Problem 14 - Chemistry The Central Science In Si Units

Question

Which of the following statements is false? (a) An Arrhenius base increases the concentration of \(\mathrm{OH}^{-}\) in water. (b) A Bronsted-Lowry base is a proton acceptor. (c) Water can act as a Brønsted-Lowry acid. (d) Water can act as a Brønsted-Lowry base. (e) Any compound that contains an -OH group acts as a Bronsted-Lowry base.

Step-by-Step Solution

Answer

Statement (e) is false.

Step 1: Understanding Arrhenius Bases

An Arrhenius base is defined as a substance that, when dissolved in water, increases the concentration of hydroxide ions (\(\mathrm{OH}^{-}\)). This means statement (a) is true.

Step 2: Understanding Bronsted-Lowry Acids and Bases

In Bronsted-Lowry theory, a base is a proton acceptor and an acid is a proton donor. Therefore, statement (b) is true because a Bronsted-Lowry base does accept protons.

Step 3: Water as a Bronsted-Lowry Acid and Base

Water is an amphoteric substance, which means it can act as both an acid and a base. Water molecules can donate a proton to become \(\mathrm{OH}^{-}\) or accept a proton to form \(\mathrm{H_3O^{+}}\). Thus, both statements (c) and (d) are true.

Step 4: Analyzing Hydroxyl Group Compounds

Not all compounds containing an -OH group act as Bronsted-Lowry bases. Some -OH containing compounds, like alcohols, do not readily accept protons. Therefore, statement (e) is false.

Key Concepts

Arrhenius base

Arrhenius bases are a foundational concept in acid-base chemistry. They are substances that increase the concentration of hydroxide ions (\(\mathrm{OH}^{-}\)) when dissolved in water. This increase in \(\mathrm{OH}^{-}\) ions is what characterizes a solution as being basic according to the Arrhenius definition.
When you dissolve a typical Arrhenius base like sodium hydroxide (NaOH) in water, it dissociates into sodium ions and hydroxide ions. This increase in hydroxide ions raises the pH of the solution, making it more basic than neutral water.

• Key point: Arrhenius bases introduce \(\mathrm{OH}^{-}\) ions into the solution.
• Examples: NaOH, KOH, and Ca(OH)\(_2\).

Arrhenius's theory focuses primarily on reactions in aqueous solutions, which is a limitation when considering broader acid-base reactions that occur in non-aqueous environments.

Brønsted-Lowry acid

The Brønsted-Lowry theory expands on the concept of acids and bases beyond the Arrhenius definition. It defines an acid as a proton donor.
In this theory, protons are represented as hydrogen ions (H\(^+\)). When an acid donates a proton, it becomes its conjugate base. For example, when hydrochloric acid (HCl) donates a proton to water, it turns into chloride ion (Cl\(^-\)), and water turns into the hydronium ion (H\(_3\)O\(^+\)).

• Key point: An acid donates a proton (H\(^+\)).
• Examples: HCl, H\(_2\)SO\(_4\), and NH\(_4^+\).

The Brønsted-Lowry model is more versatile than the Arrhenius model as it can explain proton transfer in any solvent, not just water.

alkaline substances

The term 'alkaline' is often used to describe bases, particularly in reference to their property of having a high pH. Alkaline substances are basic and typically contain metal oxides or hydroxides, like those from the Group I or Group II elements of the periodic table.
When dissolved in water, these substances increase the concentration of hydroxide ions (\(\mathrm{OH}^{-}\)), raising the solution's pH. An alkaline solution has a pH greater than 7.

• Key point: Alkaline substances increase the pH of a solution.
• Examples: NaOH (sodium hydroxide) and Mg(OH)\(_2\) (magnesium hydroxide).

Alkalinity is important not only in chemistry but also in everyday life. It's used for applications ranging from neutralizing acidic soils in agriculture to manufacturing soap.

hydroxyl group

The hydroxyl group, represented by -OH, is a common functional group found in many chemical compounds. While it is present in many bases, not all compounds containing a hydroxyl group are base-like in behavior according to the Brønsted-Lowry theory.
Compounds like alcohols (ethanol, for instance) contain hydroxyl groups but do not readily accept protons to act as bases. This is crucial to distinguish because only some -OH containing compounds, such as metal hydroxides, behave as bases.

• Key point: Not all hydroxyl-containing compounds are bases.
• Examples: Methanol (not a base), NaOH (a base).

This emphasizes the importance of context when considering chemical behavior: the mere presence of a hydroxyl group doesn't dictate a compound's ability to act as a base; its reactivity depends on its structure and the surrounding conditions.