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Chapter 6: Problem 16

Which of the following is not a base according to any of the theories? (a) \(\mathrm{Mg}(\mathrm{OH})_{2}\) (b) \(\mathrm{NH}_{3}\) (c) \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) (d) \(\mathrm{BF}_{3}\)

Short Answer

Expert verified
Answer: The compound that is not a base according to any of the theories is (d) BF鈧�.

Step by step solution

01

1. The Arrhenius Theory

According to the Arrhenius theory, a base is a compound that increases the concentration of hydroxide ions (OH-) when dissolved in water. Let's check each compound: (a) \(\mathrm{Mg}(\mathrm{OH})_{2}\): This compound dissociates into Mg2+ ions and 2 OH- ions when dissolved in water, so it is a base according to the Arrhenius theory. (b) \(\mathrm{NH}_{3}\): Ammonia doesn't contain hydroxide ions directly, so it may not seem like a base in the Arrhenius theory. However, it reacts with water to produce hydroxide ions: \(\mathrm{NH_{3}(aq) + H_{2}O(l) \rightarrow NH_{4}^{+}(aq) + OH^{-}(aq)}\), so it is considered a base according to the Arrhenius theory. (c) \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\): This ion does not increase the concentration of hydroxide ions in the solution, so it is not a base according to the Arrhenius theory. (d) \(\mathrm{BF}_{3}\): Boron trifluoride does not increase hydroxide ions concentration in the solution, so it is not a base according to the Arrhenius theory.
02

2. The Br酶nsted-Lowry Theory

According to the Br酶nsted-Lowry theory, a base is a proton (H+) acceptor. Let's check each compound: (a) \(\mathrm{Mg}(\mathrm{OH})_{2}\): Hydroxide ions in this compound can accept protons, so it is a base according to the Br酶nsted-Lowry theory. (b) \(\mathrm{NH}_{3}\): Ammonia can accept a proton to form \(\mathrm{NH}_{4}^{+}\), so it is a base according to the Br酶nsted-Lowry theory. (c) \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\): This ion can accept a proton to form \(\mathrm{H}_{3}\mathrm{PO}_{4}\), so it is a base according to the Br酶nsted-Lowry theory. (d) \(\mathrm{BF}_{3}\): Boron trifluoride does not accept protons, so it is not a base according to the Br酶nsted-Lowry theory. Now, we see that compounds (a), (b), and (c) are considered bases in at least one of the theories. In contrast, compound (d) \(\mathrm{BF}_{3}\) doesn't fit the base criteria in any of the theories. Therefore, we can conclude:
03

Answer

The compound that is not a base according to any of the theories is (d) \(\mathrm{BF}_{3}\).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Arrhenius Theory
The Arrhenius theory of acids and bases is akin to a doorway into understanding reactions in aqueous solutions. Swedish scientist Svante Arrhenius proposed this theory in 1884. It defines a base as a substance that, when dissolved in water, increases the concentration of hydroxide ions (OH^-). These ions are what give a solution its basic (or alkaline) properties.

For example, magnesium hydroxide (Mg(OH)_2), a common antacid, dissociates in water to produce OH^- ions. The presence of these OH^- ions confirms Mg(OH)_2 as a base under the Arrhenius theory. However, substances like ammonia (NH_3) that don't release hydroxide ions directly but can create them through reactions with water are also considered bases by this definition.
Br酶nsted-Lowry Theory
Expanding our understanding of bases, the Br酶nsted-Lowry theory, introduced by scientists Johannes Nicolaus Br酶nsted and Thomas Martin Lowry in 1923, doesn't limit itself to aqueous solutions. Instead, it defines bases broadly as substances that can accept a proton, which is simply a hydrogen ion (H^+).

This definition includes substances like ammonia (NH_3), which accepts a proton to become ammonium (NH_4^+), and dihydrogen phosphate (H_2PO_4^-), which can accept a proton to form phosphoric acid (H_3PO_4). But not all compounds can accept protons. For example, boron trifluoride (BF_3) cannot, which is why it does not qualify as a base under the Br酶nsted-Lowry definition.
Proton Acceptor
At the heart of the Br酶nsted-Lowry theory is the concept of a 'proton acceptor.' This is essentially a base's defining role鈥攁ccepting protons from acids in a proton transfer reaction. While this may sound simple, it's a fundamental aspect of countless chemical reactions, including those in biological systems.

Substances like NH_3 showcase this role beautifully by readily accepting a proton to form NH_4^+. Even the dihydrogen phosphate ion (H_2PO_4^-) stands as a proton acceptor, although it may not be intuitive to think of a negatively charged ion as a base without understanding its potential to form phosphoric acid.
Hydroxide Ions
Often symbolized as OH^-, hydroxide ions are negatively charged molecules consisting of an oxygen atom bonded to a hydrogen atom. They play an essential role in chemistry, particularly in defining the basicity of a substance according to the Arrhenius theory. If a substance can produce OH^- ions when dissolved in water, it increases the solution's pH level (making it more basic).

Compounds like Mg(OH)_2 are typical bases that release hydroxide ions into solutions, while NH_3 interacts with water to produce hydroxide ions indirectly. Regardless of their source, hydroxide ions are responsible for the characteristic properties of basic solutions, such as a slippery feel and the ability to turn red litmus paper blue.

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Most popular questions from this chapter

Which of the following species cannot have conjugate base? (a) \(\mathrm{HSO}_{4}^{-}\) (b) \(\mathrm{HPO}_{4}^{-2}\) (c) \(\mathrm{PO}_{4}^{-3}\) (d) \(\mathrm{H}_{2} \mathrm{PO}_{2}^{-}\)

According to Lewis acid-base concept, which is a correct statement? (a) Species in which central atom has completed octet cannot act as acid. (b) All negatively charged ions are acids. (c) Molecule in which the central atom has vacant \(" \mathrm{~d}^{\prime \prime}\) orbitals acts as acid. (d) All positively changed ions are bases.

Match the entries given in column A with appropriate ones from column B. $$ \begin{array}{lll} \hline \text { Column A } & \text { Column B } \\ \hline \text { A. } & \text { Sodium chloride } & \text { ( ) a. Baking powder } \\ \text { B. } & \text { Tartaric acid } & \text { () b. Freezing mixture in ice } \\\\\text { creams } \\ \text { C. } & \text { Potassium nitrate } & \text { () c. Antacid in medicine } \\ \text { D. } & \begin{array}{l} \text { Sodium } \\ \text { bicarbonate } \end{array} & \text { () d. Gun powder } \\ \text { E. } & \text { Citric acid } & \text { () e. Bleaching powder } \\ \text { F. } & \begin{array}{l} \text { Calcium } \\ \text { hydroxide } \end{array} & \text { () } f \text { . Flavouring agent } \\ \hline \end{array} $$

Which among the following does not undergo hydrolysis? (a) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{COOH}\) (c) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\) (d) \(\mathrm{K}_{2} \mathrm{SO}_{4}\)

Which among the following pairs of acid and base are weak respectively? (a) \(\mathrm{HCOOH}\) and \(\mathrm{NH}_{4} \mathrm{OH}\) (b) \(\mathrm{HCl}\) and \(\mathrm{NaOH}\) (c) \(\mathrm{CH}_{3} \mathrm{COOH}\) and \(\mathrm{KOH}\) (d) \(\mathrm{HClO}_{4}\) and \(\mathrm{CsOH}\)
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