Consider the following acid-neutralization reactions involving the strong base NaOH(aq): HNO31aq2 + NaOH1aq2¡NaNO31aq2 + H2O1l2 HCl1aq2 + NaOH1aq2¡NaCl1aq2 + H2O1l2 NH4+1aq2 + NaOH1aq2¡NH31aq2 + Na+1aq2 + H2O1l2 (d) In the third equation NH4 +1aq2 is acting as an acid. Based on the value of H° for this reaction, do you think it is a strong or a weak acid? Explain.

Verified step by step guidance

1. Identify the reaction type: The reaction involving NH4+ and NaOH is an acid-base neutralization reaction.
2. Analyze the products: The products of the reaction are NH3 (ammonia), Na+ (sodium ion), and H2O (water). The formation of NH3, a weak base, suggests the reactant NH4+ is its conjugate acid.
3. Understand the role of NH4+: In this reaction, NH4+ donates a proton (H+) to the OH- ion from NaOH, forming water and NH3. This behavior is characteristic of an acid.
4. Consider the strength of NH4+ as an acid: Strong acids typically dissociate completely in water to form stable, non-reactive conjugate bases. NH3, however, is known to be a weak base that can accept a proton under certain conditions, indicating NH4+ is a weak acid.
5. Relate to the enthalpy change (H°): If the reaction enthalpy (H°) is not highly negative, it suggests that the reaction does not proceed with a significant release of energy, typical of weak acid reactions with weak bases.

Key Concepts

Acid-Base Theory

Acid-base theory explains the behavior of acids and bases in chemical reactions. According to the Brønsted-Lowry theory, acids are proton donors, while bases are proton acceptors. In the given reaction, NH4+ acts as an acid by donating a proton to NaOH, forming NH3 and water. Understanding this theory is crucial for identifying the role of substances in acid-base reactions.

Strength of Acids

The strength of an acid is determined by its ability to dissociate in solution and release protons (H+ ions). Strong acids, like HCl and HNO3, completely dissociate in water, while weak acids, like NH4+, only partially dissociate. The degree of dissociation can be inferred from the reaction's enthalpy change (ΔH°), which indicates whether the reaction is favorable and how readily the acid donates protons.

Enthalpy Change (ΔH°)

Enthalpy change (ΔH°) is a measure of the heat absorbed or released during a chemical reaction at constant pressure. A negative ΔH° indicates that the reaction is exothermic, suggesting that the products are more stable than the reactants. In the context of acid strength, a significant release of energy when NH4+ donates a proton may imply that it is a weak acid, as strong acids typically have more favorable dissociation energetics.