Question

Are a \(1 \mathrm{M}\) solution of ammonia, \(\mathrm{NH}_{3}\), and a \(1 \mathrm{M}\) solution of lithium hydroxide, I iOH, equally basic, or is one solution more basic than the other? Explain.

Short answer

A $1 \mathrm{M}$ solution of lithium hydroxide (LiOH) is more basic than a $1 \mathrm{M}$ solution of ammonia (NH3). This is because LiOH is a strong base, meaning it completely dissociates in water, creating a higher concentration of hydroxide ions (OH⁻). In contrast, NH3 is a weak base, only partially dissociating in water, resulting in a lower concentration of hydroxide ions (OH⁻).

Step-by-step solution

Identify the substances as weak or strong bases

Ammonia (NH3) is a weak base because it only partially dissociates in water, while lithium hydroxide (LiOH) is a strong base, as it completely dissociates in water when dissolved.

Write the dissociation reactions for each base

For ammonia, the dissociation reaction in water is as follows: \(NH_{3} (aq) + H_{2}O (l) \rightleftharpoons NH_{4}^{+} (aq) + OH^{-} (aq)\) For lithium hydroxide, the dissociation reaction in water is as follows: \(LiOH (aq) \rightarrow Li^{+} (aq) + OH^{-} (aq)\)

Compare the formation of hydroxide ions from each dissociation reaction

Ammonia is a weak base, so the dissociation reaction does not proceed completely to the right. This means that the concentration of hydroxide ions (OH⁻) formed in an ammonia solution would be less than 1 M. On the other hand, lithium hydroxide is a strong base and its dissociation reaction proceeds completely to the right. This means that for every 1 M of LiOH, there will be 1 M of hydroxide ions (OH⁻) formed in the solution.

Determine which solution is more basic

Since the concentration of hydroxide ions (OH⁻) is higher in a lithium hydroxide (LiOH) solution compared to an ammonia (NH3) solution, the 1 M lithium hydroxide solution is more basic than the 1 M ammonia solution.

Key concepts

Weak and Strong Bases

Understanding the difference between weak and strong bases is crucial in predicting their behavior in solution. A strong base, such as lithium hydroxide (LiOH), completely dissociates in water to release all of its hydroxide ions (OH⁻). This means that in a 1 M solution of LiOH, there will indeed be a concentration of 1 M OH⁻ ions.

In contrast, a weak base like ammonia (NH₃) only partially dissociates, establishing a dynamic equilibrium between the undissociated ammonia and the products of its dissociation, ammonium ions (NH₄⁺) and hydroxide ions (OH⁻). As a result, the concentration of OH⁻ ions in a weak base solution is significantly less than the concentration of the base itself. This partial dissociation is the reason why a 1 M solution of ammonia is less basic compared to a 1 M solution of lithium hydroxide.

Dissociation Reactions

Dissociation reactions are fundamental to understanding how acids and bases behave in water. When bases dissolve in water, they dissociate into ions. A strong base like lithium hydroxide will dissociate completely:
\[ LiOH (aq) \rightarrow Li^{+} (aq) + OH^{-} (aq) \]
This reaction does not reach equilibrium and proceeds in a single direction to completion. On the other hand, a weak base such as ammonia undergoes a reversible reaction:
\[ NH_{3} (aq) + H_{2}O (l) \rightleftharpoons NH_{4}^{+} (aq) + OH^{-} (aq) \]
Here, the double arrow indicates a state of equilibrium where both the reactants and products are present at all times. Students need to understand that these reversible reactions significantly impact the concentration of hydroxide ions in solution, which in turn affects the basicity of the solution.

Hydroxide Ion Concentration

Hydroxide ion concentration is a direct measure of the basicity of a solution. In the context of the original exercise, the solution which has a higher concentration of these OH⁻ ions will be more basic. Since the strong base LiOH fully dissociates, it releases a high concentration of hydroxide ions equal to its original concentration. In comparison, the weak base ammonia undergoes partial dissociation in water, which results in a lower hydroxide ion concentration than the initial ammonia concentration.

This difference in hydroxide ion availability is the reason why LiOH will render a solution more basic than an equivalent concentration of NH₃. It is important for students to grasp that the pH scale, which measures the acidity or basicity of a solution, is fundamentally related to the concentration of these hydroxide ions.

Chemical Equilibrium

Chemical equilibrium is a state when the rate of the forward reaction equals the rate of the reverse reaction, meaning that the concentrations of reactants and products remain constant over time. This concept is vital in our context, as it applies to the dissociation of weak bases like ammonia. The recognition of ammonia engaging in a state of equilibrium when it dissociates:
\[ NH_{3} (aq) + H_{2}O (l) \rightleftharpoons NH_{4}^{+} (aq) + OH^{-} (aq) \]
illustrates that the reaction does not go to completion. Le Chatelier's principle plays a role in how the equilibrium can shift in response to changes in conditions, such as concentration and temperature. For students, understanding this dynamic balance is key to predicting the behavior of weak bases in solution and how they differ from strong bases, which do not establish an equilibrium in water.