since ethyne is a weaker acid than water, the equilibrium will favor the reactants (HC≡CH and HO⁻).
5. Step 5: Draw the equilibrium arrows. Use equilibrium arrows to indicate that the reaction favors the reactants. The longer arrow should point toward the reactants (HC≡CH and HO⁻), and the shorter arrow should point toward the products (HC≡C⁻ and H₂O).

Key Concepts

Acid-Base Reactions

Acid-base reactions involve the transfer of protons (H+) between reactants. In this context, an acid donates a proton while a base accepts it. Understanding the strength of acids and bases, indicated by their pKa values, is crucial for predicting the direction of the equilibrium in these reactions.

pKa and Acid Strength

The pKa value is a quantitative measure of the strength of an acid in solution; lower pKa values indicate stronger acids. In the given question, ethyne (pKa 25) is a weaker acid compared to water (pKa 15.7), which means it is less likely to donate a proton. This concept is essential for determining which side of the reaction is favored.

Equilibrium in Chemical Reactions

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products. The position of equilibrium can be influenced by the relative strengths of the acids and bases involved, which is critical for drawing the correct equilibrium arrows in the reaction between ethyne and hydroxide ion (HO−).