Question

Which of the following represent conjugate acidbase pairs? For those pairs that are not conjugates, write the correct conjugate acid or base for each species in the pair. a. \(\mathrm{HSO}_{4}^{-}, \mathrm{SO}_{4}^{2-}\) b. \(HBr, Br\) c. \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}, \mathrm{PO}_{4}^{3-}\) d. \(\mathrm{HNO}_{3}, \mathrm{NO}_{2}^{-}\)

Short answer

Conjugate acid-base pairs: (a) \(\mathrm{HSO}_{4}^{-}\) and \(\mathrm{SO}_{4}^{2-}\), (b) \(HBr\) and \(Br\). Non-conjugate pairs with corrected conjugates: (c) \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\) and \(\mathrm{HPO}_{4}^{2-}\), (d) \(\mathrm{HNO}_{3}\) and \(\mathrm{NO}_{3}^{-}\).

Step-by-step solution

Identify the acid and base in each pair

In each pair, determine the species that have gained or lost a proton. This will help us identify the conjugate acid and base within the pairs later on.

Determine if the pairs are conjugates

For each pair, check if the two species differ by only a proton (H+ ion) – one needing to donate and another needing to accept the proton. If they do, we will confirm that they are conjugate acid-base pairs.

Find the correct conjugate acid or base for non-conjugate pairs

For those pairs that are not conjugates, we will write the correct conjugate acid if the given species is a base, or correct conjugate base if the given species is an acid. Now, let's apply these steps to each pair. a. \(\mathrm{HSO}_{4}^{-}, \mathrm{SO}_{4}^{2-}\) Step 1: HSO4- has lost a proton, and SO42- has lost two protons, therefore HSO4- is the acid and SO42- is the base. Step 2: HSO4- and SO42- differ by a proton - they are conjugate acid-base pairs. b. \(HBr, Br\) Step 1: HBr has not lost any protons, and Br has lost one proton, therefore HBr is the acid and Br is the base. Step 2: HBr and Br differ by a proton - they are conjugate acid-base pairs. c. \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}, \mathrm{PO}_{4}^{3-}\) Step 1: H2PO4- has lost one proton, and PO43- has lost three protons, therefore H2PO4- is the acid and PO43- is the base. Step 2: H2PO4- and PO43- do not differ by just one proton - they are not conjugate acid-base pairs. Step 3: The correct conjugate base for H2PO4- is HPO42-, and the correct conjugate acid for PO43- is HPO42-. d. \(\mathrm{HNO}_{3}, \mathrm{NO}_{2}^{-}\) Step 1: HNO3 has not lost any protons, and NO2- has lost one proton, therefore HNO3 is the acid and NO2- is the base. Step 2: HNO3 and NO2- do not differ by just one proton - they are not conjugate acid-base pairs. Step 3: The correct conjugate base for HNO3 is NO3-, and the correct conjugate acid for NO2- is HNO2.

Key concepts

Acid-Base Reactions

Acid-base reactions are fundamental processes in chemistry, where an acid and a base interact. These reactions often result in the transfer of a proton ( H^+ ) from the acid to the base. Understanding these reactions helps in predicting the products formed in various chemical reactions.
Acid-base reactions usually occur in aqueous solutions and result in the formation of water and a salt. For example, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), the result is water (H2O) and sodium chloride (NaCl).

• The role of acids is to donate protons.
• The role of bases is to accept protons.

By identifying how protons are transferred, we can better understand the nature of the substances involved in these reactions.

Bronsted-Lowry Theory

The Bronsted-Lowry theory, proposed by Johannes Bronsted and Thomas Lowry in 1923, provides a more generalized definition of acids and bases. According to this theory:

• An acid is a species that donates a proton ( H^+ ).
• A base is a species that accepts a proton.

This theory broadens the range of acid-base reactions and provides a better explanation for reactions that don't involve the formation of water. For example, the reaction between ammonia (NH3) and hydrogen chloride (HCl) can be explained using this theory.
The Bronsted-Lowry theory also introduces the concept of conjugate acid-base pairs. In a reaction, the acid transforms into its conjugate base, while the base transforms into its conjugate acid by gaining or losing a proton.

Acid Dissociation

Acid dissociation refers to the process in which an acid releases a proton ( H^+ ) into a solution. This is a crucial step in the process of an acid-base reaction. The strength of an acid is determined by its ability to dissociate and release protons.
Some acids partially dissociate in solution, while others completely dissociate. The extent of dissociation depends on various factors such as the inherent strength of the acid and the surrounding solvent.
For example, hydrochloric acid (HCl) completely dissociates in water, making it a strong acid. Conversely, acetic acid (CH3COOH) partially dissociates, classifying it as a weak acid.

Base Dissociation

Base dissociation is the process by which a base accepts a proton ( H^+ ) from the surrounding solution. This process is central to many chemical reactions that involve bases.
The strength of a base is determined by its tendency to accept protons. Like acids, bases can also be classified as strong or weak based on their dissociation capabilities.

• Strong bases, such as sodium hydroxide (NaOH), readily accept protons and completely dissociate in solutions.
• Weak bases, like ammonia (NH3), partially dissociate and are less eager to accept protons compared to strong bases.

The concept of base dissociation helps in understanding the behavior of bases in acid-base reactions and their role in balancing chemical equations.