Problem 25 - Chemistry: The Central Science in SI Units, Global Edition

Question

Which ions remain in solution, unreacted, after each of the following pairs of solutions is mixed? (a) potassium carbonate and magnesium sulfate (b) lead nitrate and lithium sulfide (c) ammonium phosphate and calcium chloride

Step-by-Step Solution

Answer

(a) The ions that remain in solution and unreacted after the reaction between potassium carbonate and magnesium sulfate are \(K^+\) and \(SO_4^{2-}\). (b) The ions that remain in solution and unreacted after the reaction between lead nitrate and lithium sulfide are \(Li^+\) and \(NO_3^-\). (c) The ions that remain in solution and unreacted after the reaction between ammonium phosphate and calcium chloride are \(NH_4^+\) and \(Cl^-\).

Step 1: (a) Potassium carbonate and magnesium sulfate

Step 1: Write the molecular equation Potassium carbonate reacts with magnesium sulfate to form potassium sulfate and magnesium carbonate. The balanced molecular equation is: \[K_2CO_3(aq) + MgSO_4(aq) \rightarrow 2 K^+SO_4^-(aq) + MgCO_3(s)\] Step 2: Write the complete ionic equation \[2K^+(aq) + CO_3^{2-}(aq) + Mg^{2+}(aq) + SO_4^{2-}(aq) \rightarrow 2K^+(aq) +SO_4^{2-}(aq) + MgCO_3(s)\] Step 3: Cancel the spectator ions and write the net ionic equation The spectator ions are \(K^+\) and \(SO_4^{2-}\). The net ionic equation is: \[CO_3^{2-}(aq) + Mg^{2+}(aq) \rightarrow MgCO_3(s)\] Step 4: Identify the ions that remain in solution and unreacted The ions that remain in solution and unreacted are \(K^+\) and \(SO_4^{2-}\).

Step 2: (b) Lead nitrate and lithium sulfide

Step 1: Write the molecular equation Lead nitrate reacts with lithium sulfide to form lead sulfide and lithium nitrate. The balanced molecular equation is: \[Pb(NO_3)_2(aq) + 2Li_2S(aq) \rightarrow PbS(s) + 2LiNO_3(aq)\] Step 2: Write the complete ionic equation \[Pb^{2+}(aq) + 2NO_3^-(aq) + 4Li^+(aq) + 2S^{2-}(aq) \rightarrow PbS(s) + 4Li^+(aq) + 2NO_3^-(aq)\] Step 3: Cancel the spectator ions and write the net ionic equation The spectator ions are \(Li^+\) and \(NO_3^-\). The net ionic equation is: \[Pb^{2+}(aq) + S^{2-}(aq) \rightarrow PbS(s)\] Step 4: Identify the ions that remain in solution and unreacted The ions that remain in solution and unreacted are \(Li^+\) and \(NO_3^-\).

Step 3: (c) Ammonium phosphate and calcium chloride

Step 1: Write the molecular equation Ammonium phosphate reacts with calcium chloride to form calcium phosphate and ammonium chloride. The balanced molecular equation is: \[(NH_4)_3PO_4(aq) + 3CaCl_2(aq) \rightarrow Ca_3(PO_4)_2(s) + 6NH_4Cl(aq)\] Step 2: Write the complete ionic equation \[3NH_4^+(aq) + PO_4^{3-}(aq) + 3Ca^{2+}(aq) + 6Cl^-(aq) \rightarrow Ca_3(PO_4)_2(s) + 6NH_4^+(aq) + 6Cl^-(aq)\] Step 3: Cancel the spectator ions and write the net ionic equation The spectator ions are \(NH_4^+\) and \(Cl^-\). The net ionic equation is: \[PO_4^{3-}(aq) + 3Ca^{2+}(aq) \rightarrow Ca_3(PO_4)_2(s)\] Step 4: Identify the ions that remain in solution and unreacted The ions that remain in solution and unreacted are \(NH_4^+\) and \(Cl^-\).

Key Concepts

Spectator Ions

In chemical reactions, especially those taking place in aqueous solutions, you may often encounter the concept of spectator ions. These ions do not participate in the chemical change or the forming of new compounds. Instead, they simply "watch" the reaction happen without undergoing any change themselves.
To identify spectator ions, one needs to look at the complete ionic equation, which shows all the ions present in the solution. By observing which ions remain unchanged on both sides of the equation, you can spot the spectator ions.

• For example, when potassium carbonate reacts with magnesium sulfate, the spectator ions are \( K^+ \) and \( SO_4^{2-} \) as they appear on both the reactant and product sides without undergoing a change.
• In a reaction between lead nitrate and lithium sulfide, \( Li^+ \) and \( NO_3^- \) are the spectator ions, remaining unchanged as well.

Understanding spectator ions is essential as it helps you simplify reaction equations to their core reactive components, known as the net ionic equation.

Reaction Equations

Reaction equations are a useful way to show what happens during a chemical reaction. There are different types of equations, such as molecular, complete ionic, and net ionic equations.
Molecular equations show the complete formulas of all reactants and products, not broken into ions. For instance, when mixing potassium carbonate with magnesium sulfate, the molecular equation is:\[ K_2CO_3(aq) + MgSO_4(aq) \rightarrow MgCO_3(s) + K_2SO_4(aq) \]
Complete ionic equations, on the other hand, are exploded views that display all the ions present, e.g., for the above molecular equation:\[ 2K^+(aq) + CO_3^{2-}(aq) + Mg^{2+}(aq) + SO_4^{2-}(aq) \rightarrow MgCO_3(s) + 2K^+(aq) + SO_4^{2-}(aq) \]

• Net ionic equations strip down the equation to include only those components that undergo chemical change. Thus, for the same reaction: \[ CO_3^{2-}(aq) + Mg^{2+}(aq) \rightarrow MgCO_3(s) \]

Using these equations effectively helps in understanding the actual chemistry behind reactions and identifying the active participants in a solution.

Chemical Solutions

Chemical solutions are homogeneous mixtures made up of two or more substances. Typically, these include a solute and a solvent, with the solute being the substance dissolved, and the solvent being the substance that does the dissolving, usually a liquid like water.
Solutions play a pivotal role in the world of chemistry, particularly when it comes to reactions occurring in aqueous environments (those involving water as the solvent). Here's why they are essential:

• In reactions, solutions enable the mixing of reactants at the molecular level, facilitating interactions that might not easily happen in a solid form.
• Ionization is another critical process happening in solutions; compounds dissociate into ions, which are the charge carriers. For example, potassium carbonate ionizes in water to form \( 2K^+(aq) \) and \( CO_3^{2-}(aq) \).
• Solutions often result in precipitates; when two solutions are mixed, if an insoluble product forms, it precipitates out of the solution. This can be seen when mixing calcium chloride with ammonium phosphate, resulting in calcium phosphate, \( Ca_3(PO_4)_2 \), as a solid.

Understanding chemical solutions helps grasp how reactions occur in real-world conditions and facilitates expectations of outcomes in diverse chemical processes.