Question

Pyridine, \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{~N}\), is a weak base. (a) Write the chemical equation for the reaction between pyridine and water. (b) List all species present in an aqueous solution of pyridine in order of concentration, highest to lowest. (c) In a 0.100 M pyridine solution, \(3.2 \%\) of the pyridine has reacted with water to form products. Calculate the concentration of all species present (except water) in \(1 \mathrm{~L}\) of a \(0.100\) M pyridine solution. (d) What is the pH of this solution?

Short answer

(a) The chemical equation for the reaction between pyridine and water is: \(C_5H_5N + H_2O \leftrightarrows C_5H_5NH^+ + OH^-\). (b) The species present in an aqueous solution of pyridine, in order of highest to lowest concentration, are: \(C_5H_5N\), \(H_2O\), \(C_5H_5NH^+\), and \(OH^-\). (c) In a 0.100 M pyridine solution, the concentrations of the species are: \(C_5H_5N = 0.0968\) M, \(C_5H_5NH^+ = 0.0032\) M, and \(OH^- = 0.0032\) M. (d) The pH of this solution is 11.51.

Step-by-step solution

Write the chemical equation

Pyridine (\(C_5H_5N\)) is a weak base and reacts with water as follows: \[C_5H_5N + H_2O \leftrightarrows C_5H_5NH^+ + OH^-\]

List all species present in an aqueous solution of pyridine

In an aqueous solution of pyridine, the following species would be present: 1. Pyridine (\(C_5H_5N\)) 2. Water (\(H_2O\)) 3. Pyridinium ion (\(C_5H_5NH^+\)) 4. Hydroxide ion (\(OH^-\))

Calculate the concentration of all species

Given that 3.2% of the pyridine has reacted, we need to find the new concentrations of each species in a 0.100 M solution. For a 1L solution: - \(C_5H_5N\): \(0.100\times(1-0.032) = 0.0968\)M - \(H_2O\): The concentration of water remains unchanged and is not considered in calculations. - \(C_5H_5NH^+\): The concentration of pyridinium ion produced would be equal to the concentration of pyridine reacted, which is \(0.100\times0.032 = 0.0032\)M. - \(OH^-\): The concentration of hydroxide ion would also be equal to the concentration of pyridinium ion i.e., \(0.0032\)M.

Calculate the pH

Now that we have the concentration of hydroxide ions (\(OH^-\)), we can determine the pH of the solution. First, we will calculate the pOH using the hydroxide ion concentration: \(pOH = -\log(OH^-) = -\log(0.0032) = 2.49\) Next, we can use the relationship between pH and pOH to find the pH: \(pH + pOH = 14\) \(pH = 14-pOH = 14-2.49 = 11.51\) Therefore, the pH of the solution is 11.51.

Key concepts

Chemical Equations

A chemical equation is a symbolic representation of a chemical reaction, showing which reactants transform into which products. In the context of the reaction between pyridine and water, the equation is written as follows:

\[C_5H_5N + H_2O \leftrightarrows C_5H_5NH^+ + OH^-\]
This shows that pyridine \(C_5H_5N\) reacts with water \(H_2O\) to form the pyridinium ion \(C_5H_5NH^+\) and hydroxide ion \(OH^-\). The double arrows indicate that the reaction can proceed in both forward and reverse directions, which is characteristic of a weak base in water, leading to an equilibrium state.

Aqueous Solution Concentration

Understanding aqueous solution concentration is crucial in chemistry education. Concentration refers to the amount of a substance, in this case, pyridine, dissolved in a certain volume of solvent, typically water. It is often expressed in molarity \(M\), which is moles \(n\) of solute per liter \(L\) of solution.

For a 0.100 M pyridine solution, and given that 3.2% has reacted, the concentration of unreacted pyridine remains at 0.0968 M; this is because the initial concentration was 0.100 M, and only a small fraction has converted into products. In solutions of weak bases, the concentrations of products are usually much lower than the concentration of the unreacted weak base, as reflected in the exercise.

pH Calculation

pH calculation is a way to express the acidity or basicity of a solution. The pH scale ranges from 0 to 14, where 7 is neutral, values less than 7 indicate acidity, and values greater than 7 indicate basicity. The pH of a solution can be calculated from the concentration of hydrogen \(H^+\) or hydroxide \(OH^-\) ions present.

For weak bases like pyridine, the pH is related to the \(OH^-\) concentration using the formula \(pOH = -\text{log}(OH^-)\), and then you can find pH by subtracting pOH from 14. By following these steps, students can grasp the quantitative measure of how basic their pyridine solution is.

Weak Bases

Weak bases are substances that do not completely dissociate into ions in an aqueous solution. They are usually characterized by their smaller equilibrium constants \(K_b\) values, indicating less dissociation compared to strong bases.

Pyridine is an example of a weak base. In water, it reacts partially to produce pyridinium ions \(C_5H_5NH^+\) and hydroxide ions \(OH^-\). The reaction doesn't go to completion, and thus, the solution will contain significant amounts of both the weak base \(C_5H_5N\) and its ions.

Acid-Base Reactions

Acid-base reactions involve the exchange of protons between reactants. In the case of weak bases, such as pyridine, the base accepts a proton from water, leading to the formation of a hydroxide ion and the corresponding conjugate acid of the weak base.

The understanding of acid-base reactions is essential in various chemical processes, including buffer systems in biological contexts. For pyridine's reaction with water, both the forward reaction, where the base accepts a proton, and the reverse reaction, where the conjugate acid donates a proton back to the hydroxide ion, are important in establishing an equilibrium state.