Question

Consider four beakers. Beaker A has an aqueous solution of \(\mathrm{NaOH}\) in which the \(\mathrm{OH}^{-}\) ions are represented by blue circles. Beaker \(\mathrm{B}\) has a weak acid; HX is represented by red circles. Beaker \(\mathrm{C}\) has a weak acid; \(\mathrm{H}_{2} \mathrm{X}\) is represented by green circles. Beaker \(\mathrm{D}\) has a weak acid; \(\mathrm{H}_{3} \mathrm{X}\) is represented by yellow circles. \(\mathrm{X}^{-}\) ions are represented by triangles. Match the pictorial representations with the reactions given below. $$ \begin{array}{l} \text { (a) } \mathrm{HX}(a q)+\mathrm{OH}^{-}(a q) \longrightarrow \mathrm{X}^{-}(a q)+\mathrm{H}_{2} \mathrm{O} \\ \text { (b) } \mathrm{H}_{2} \mathrm{X}(a q)+2 \mathrm{OH}^{-}(a q) \longrightarrow \mathrm{X}^{-}(a q)+2 \mathrm{H}_{2} \mathrm{O} \\ \text { (c) } \mathrm{H}_{3} \mathrm{X}(a q)+3 \mathrm{OH}^{-}(a q) \longrightarrow \mathrm{X}^{-}(a q)+3 \mathrm{H}_{2} \mathrm{O} \end{array} $$

Short answer

Question: Match the given reactions with the correct beaker representations (A, B, C, and D) based on the analysis. Answer: Beaker B corresponds to reaction (a), Beaker C corresponds to reaction (b), and Beaker D corresponds to reaction (c). Beaker A contains the aqueous solution of NaOH.

Step-by-step solution

Analyzing the given reactions

We are given three reactions: (a) HX(aq) + OH⁻(aq) → X⁻(aq) + H2O (b) H2X(aq) + 2OH⁻(aq) → X⁻(aq) + 2H2O (c) H3X(aq) + 3OH⁻(aq) → X⁻(aq) + 3H2O We need to match these reactions with the beakers A, B, C, and D.

Identify the beaker with NaOH solution

Beaker A is the one containing the aqueous solution of NaOH. In the NaOH solution, the OH⁻ ions are represented by blue circles. We will be using this to find out the amount of blue circles required for each reaction to match the pictorial representations.

Match the reactions with appropriate acids in the beakers

In reaction (a), 1 red circle (HX) reacts with 1 blue circle (OH⁻) to form 1 triangle (X⁻) and 1 molecule of water. Hence, this reaction should represent Beaker B (with weak acid HX). In reaction (b), 1 green circle (H2X) reacts with 2 blue circles (2OH⁻) to form 1 triangle (X⁻) and 2 molecules of water. Hence, this reaction should represent Beaker C (with weak acid H2X). In reaction (c), 1 yellow circle (H3X) reacts with 3 blue circles (3OH⁻) to form 1 triangle (X⁻) and 3 molecules of water. Hence, this reaction should represent Beaker D (with weak acid H3X).

Final matching of reactions with beakers

Now that we have analyzed the reactions and matched them with the beakers, the final matching will be: Beaker A: Aqueous solution of NaOH Beaker B: Reaction (a) - HX (red circles) reacting with OH⁻ (blue circles) Beaker C: Reaction (b) - H2X (green circles) reacting with OH⁻ (blue circles) Beaker D: Reaction (c) - H3X (yellow circles) reacting with OH⁻ (blue circles)

Key concepts

Aqueous Solutions

Imagine you have a glass of water and you stir in some salt. What happens? The salt dissolves, and you're left with saltwater—a simple example of an aqueous solution. In chemistry, aqueous solutions are a big deal. They are mixtures where water acts as the solvent.

A solvent is a substance that has the ability to dissolve other substances—called solutes—to form a solution. In the case of our saltwater, water is the solvent, and salt is the solute. When we're dealing with reactions in chemistry, particularly acid-base reactions, they often happen in aqueous solutions. This is because water can dissolve a wide variety of compounds, making it an ideal medium for these reactions to occur.

Weak Acids

Understanding Weak Acids

When you hear the term acid, you might think of something strong and corrosive. However, not all acids are created equal. Some are strong, sure, but many are what we call 'weak acids.' A weak acid is one that doesn't fully dissociate in water. Instead, it reaches an equilibrium where both the acid and its ionized form coexist in the solution.

This distinction is crucial because it influences how the acid behaves and reacts. In the example from the exercise, HX, H2X, and H3X are all weak acids that partially dissociate to produce hydrogen ions (H+) and their respective conjugate bases (X-). But they do this to varying degrees, which is why each acid reacts differently with the hydroxide ions (OH-).

Weak acids are common in everyday life. Citric acid in lemons, acetic acid in vinegar, and carbonic acid in soda are just a few examples. They're essential in biochemistry and environmental science, too, affecting everything from the pH of blood to the acidity of rain.

Hydroxide Ion Reactions

The hydroxide ion (OH⁻) is a negatively charged molecule consisting of one oxygen and one hydrogen atom. It plays a key role in acid-base chemistry, particularly in reactions with acids, as seen in our textbook example.

In an acid-base reaction, the acid donates a proton (H+), and the base accepts it. The hydroxide ion is a common base that reacts with acids to form water (H2O) and the corresponding salt. When an acid is introduced to a solution with hydroxide ions, these ions will quickly grab H+ ions from the acid, neutralizing it, and forming water in the process.

In our exercise, the number of hydroxide ions (blue circles) reacting varies according to the number of hydrogen ions the weak acids can offer. Beakers B, C, and D contained acids that required 1, 2, and 3 hydroxide ions respectively to neutralize and form water. These reactions demonstrate how the hydroxide ions work in tandem with weak acids in aqueous solutions.