Problem 23 - Chemistry: The Central Science

Question

Name the spectator ions in any reactions that may be involved when each of the following pairs of solutions are mixed. (a) \(\mathrm{Na}_{2} \mathrm{CO}_{3}(\mathrm{aq})\) and \(\mathrm{MgSO}_{4}(a q)\) (b) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(a q)\) and \(\mathrm{Na}_{2} \mathrm{~S}(a q)\) (c) \(\left(\mathrm{NH}_{4}\right)_{3} \mathrm{PO}_{4}(a q)\) and \(\mathrm{CaCl}_{2}(a q)\)

Step-by-Step Solution

Answer

Spectator ions for each reaction: (a) \(Na^+(aq), SO_4^{2-}(aq)\) (b) \(Na^+(aq), NO_3^-(aq)\) (c) \(NH_4^+(aq), Cl^-(aq)\)

Step 1: Write down the chemical equations

For each of the given pairs of solutions, write down the chemical equation for the reaction that may occur. (a) Sodium carbonate and magnesium sulfate: \[Na_2CO_3(aq) + MgSO_4(aq) \rightarrow MgCO_3(s) + 2Na^+(aq) + SO_4^{2-}(aq)\] (b) Lead(II) nitrate and sodium sulfide: \[Pb(NO_3)_2(aq) + Na_2S(aq) \rightarrow PbS(s) + 2Na^+(aq) + 2NO_3^-(aq)\] (c) Ammonium phosphate and calcium chloride: \[(NH_4)_3PO_4(aq) + 3CaCl_2(aq) \rightarrow Ca_3(PO_4)_2(s) + 6NH_4^+(aq) + 6Cl^-(aq)\]

Step 2: Identify the spectator ions

Spectator ions are those that are present in both the reactants and products of the chemical reaction. They do not take part in the reaction and are simply present. (a) For the reaction between sodium carbonate and magnesium sulfate: Spectator ions: \(Na^+(aq), SO_4^{2-}(aq)\) (b) For the reaction between lead(II) nitrate and sodium sulfide: Spectator ions: \(Na^+(aq), NO_3^-(aq)\) (c) For the reaction between ammonium phosphate and calcium chloride: Spectator ions: \(NH_4^+(aq), Cl^-(aq)\)

Key Concepts

Chemical Reactions in Solutions

When we talk about chemical reactions in solutions, we're dealing with the process wherein reactants are dissolved in a solvent, usually water, to create a solution. In the realm of chemistry, these solutions allow for the dispersal of ions or molecules, providing a medium for them to react with each other more efficiently.

In typical aqueous reactions, substances can dissociate into ions, and these ions can interact to form new compounds, some of which might be insoluble and precipitate out of solution. For students looking to improve their understanding, it's important to focus on the soluble reactants and the rules of solubility to predict the products.

For example, when a solution of sodium carbonate is mixed with magnesium sulfate, both compounds dissolve in water to form their respective ions. These ions can then interact to produce a precipitate if the product is an insoluble compound, as is in this case where magnesium carbonate forms.

Ionic Equations

Moving deeper into reactions, ionic equations offer a closer look by representing dissolved ionic compounds as separate ions. Full ionic equations include all the ions, even those that do not participate directly in the reaction, known as spectator ions.

In contrast, net ionic equations are simplified versions that only show the ions and molecules directly involved in the chemical change, ignoring the spectator ions. This makes it clearer to see what the actual chemical change is. For students, identifying the net ionic equation requires understanding which ions form a precipitate and which remain in solution unchanged.

Tip for Students:

• When writing full ionic equations, first ensure that all soluble reactants are written as their dissociated ions. Then, identify the solid product, which will remain in its molecular form.
• To find the net ionic equation, cancel out ions that appear on both sides of the equation, as these are the spectators.

Double Displacement Reactions

In double displacement reactions, you'll find that two compounds exchange components, often leading to the formation of a precipitate, a gas, or a weak electrolyte. These reactions have a general form of AB + CD → AD + CB, where A and C are cations, while B and D are anions.

For students aiming to grasp these reactions, it's essential to recognize the patterns that lead to an actual exchange. Normally, you can predict the result of a double displacement reaction by knowing the solubility rules and identifying whether any of the products is insoluble or not normally found in solutions.

A typical classroom example: when aqueous solutions of ammonium phosphate and calcium chloride are mixed, the calcium and phosphate ions combine to form calcium phosphate, a solid precipitate, while ammonium and chloride ions remain in the solution as spectator ions. This transformation is a classic example of a double displacement reaction.