Problem 150 - Chemistry

Question

Which of the following represent conjugate acid-base pairs? For those pairs that are not conjugates, write the correct conjugate acid or base for each species in the pair. a. \(\mathrm{H}_{2} \mathrm{O}, \mathrm{OH}\) c. \(\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{H}_{2} \mathrm{PO}_{4}^{-}\) b. \(\mathrm{H}_{2} \mathrm{SO}_{4}, \mathrm{SO}_{4}^{2-}\) d. \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}, \mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}\)

Step-by-Step Solution

Answer

Pairs A, C, and D represent conjugate acid-base pairs. Pair B is not a conjugate acid-base pair, and the correct conjugate base for \(\mathrm{H}_2\mathrm{SO}_4\) is \(\mathrm{HSO}_4^-\).

Step 1: Pair A: \(\mathrm{H}_{2} \mathrm{O}, \mathrm{OH}^-\)

First, we need to evaluate if these species are related. In this case, the species could be an acid and its conjugate base. We find the conjugate base of \(\mathrm{H}_{2} \mathrm{O}\) by removing a proton: \[\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{OH}^- + H^+\] Since the conjugate species is the same as the given \(\mathrm{OH}^-\), pair A represents a conjugate acid-base pair.

Step 2: Pair B: \(\mathrm{H}_{2} \mathrm{SO}_{4}, \mathrm{SO}_{4}^{2-}\)

Now, we evaluate if these species are related. The species could be an acid and its conjugate base. We find the conjugate base of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) by removing a proton: \[\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{HSO}_{4}^- + H^+\] The conjugate base found is \(\mathrm{HSO}_{4}^-\), which is different from the given \(\mathrm{SO}_{4}^{2-}\). Thus, pair B does not represent a conjugate acid-base pair, and the correct conjugate base for \(\mathrm{H}_2\mathrm{SO}_4\) is \(\mathrm{HSO}_4^-\).

Step 3: Pair C: \(\mathrm{H}_{3} \mathrm{PO}_{4}, \mathrm{H}_{2} \mathrm{PO}_{4}^{-}\)

We need to check if these species are related. The species could be an acid and its conjugate base. We find the conjugate base of \(\mathrm{H}_{3} \mathrm{PO}_{4}\) by removing a proton: \[\mathrm{H}_{3} \mathrm{PO}_{4} \rightarrow \mathrm{H}_{2} \mathrm{PO}_{4}^- + H^+\] Since the conjugate species is the same as the given \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\), pair C represents a conjugate acid-base pair.

Step 4: Pair D: \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}, \mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}\)

Let's see if these species are related. They could be an acid and its conjugate base. We find the conjugate base of \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\) by removing a proton: \[\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2} \rightarrow \mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^- + H^+\] Since the conjugate species is the same as the given \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}\), pair D represents a conjugate acid-base pair. In summary, pairs A, C, and D are conjugate acid-base pairs, while pair B is not. The correct conjugate base for pair B is \(\mathrm{HSO}_{4}^-\).

Key Concepts

Acids and Bases

Acids and bases are fundamental concepts in chemistry. Understanding these substances begins with recognizing what defines an acid or a base. Acids are substances that can donate a proton ( H^+ ) to another substance. A common example of an acid is hydrochloric acid ( HCl ). On the other hand, bases are substances that can accept a proton from another substance. An example of a base is sodium hydroxide ( NaOH ).

These two types of substances play crucial roles in various chemical reactions and processes. Many reactions in chemistry involve the transfer of protons between acids and bases. This transfer influences the acidity or basicity of a solution, which is crucial in fields like biochemistry, environmental science, and industrial processes.

Proton Transfer

Proton transfer is central to the behavior of acids and bases. When an acid reacts with a base, the acid donates a proton to the base. This process is crucial because it forms the basis of many chemical reactions.

• In the case of water, it can act both as a base and an acid. When water ( H_2O ) acts as an acid, it donates a proton and forms hydroxide ( OH^- ).
• Similarly, when it accepts a proton, it forms hydronium ( H_3O^+ ).

The concept goes beyond just mixing solutions. Proton transfer can help determine the pH of a solution, indicating its acidity or alkalinity level. By understanding which direction the proton moves in a reaction, scientists can predict how substances will behave in both simple and complex chemical equations.

Chemical Equations

Chemical equations describe the transformation of reactants into products. In the context of acids and bases, these reactions often involve the transfer of protons. An accurate chemical equation will list both the donated proton and the resulting changes in structure of the substances involved.

For instance, when hydrochloric acid (HCl) is mixed with sodium hydroxide (NaOH), a neutralization reaction occurs:\[HCl + NaOH \rightarrow NaCl + H_2O\]This equation summarizes the proton transfer process, where HCl donates a proton to the hydroxide ion (OH^-), forming water. Chemical equations help visualize and quantify these reactions, making them a powerful tool in understanding chemical processes.

• They must be balanced, with equal numbers of atoms on each side of the equation.
• They provide valuable information on the amounts of each substance used and produced.

Conjugate Species

Conjugate species are pairs of molecules or ions related by the loss or gain of a single proton. In any proton transfer reaction, the acid transforms into its conjugate base, and the base transforms into its conjugate acid. For instance, when H_2O loses a proton, it forms its conjugate base, OH^- .

• A conjugate acid-base pair consists of two species that differ only by one proton.
• Identifying these pairs helps predict the direction of acid-base reactions.
• Conjugate pairs are integral to maintaining equilibrium in a solution.

This concept explains the reversible nature of many reactions. By understanding conjugate species, chemists can design reactions and products with desired properties and behaviors. Understanding these relationships illuminates the interconnected nature of chemical processes, particularly in biological systems where acid-base balancing is critical.