Question

(a) Give the conjugate base of the following Bronsted-Lowry acids: (i) HCOOH, (ii) \(\mathrm{HPO}_{4}^{2-} .\) (b) Give the conjugate acid of the following Bronsted-Lowry bases: (i) SO \(_{4}^{2-}\) (ii) \(\mathrm{CH}_{3} \mathrm{NH}_{2} .\)

Short answer

(a) The conjugate bases of HCOOH and \(HPO_4^{2-}\) are \(HCOO^-\) and \(PO_4^{3-}\), respectively. (b) The conjugate acids of \(SO_4^{2-}\) and CH3NH2 are \(HSO_4^-\) and \(CH_3NH_3^+\), respectively.

Step-by-step solution

(a) Conjugate base of HCOOH (formic acid)

We're given the Bronsted-Lowry acid HCOOH. To find the conjugate base, we need to remove one proton (H+) from the molecule. The resulting conjugate base is \(HCOO^-\): HCOOH \( \rightarrow \) H+ + \(HCOO^-\)

(a) Conjugate base of \(HPO_4^{2-}\)

The given Bronsted-Lowry acid is \(HPO_4^{2-}\). To find the conjugate base, we need to remove one proton (H+). The resulting conjugate base is \(PO_4^{3-}\): \(HPO_4^{2-} \rightarrow \) H+ + \(PO_4^{3-}\)

(b) Conjugate acid of \(SO_4^{2-}\)

We're given the Bronsted-Lowry base \(SO_4^{2-}\). To find the conjugate acid, we need to add one proton (H+) to the molecule. The resulting conjugate base is \(HSO_4^-\): \(SO_4^{2-} + \) H+ \( \rightarrow HSO_4^-\)

(b) Conjugate acid of CH3NH2 (methylamine)

The given Bronsted-Lowry base is CH3NH2. To find the conjugate acid, we need to add one proton (H+) to the molecule. The resulting conjugate base is \(CH_3NH_3^+\): CH3NH2 + H+ \( \rightarrow CH_3NH_3^+\)

Key concepts

Understanding the Bronsted-Lowry Theory

The Bronsted-Lowry theory is a fundamental concept in the chemistry of acids and bases. It defines an acid as a substance that can donate a proton (a hydrogen ion, H+), and a base as a substance that can accept a proton. When an acid donates a proton, it forms what is known as its conjugate base. Conversely, when a base accepts a proton, it forms its conjugate acid.

For instance, in the case of formic acid (HCOOH), when it donates a proton, it transforms into its conjugate base, formate (HCOO-):
HCOOH → H+ + HCOO-
This reaction shows the acid-base pair of formic acid and formate.

Similarly, for the dihydrogen phosphate ion (HPO4^2-), when it loses a proton, it becomes phosphate (PO4^3-), which is the conjugate base:
HPO4^2- → H+ + PO4^3-
These reactions demonstrate how the transfer of protons is central in the Bronsted-Lowry theory of acids and bases.

Navigating Acid-Base Reactions

Acid-base reactions, according to the Bronsted-Lowry theory, involve the transfer of protons from acids to bases. This proton transfer is what constitutes the chemical reaction. The strength of acids and bases is determined by their tendency to donate or accept protons. Strong acids readily lose their protons, while strong bases readily accept protons.

For the sulfate ion (SO4^2-), a typical base, when it accepts a proton, it becomes the hydrogen sulfate ion (HSO4-), its conjugate acid:
SO4^2- + H+ → HSO4^-
On the other hand, a common base such as methylamine (CH3NH2) when it gains a proton, forms its conjugate acid, the methylammonium ion (CH3NH3+):
CH3NH2 + H+ → CH3NH3+
These examples illustrate the reversible nature of acid-base interactions, and the concept of conjugate pairs is pivotal for understanding these chemical processes.

Deciphering Chemical Equilibrium in Acids and Bases

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, resulting in no net change in the concentration of reactants and products over time. In the context of acid-base chemistry, an equilibrium is established between the conjugate acid-base pairs. For instance, when formic acid donates a proton, the reaction can reach a point where the rate at which formic acid (HCOOH) is losing protons is equal to the rate at which its conjugate base (HCOO-) is gaining protons, creating a dynamic balance.

This state of balance can be represented by a double arrow in the chemical equation:
HCOOH ↔ H+ + HCOO-
Understanding equilibrium is crucial as it helps predict the concentrations of acids and bases at any given point and manipulate conditions to favor the production or reduction of certain substances.

The concept of acid-base equilibrium ties back to the idea of conjugate pairs, as the strength of an acid or base influences the position of equilibrium in these reactions. A strong acid will have a weaker conjugate base, and a weak acid will have a stronger conjugate base, impacting the equilibrium position and highlighting the interconnectedness of these concepts.