Question

Write the formula for the conjugate acid of each of the following. (a) \(\mathrm{F}^{-}\) (b) \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}\) (c) \(\mathrm{HCrO}_{4}^{-}\) (d) \(\mathrm{CN}^{-}\)

Short answer

The conjugate acids for the given compounds are: (a) \(\mathrm{HF}\), (b) \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NH}^{+}\), (c) \(\mathrm{H}_{2} \mathrm{CrO}_{4}\), (d) \(\mathrm{HCN}\).

Step-by-step solution

Understanding a Conjugate Acid

To find the conjugate acid of a species, you need to add a proton (an H+ ion) to it. The conjugate acid will have one more hydrogen and a charge that is one unit more positive than the original species.

Identify the Compound and Add a Proton for (a) \(\mathrm{F}^{-}\)

The compound in question is fluoride ion, \(\mathrm{F}^{-}\). To form the conjugate acid, add a proton to the fluoride ion, resulting in \(\mathrm{HF}\).

Identify the Compound and Add a Proton for (b) \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{N}\)

The compound is pyridine, \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{N}\). To form its conjugate acid, add a proton to the nitrogen atom, resulting in \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NH}^{+}\).

Identify the Compound and Add a Proton for (c) \(\mathrm{HCrO}_{4}^{-}\)

This compound is the hydrogen chromate ion, \(\mathrm{HCrO}_{4}^{-}\). To find its conjugate acid, add a proton, giving \(\mathrm{H}_{2} \mathrm{CrO}_{4}\).

Identify the Compound and Add a Proton for (d) \(\mathrm{CN}^{-}\)

The compound is the cyanide ion, \(\mathrm{CN}^{-}\). To form the conjugate acid, add a proton, resulting in \(\mathrm{HCN}\).

Key concepts

Acid-Base Chemistry

Acid-base chemistry is a fundamental aspect of chemical science, encompassing the study of the properties and reactions of acids and bases. In this context, it's important to understand that acids are substances that can donate a proton (H⁺) in a chemical reaction, while bases are substances that can accept a proton.

When an acid donates a proton, it forms what is known as a conjugate base, which can potentially accept a proton in the future. Conversely, when a base accepts a proton, it becomes a conjugate acid. The strength of an acid or base is often discussed in terms of its ability to donate or accept protons. Strong acids will almost completely disassociate in water, donating their protons to water molecules, while weak acids disassociate to a lesser extent. Similarly, strong bases are excellent proton acceptors, while weak bases are less so.

To reinforce these concepts, let's consider a real-world analogy. Imagine acids as generous people at a party handing out gifts (protons) to guests (other reactants). The guests who receive the gifts become much happier (more positive), much like a base becomes a conjugate acid upon accepting a proton, now carrying a positive charge if it was originally neutral.

Protonation

Protonation is the addition of a proton to an atom, molecule, or ion. It is a crucial step in many chemical reactions, including those in acid-base chemistry.

Understanding protonation is like learning the rules of catching in a game of ball. Just as a player catches a ball to gain control of it, a base accepts a proton to become protonated. This transformation involves a change in charge and chemical properties. In the case of fluoride ion (F^-), when it accepts a proton, the end product is hydrogen fluoride (HF), a molecule rather than an ion, since the negative charge of the fluoride ion is neutralized by the positively charged proton.

Guidelines for Protonation

When considering protonation, remember that it:

• Involves the transfer of a proton to another species.
• Results in the formation of a conjugate acid from a base.
• Can lead to a change in the charge of the base, often making it more positive.
• Alters the chemical reactivity and properties of the substance being protonated.

By following these guidelines, students can approach problems involving protonation with greater confidence and success.

Chemical Equilibrium

Chemical equilibrium is a state in a chemical reaction where the rate of the forward reaction equals the rate of the reverse reaction. This means that the concentrations of the reactants and products remain constant over time, not that they are necessarily equal to each other.

Think of chemical equilibrium like a game of tug-of-war where each team is equally strong. Neither side wins because the amount of force exerted by one team is matched by the other, creating a state of balance. In chemical terms, though reactants are still converting to products and vice versa, the overall concentration of these substances no longer changes. This is represented in the dynamic but stable state of equilibrium.

Equilibrium and Conjugate Pairs

When an acid reacts with a base, they form what's known as a conjugate acid-base pair, and this reaction can reach equilibrium. For example, when hydrogen chromate ion (HCrO₄^-) reacts with water, it can both accept and donate a proton. Depending on the direction of the reaction, it can form either the dichromate ion (Cr₂O₇^2-) or the chromic acid (H₂CrO₄). At equilibrium, both the forward and reverse reactions proceed at the same rate, resulting in no net change in the concentration of the hydrogen chromate ion and its conjugate forms.