Question

Which of the following hydroxides is acidic? (a) \(\mathrm{Al}(\mathrm{OH})_{3}\) (b) \(\mathrm{Ga}(\mathrm{OH})_{3}\) (c) \(\mathrm{Tl}(\mathrm{OH})_{3}\) (d) \(\mathrm{B}(\mathrm{OH})_{3}\)

Short answer

\mathrm{B}(\mathrm{OH})_{3} is the acidic hydroxide.

Step-by-step solution

Identifying Acidic Hydroxides

To determine which hydroxide is acidic, knowledge of periodic trends and chemical behavior is required. Acidic hydroxides are typically associated with non-metallic elements or metalloids. We know that aluminium (Al), gallium (Ga), and thallium (Tl) behave as metals while boron (B) is a metalloid. The acidic nature increases with the increase in the non-metallic character. Hence, among the given options, boron hydroxide \(\mathrm{B}(\mathrm{OH})_{3}\) is more likely to show acidic properties because boron is a metalloid and shows more non-metallic character than the other elements listed.

Determining the Nature of Hydroxides

The acidic nature of metal hydroxides is also related to the oxidation state of the metal and its electronegativity. Boron being a metalloid with a higher electronegativity compared to other metals in the list forms an electron-deficient hydroxide that can accept electron pairs, behaving as a Lewis acid. Therefore, among the given options, \(\mathrm{B}(\mathrm{OH})_{3}\) is acidic in nature.

Conclusion

Based on the non-metallic character and electronegativity, \(\mathrm{B}(\mathrm{OH})_{3}\) is identified as the acidic hydroxide among the options provided.

Key concepts

Chemical Behavior of Hydroxides

When diving into the chemical behavior of hydroxides, we discover that not all hydroxides are made equal. Understanding hydroxides requires a look into their reactivity and properties. Acidic hydroxides, for instance, have the ability to donate a proton (H+) or accept an electron pair, and they are often linked to elements with non-metallic traits.

To make this more consumable, consider how water interacts with different substances. Some hydroxides can make the solution basic by releasing hydroxide ions (OH-), while acidic hydroxides react with the water to form hydronium ions (H3O+), leading to an acidic solution. That's why when you mix boron hydroxide with water, it behaves differently compared to, say, sodium hydroxide, which is a classic base.

Periodic Trends and Acidic Hydroxides

The periodic table is like a map that helps us predict the behavior of elements. One of these predictions involves how acidity changes across the table. Generally, elements that are more to the right and up on the periodic table tend to form acidic hydroxides. This is a periodic trend where the non-metallic character increases as you move from left to right and from bottom to top.

Using our problem as a reference point, boron is located higher and to the right of the other elements listed, putting it firmly within the zone where elements are more likely to form acidic hydroxides.

Lewis Acids and Electron Deficiency

In the world of chemistry, Lewis acids are like social butterflies that are eager to accept a pair of electrons. Acidic hydroxides are great examples of Lewis acids. They are electron deficient and readily latch onto a pair of electrons from a donor. This is due to incomplete electron shells, which make them feel a bit insecure and on the lookout for extra electrons.

Think of boron hydroxide as the 'Lewis acid' at the party; it's the one that is short on electrons and is keen to accept help from electron-rich neighbors like water molecules to reach a full, stable configuration.

Non-Metallic Character and Acidity

Elements with a strong non-metallic character love to hold on to their electrons and can even attract more. This property also contributes to the acidic nature of the hydroxides they form. To put it simply, elements that are non-metallic are more likely to produce acidic hydroxides. That's because they are better at keeping their electrons close and can even reach out to grab more, just like how Boron does in its hydroxide form, making it acidic.

Elements that are metallic typically form bases rather than acids. That's why boron, with its non-metallic personality in the playground of elements, tends to produce an acidic hydroxide when it plays with water.

Oxidation State and Acidity

The oxidation state can give us valuable hints about an element's potential to form acidic hydroxides. An element with a high oxidation state has more of a 'charge' and 'demand' for electrons, which can lead to it exhibiting acidic properties when combined with hydroxide ions.

Oxidation states are like the 'plus-one' at an electric party—the higher they are, the more pull they have on electrons from around them, which in turn increases the acidity of their hydroxides. For boron, its oxidation state allows it to play the role of a powerful Lewis acid, easily mingling with electron donors to create an acidic solution.