Problem 40 - Chemistry The Molecular Science

Question

Classify each of these as an acid or a base. Which are strong and which are weak? What ions are produced when each is dissolved in water? (a) \(\mathrm{KOH}\) (b) \(\mathrm{Mg}(\mathrm{OH})_{2}\) (c) \(\mathrm{HClO}\) (d) \(\mathrm{HBr}\) (e) \(\mathrm{LiOH}\) (f) \(\mathrm{H}_{2} \mathrm{SO}_{3}\)

Step-by-Step Solution

Answer

(a) KOH: strong base; (b) Mg(OH)2: weak base; (c) HClO: weak acid; (d) HBr: strong acid; (e) LiOH: strong base; (f) H2SO3: weak acid.

Step 1: Classify KOH

Potassium hydroxide (\(\mathrm{KOH}\)) is a base. It is considered a strong base because it dissociates completely in water to produce \(\mathrm{K}^+\) and \(\mathrm{OH}^-\) ions.

Step 2: Classify Mg(OH)2

Magnesium hydroxide (\(\mathrm{Mg(OH)}_2\)) is a base. It is a weak base because it does not dissociate completely in water. It partially ionizes to form \(\mathrm{Mg}^{2+}\) and \(\mathrm{OH}^-\) ions.

Step 3: Classify HClO

Hypochlorous acid (\(\mathrm{HClO}\)) is an acid. It is a weak acid because it does not completely dissociate in water. It partially ionizes to form \(\mathrm{H}^+\) and \(\mathrm{ClO}^-\) ions.

Step 4: Classify HBr

Hydrobromic acid (\(\mathrm{HBr}\)) is an acid. It is a strong acid because it dissociates completely in water to form \(\mathrm{H}^+\) and \(\mathrm{Br}^-\) ions.

Step 5: Classify LiOH

Lithium hydroxide (\(\mathrm{LiOH}\)) is a base. It is a strong base, though not as common as other strong bases, because it dissociates completely in water to produce \(\mathrm{Li}^+\) and \(\mathrm{OH}^-\) ions.

Step 6: Classify H2SO3

Sulfurous acid (\(\mathrm{H}_2\mathrm{SO}_3\)) is an acid. It is a weak acid because it does not completely dissociate in water. It partially ionizes to form \(\mathrm{H}^+\) and \(\mathrm{HSO}_3^-\) ions.

Key Concepts

Strong acids and bases

In acid-base chemistry, strong acids and bases play a pivotal role because they fully dissociate in water. This means they break apart almost completely into their constituent ions when dissolved. For strong acids like hydrobromic acid ( HBr ), this means producing a complete set of H^+ (hydrogen ions) and Br^- (bromide ions) upon dissolution. Similarly, strong bases such as potassium hydroxide ( KOH ) and lithium hydroxide ( LiOH ) fully dissociate to yield K^+ , OH^- , and Li^+ ions, respectively.
Examples of Strong Acids:

• Hydrochloric acid ( HCl )
• Sulfuric acid ( H_2SO_4 )
• Nitric acid ( HNO_3 )
• Hydrobromic acid ( HBr )

Examples of Strong Bases:
• Sodium hydroxide ( NaOH )
• Potassium hydroxide ( KOH )
• Lithium hydroxide ( LiOH )
• Calcium hydroxide ( Ca(OH)_2 )

These compounds are especially important in scientific and industrial applications because of their ability to influence solutions very powerfully due to their complete ionization.

Weak acids and bases

Contrasting with strong acids and bases, weak acids and bases do not fully dissociate in water. Instead, they only partially ionize, which means that a significant number of molecules remain undissociated in the solution.
For weak acids like hypochlorous acid ( HClO ) and sulfurous acid ( H_2SO_3 ), you will generally find that only a fraction of these molecules release hydrogen ions ( H^+ ) into the solution. This results in a solution that is less acidic compared to one with a strong acid at the same concentration. In the case of HClO , it forms H^+ and ClO^- ions, but not completely.
Similarly, weak bases like magnesium hydroxide ( Mg(OH)_2 ) dissolve in water to form only a small amount of Mg^{2+} and OH^- ions. Most of the magnesium hydroxide remains intact in the mixture.
Examples of Weak Acids:

• Acetic acid ( CH_3COOH )
• Hypochlorous acid ( HClO )
• Sulfurous acid ( H_2SO_3 )

Examples of Weak Bases:
• Ammonia ( NH_3 )
• Magnesium hydroxide ( Mg(OH)_2 )
• Aluminum hydroxide ( Al(OH)_3 )

Understanding this partial ionization is crucial when predicting chemical behavior in reactions.

Ionization in water

Ionization is the process by which a compound breaks apart into its constituent ions when dissolved in water. This process is central to understanding how acids and bases behave in aqueous solutions. For substances considered strong, like HBr or KOH , ionization is complete, meaning they dissociate fully into ions, leaving no molecules intact.
In the case of HBr , you will find it breaks into H^+ and Br^- ions, while KOH forms K^+ and OH^- ions. This full ionization leads to high conductivity and strong reactions with other substances.
Weak acids and bases, on the other hand, exhibit partial ionization. For instance, when HClO is added to water, only some of the molecules separate into H^+ and ClO^- ions, with plenty remaining as whole HClO molecules.
The extent of ionization is dictated by the acid or base's inherent properties; strong electrolytes ionize completely, while weak electrolytes do not. The degree of ionization is also influenced by the concentration of the acid or base in the solution and the presence of other substances.
Key takeaways include:

• Strong acids and bases ionize completely, leading to high conductivities.
• Weak acids and bases only partially ionize, resulting in lower ion concentrations.
• The degree of ionization affects solution properties such as pH and electrical conductivity.

Thus, the concept of ionization allows scientists to predict and control the outcomes of chemical reactions in water.