Problem 25 - Chemistry The Central Science In Si Units

Question

Predict the products of the following acid-base reactions, and predict whether the equilibrium lies to the left or to the right of the reaction arrow: (a) \(\mathrm{O}^{2-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons\) (b) \(\mathrm{CH}_{3} \mathrm{COOH}(a q)+\mathrm{HS}^{-}(a q) \rightleftharpoons\) (c) \(\mathrm{NO}_{2}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons\)

Step-by-Step Solution

Answer

(a) Equilibrium lies to the right. (b) Equilibrium slightly left/balanced. (c) Equilibrium slightly left.

Step 1: Identify the nature of the reactants

For each reaction, identify the nature of reactants as either acids or bases.\( \text{(a) } \mathrm{O}^{2-} \) is a strong base, while \( \mathrm{H}_{2} \mathrm{O} \) is a weaker acid.\( \text{(b) } \mathrm{CH}_{3} \mathrm{COOH} \) is an acid, while \( \mathrm{HS}^- \) is a conjugate base.\( \text{(c) } \mathrm{NO}_{2}^{-} \) is a weak base, and \( \mathrm{H}_{2} \mathrm{O} \) acts as both an acid and a base.

Step 2: Predict the products for each reaction

Determine what the products will be when acids react with bases.\( \text{(a) } \mathrm{O}^{2-}(aq) + \mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons 2\mathrm{OH}^-(aq) \) Point for likely product since \( \mathrm{O}^{2-} \) attracts protons to form hydroxide ions.\( \text{(b) } \mathrm{CH}_{3} \mathrm{COOH} + \mathrm{HS}^- \rightleftharpoons \mathrm{HS}^- + \mathrm{CH}_{3} \mathrm{COO}^- \).\( \text{(c) } \mathrm{NO}_{2}^- + \mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{HNO}_{2} + \mathrm{OH}^- \).

Step 3: Assess equilibrium position

Observe the strength of acids and bases: strong acids/bases drive equilibrium.\( \text{(a) } \mathrm{O}^{2-} \) forming \( \mathrm{OH}^- \) strongly suggests the equilibrium will lie to the right.\( \text{(b) } \mathrm{CH}_{3} \mathrm{COOH} \) is only a weak acid, thus the equilibrium will be balanced but can lean slightly left.\( \text{(c) } \mathrm{NO}_{2}^- \) and \( \mathrm{HNO}_2 \) are both weak; it may lean slightly left as well.

Key Concepts

Equilibrium Position

In acid-base reactions, understanding the equilibrium position is crucial. Equilibrium refers to the state where the rate of the forward reaction equals the rate of the reverse reaction. This means the concentrations of reactants and products remain constant over time, though not necessarily equal.
In a chemical reaction, the position of equilibrium indicates whether the reactants or products are favored. If the equilibrium lies to the right, it indicates more products are formed at equilibrium. Conversely, if it lies to the left, the reactants remain predominant.
To determine equilibrium position, consider the relative strengths of the acids and bases involved. Stronger reactants tend to drive the equilibrium towards the formation of more products (right). Weaker acids and bases may result in an equilibrium that favors the reactants (left). Equilibrium is dynamic, allowing adjustments in response to changes like concentration or temperature. This helps illustrate how chemical reactions are often not strictly one-directional but instead reach a state of balance.

Strong and Weak Acids and Bases

Acid and base strengths are foundational to predicting reaction outcomes. Strong acids and bases dissociate completely in solution. This means they release more hydrogen ions (H+) or hydroxide ions (OH-) and can drive reactions forward more forcefully than their weaker counterparts.
Characteristics of Strong Acids/Bases:

• Completely ionize in water.
• Stable ions that remain in solution.
• Drive equilibrium to the right, producing more products.

Weak Acids/Bases: These do not fully dissociate in solution. They tend to form an equilibrium where the undissociated form remains significant. Weak acids and bases often result in a reaction where the equilibrium position is more balanced, or favors reactants.
Understanding the distinction between strong and weak acids and bases helps anticipate reaction behavior. For example, in reaction (a), the strong base oxygen ion ( O^{2-} ) reacts with water to produce a more significant amount of hydroxide ions, driving the equilibrium right.

Acid and Base Reactants

In any acid-base reaction, identifying the reactants helps predict the products and their subsequent interaction. An acid reacts by donating a proton (H+) to a base. Conversely, a base receives a proton. Understanding this foundational concept aids in predicting which species in a given reaction acts as the acid and which as the base.
Acid Reactants:

• Release H+ ions in solution.
• Can be identified if they have protons available for donation.
• Examples include water, acetic acid (\(CH_3COOH\)), and many others.

Base Reactants:

• Receive H+ ions, often forming water or hydroxide ions when reacting with acids.
• Include compounds like \(OH^-\), \(HS^-\), and other ions or molecules capable of accepting protons.

Grasping which reactants are acids or bases is key for predicting the main products of every acid-base reaction and understanding the position of equilibrium. The direction and magnitude of equilibrium shifts often depend on the relative strength of these acid and base reactants.