Question

(a) Give the conjugate base of the following BrønstedLowry acids: (i) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\). (ii) \(\mathrm{HPO}_{4}{ }^{2-}\). (b) Give the conjugate acid of the following Brønsted-Lowry bases: (i) \(\mathrm{CO}_{3}{ }^{2-}\), (ii) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2}\).

Short answer

(a) (i) \(C_6H_5COO^-\) (ii) \(PO_4^{3-}\) (b) (i) \(HCO_3^-\) (ii) \(C_2H_5NH_3^+\)

Step-by-step solution

Understanding the terms

A Brønsted-Lowry acid is a substance that donates a proton (H⁺) in a reaction, while a Brønsted-Lowry base is a substance that accepts a proton (H⁺) in a reaction. The conjugate base of an acid is formed when the acid loses a proton. Conversely, the conjugate acid of a base is formed when the base gains a proton.

Identify the conjugate base for C6H5COOH

Given the Brønsted-Lowry acid: C6H5COOH When this acid loses a proton (H⁺), the conjugate base is formed: \(C_6H_5COOH → C_6H_5COO^- + H^+\) The conjugate base is: \(C_6H_5COO^-\)

Identify the conjugate base for HPO4^2-

Given the Brønsted-Lowry acid: \(HPO_4^{2-}\) When this acid loses a proton (H⁺), the conjugate base is formed: \(HPO_4^{2-} → PO_4^{3-} + H^+\) The conjugate base is: \(PO_4^{3-}\) For part (b):

Identify the conjugate acid for CO3^2-

Given the Brønsted-Lowry base: \(CO_3^{2-}\) When this base gains a proton (H⁺), the conjugate acid is formed: \(CO_3^{2-} + H^+ → HCO_3^-\) The conjugate acid is: \(HCO_3^-\)

Identify the conjugate acid for C2H5NH2

Given the Brønsted-Lowry base: \(C_2H_5NH_2\) When this base gains a proton (H⁺), the conjugate acid is formed: \(C_2H_5NH_2 + H^+ → C_2H_5NH_3^+\) The conjugate acid is: \(C_2H_5NH_3^+\) So, the answers are: (a) (i) Conjugate base of \(C_6H_5COOH\) is \(C_6H_5COO^-\) (a) (ii) Conjugate base of \(HPO_4^{2-}\) is \(PO_4^{3-}\) (b) (i) Conjugate acid of \(CO_3^{2-}\) is \(HCO_3^-\) (b) (ii) Conjugate acid of \(C_2H_5NH_2\) is \(C_2H_5NH_3^+\)

Key concepts

Conjugate Base

In the realm of chemistry, understanding how acids and bases transform is fundamental. The concept of a conjugate base emerges from the Brønsted-Lowry acid-base theory. It is the species left over after an acid has donated its proton to a base. Visualize it as what an acid becomes once it gives away a bit of itself—the proton.

For instance, when benzoic acid (\(C_6H_5COOH\)) donates a proton, it transforms into its conjugate base, benzoate ion (\(C_6H_5COO^-\)). This swap is not just an academic exercise; it's pivotal in predicting the behavior of substances in chemical reactions, particularly in solutions.

• A stronger acid will produce a weaker conjugate base.
• Conjugate bases often have a negative charge due to the loss of the positively charged proton.

This focus on the product of proton donation is a crucial piece of the acid-base reaction puzzle.

Conjugate Acid

Just as with conjugate bases, every Brønsted-Lowry base has a counterpart called the conjugate acid. This is the species obtained when a base accepts a proton. The result is typically a positively charged ion, given that the base acquires an extra proton with a positive charge from the acid.

A classic example is the carbonate ion (\(CO_3^{2-}\)) receiving a proton to become bicarbonate (\(HCO_3^-\)). This reveals the flip side of the acid-base interaction – the new identity the base assumes upon embracing a proton.

• The strength of the conjugate acid correlates inversely with the base that formed it—a weak base will create a strong conjugate acid.

The conjugate acid offers insights into what could happen next in a chemical scenario, highlighting the interconnectedness of substances in chemical equilibria.

Proton Transfer Reactions

The dance of acids and bases largely revolves around proton transfer reactions. These are the processes in which a proton, the nucleus of a hydrogen atom, is exchanged between a pair of molecules. They're central to the Brønsted-Lowry theory and vital in understanding chemical reactivity.

Imagine this scenario as a close encounter between two molecules that results in one of them walking away with a proton souvenir. The acid (proton donor) partners up with a base (proton acceptor), and together they form a conjugate base and a conjugate acid. This exchange can affect the nature and strength of the substances involved and is at the heart of many biochemical processes, such as enzyme function and cellular metabolism.

• Proton transfer reactions are often reversible.
• The speed and extent of these reactions influence the pH level of a solution.

The study of proton transfers is the cornerstone of predicting outcomes in acid-base chemistry.

Acid-Base Equilibria

The tug-of-war between acids and bases ultimately finds a middle ground in what chemists call acid-base equilibria. This is the point of balance in a reversible chemical reaction where the rates of the forward reaction (forming conjugate bases and acids) and the reverse reaction (re-forming the original acids and bases) are equal. It's a state of dynamic harmony where substances constantly change partners but overall concentrations remain steady.

At equilibrium, the ratio of conjugate acid to base holds valuable information—the equilibrium constant (\(K_a\) or \(K_b\)) can tell us how strong or weak an acid or base is. These constants don't just hang around like wallflowers; they're active participants in the prediction of reaction direction and the intensity of acidity or basicity in a solution.

• At equilibrium, the system may look calm, but molecules are dynamically exchanging protons.
• pH measurements are practical applications of acid-base equilibria.

Understanding this balancing act is essential for mastering acid-base reactions and their practical implications in real-world science and industry.