Question

(a) Write a balanced acid-base reaction that produces \(\mathrm{Na}_{2} \mathrm{~S}\). (b) If \(\mathrm{Na}_{2} \mathrm{~S}\) is added to an aqueous solution of \(\mathrm{H}_{2} \mathrm{~S}\) \(\left(K_{\mathrm{a}}=9.1-10^{-8}\right)\), will the pH of the solution rise or fall? Explain. 89\. (a) Would you expect a reaction to take place between \(\mathrm{HCN}(a q)\) and \(\operatorname{AgNO}_{3}(a q)\) $$ \left(K_{\mathrm{sp}} \text { for } \mathrm{AgCN}=5.97 \times 10^{-17}\right) \text { ? } $$ Explain, and if a reaction occurs, write the net ionic equation. (b) If \(\mathrm{NaCN}\) is added to distilled water, would you expect the solution to be acidic, basic, or neutral? Explain using any chemical equations that may be appropriate.

Short answer

The balanced equation for Na2S is 2 NaOH + H2S -> Na2S + 2 H2O, adding Na2S to H2S will increase pH. AgNO3 reacts with HCN to form a precipitate of AgCN, as indicated by a net ionic equation Ag+ + CN- -> AgCN. NaCN in water results in a basic solution due to the formation of OH- ions.

Step-by-step solution

- Writing a balanced acid-base reaction for the production of Na2S

Sodium sulfide (Na2S) can be produced by reacting a strong base with a weak acid. Sodium hydroxide (NaOH) is a strong base, and hydrogen sulfide (H2S) is a weak acid. The balanced reaction is as follows: 2 NaOH (aq) + H2S (aq) -> Na2S (aq) + 2 H2O (l)

- Predicting the change in pH when Na2S is added to H2S solution

Na2S is a salt that will dissociate completely in water to form Na+ and S^2- ions. The sulfide ions (S^2-) will react with water to form HS- ions and OH- ions. This reaction will produce hydroxide ions, which are basic: S^2- (aq) + H2O (l) -> HS- (aq) + OH- (aq)Since OH- ions increase the pH, the addition of Na2S to an aqueous solution of H2S will cause the pH to rise.

- Evaluating the potential reaction between HCN and AgNO3

A reaction between hydrogen cyanide (HCN) and silver nitrate (AgNO3) can be predicted by considering the solubility product constant (Ksp) of silver cyanide (AgCN). When HCN reacts with AgNO3, a precipitate of AgCN could form if the product of the concentrations of Ag+ and CN- ions exceeds the Ksp value. The balanced reaction is: HCN (aq) + AgNO3 (aq) -> AgCN (s) + HNO3 (aq)The net ionic equation for the formation of AgCN from HCN and AgNO3 is: Ag+ (aq) + CN- (aq) -> AgCN (s)Since Ksp for AgCN is 5.97 x 10^-17, which is a very small number, the reaction will proceed to form a precipitate of AgCN.

- Assessing the pH of a solution of NaCN in water

NaCN is a salt formed from the weak acid HCN and the strong base NaOH. In water, NaCN will dissociate into Na+ and CN- ions. The CN- ion is a weak base and will react with water to produce HCN and OH- ions, according to the following reaction: CN- (aq) + H2O (l) -> HCN (aq) + OH- (aq)Since this reaction produces OH- ions, the solution will be basic. Therefore, the addition of NaCN to distilled water will result in a basic solution.

Key concepts

Balanced Chemical Equations

Grasping the concept of balanced chemical equations is pivotal for understanding chemical reactions in chemistry. The laws of conservation of mass and charge dictate that the number of atoms for each element, as well as the total charge, must be the same on both the reactant and product sides of an equation. For example, when sodium hydroxide (NaOH) reacts with hydrogen sulfide (H2S), the balanced chemical equation is:
2 NaOH (aq) + H2S (aq) → Na2S (aq) + 2 H2O (l)
This equation clearly shows that there are equal numbers of each type of atom on both sides, thus preserving mass and charge. Learning to balance equations is fundamental in predicting the products of reactions, as it helps to visualize the exact amounts of reactants needed and the quantity of products formed.

pH Change in Reactions

Chemical reactions that involve acids and bases often result in a pH change within the solution. The pH scale is a measure of the acidity or basicity of an aqueous solution. When sodium sulfide (Na2S) is added to an aqueous solution, it dissociates, increasing the number of hydroxide (OH-) ions, which are basic in nature:
S^2- (aq) + H2O (l) → HS^- (aq) + OH^- (aq)
Because the production of hydroxide ions indicates increased basicity, the pH of the solution will rise, becoming more alkaline. It's important for students to understand how the addition of certain compounds can alter the pH, affecting the chemical environment and the progress of subsequent reactions.

Solubility Product Constant

The solubility product constant, or Ksp, is crucial in predicting the solubility of ionic compounds in water. It represents the extent to which a salt can dissociate into its ions in solution. A low Ksp value implies low solubility, and vice versa. For instance, the Ksp for silver cyanide (AgCN) is a very small number (5.97 x 10^-17), indicating that it is not highly soluble in water. When considering a potential reaction between hydrogen cyanide (HCN) and silver nitrate (AgNO3), the Ksp helps us determine if a precipitate will form:
Ag⁺ (aq) + CN^- (aq) → AgCN (s)
Since the product of the concentration of Ag⁺ and CN^- ions could exceed the Ksp under some conditions, AgCN will precipitate, and the reaction will proceed.

Acid-Base Properties of Salts

Understanding the acid-base properties of salts is crucial in predicting the nature of solutions. Salts are products of an acid-base reaction and can influence the pH when dissolved in water. The reaction of a salt with water can cause the solution to be acidic, neutral, or basic depending on the strengths of the parent acid and base. For example, sodium cyanide (NaCN) is formed from the weak acid HCN and the strong base NaOH. When dissolved, it undergoes hydrolysis:
CN^- (aq) + H2O (l) → HCN (aq) + OH^- (aq)
This reaction produces hydroxide ions, which are responsible for the basic nature of the resulting solution. Students should note how the origin of the salt, i.e., from a strong base and a weak acid or vice versa, determines whether the solution will be acidic, basic, or neutral.