Problem 43 - Chemistry The Central Science

Question

Magnesium carbonate, magnesium oxide, and magnesium hydroxide are all white solids that react with acidic solutions. (a) Write a balanced molecular equation and a net ionic equation for the reaction that occurs when each substance reacts with a hydrochloric acid solution. (b) By observing the reactions in part (a) could you distinguish any of the three magnesium substances from the other two? If so how? (c) If excess \(\mathrm{HCl}(a q)\) is added, would the clear solutions left behind after the reaction is complete contain the same or different ions in each case?

Step-by-Step Solution

Answer

The reactions of magnesium carbonate, magnesium oxide, and magnesium hydroxide with hydrochloric acid produce the following net ionic equations: 1. \(MgCO_{3(s)} + 2H^+_{(aq)} \rightarrow Mg^{2+}_{(aq)} + H_{2}O_{(l)} + CO_{2(g)}\) 2. \(MgO_{(s)} + 2H^+_{(aq)} \rightarrow Mg^{2+}_{(aq)} + H_{2}O_{(l)} \) 3. \(Mg(OH)_{2(s)} + 2H^+_{(aq)} \rightarrow Mg^{2+}_{(aq)} + 2H_{2}O_{(l)} \) It is not possible to distinguish the magnesium substances based solely on their reactions, as they all produce similar products. If excess hydrochloric acid is added, the clear solutions left behind will all contain the same ions: \(Mg^{2+}\) and \(Cl^-\).

Step 1: Molecular and Net Ionic Equations for Magnesium Carbonate Reaction

1. Write the molecular equation for the reaction of magnesium carbonate with hydrochloric acid: \(MgCO_{3(s)} + 2HCl_{(aq)} \rightarrow MgCl_{2(aq)} + H_{2}O_{(l)} + CO_{2(g)} \) 2. Remove spectator ions to obtain the net ionic equation: \(MgCO_{3(s)} + 2H^+_{(aq)} \rightarrow Mg^{2+}_{(aq)} + H_{2}O_{(l)} + CO_{2(g)} \)

Step 2: Molecular and Net Ionic Equations for Magnesium Oxide Reaction

1. Write the molecular equation for the reaction of magnesium oxide with hydrochloric acid: \(MgO_{(s)} + 2HCl_{(aq)} \rightarrow MgCl_{2(aq)} + H_{2}O_{(l)} \) 2. Remove spectator ions to obtain the net ionic equation: \(MgO_{(s)} + 2H^+_{(aq)} \rightarrow Mg^{2+}_{(aq)} + H_{2}O_{(l)} \)

Step 3: Molecular and Net Ionic Equations for Magnesium Hydroxide Reaction

1. Write the molecular equation for the reaction of magnesium hydroxide with hydrochloric acid: \(Mg(OH)_{2(s)} + 2HCl_{(aq)} \rightarrow MgCl_{2(aq)} + 2H_{2}O_{(l)} \) 2. Remove spectator ions to obtain the net ionic equation: \(Mg(OH)_{2(s)} + 2H^+_{(aq)} \rightarrow Mg^{2+}_{(aq)} + 2H_{2}O_{(l)} \)

Step 4: Distinguishing Magnesium Substances

It is not possible to distinguish the three magnesium substances based solely on their reactions with hydrochloric acid, as the molecular and net ionic equations all produce magnesium chloride and water, and the only difference is that magnesium carbonate also releases carbon dioxide.

Step 5: Ions in the Clear Solutions

If excess hydrochloric acid is added, all three reactions will go to completion, and the clear solutions left behind will all contain the same ions: \(Mg^{2+}\) and \(Cl^-\), as all three reactions produce magnesium chloride in solution.

Key Concepts

Molecular Equations

Molecular equations represent the complete formulas of reactants and products involved in a chemical reaction. This type of equation provides a picture of the reaction in terms of molecules or formula units. Here’s how it works when reacting magnesium compounds with hydrochloric acid:

• Magnesium carbonate reacts with hydrochloric acid to form magnesium chloride, water, and carbon dioxide: \(MgCO_{3(s)} + 2HCl_{(aq)} \rightarrow MgCl_{2(aq)} + H_{2}O_{(l)} + CO_{2(g)}\).
• Magnesium oxide reacts with hydrochloric acid to produce magnesium chloride and water: \(MgO_{(s)} + 2HCl_{(aq)} \rightarrow MgCl_{2(aq)} + H_{2}O_{(l)}\).
• Magnesium hydroxide reacts with hydrochloric acid forming magnesium chloride and water: \(Mg(OH)_{2(s)} + 2HCl_{(aq)} \rightarrow MgCl_{2(aq)} + 2H_{2}O_{(l)}\).

In molecular equations, we can see the complete picture of reactants turning into products. There's no focus on what happens to ions or how the compounds break down in water.

Net Ionic Equations

Net ionic equations simplify chemical reactions by omitting spectator ions and showing only the species that actually participate in the reaction. This allows us to focus on the core chemical change occurring in the solution.

• For magnesium carbonate reacting with hydrochloric acid, the net ionic equation highlights carbon dioxide formation: \(MgCO_{3(s)} + 2H^+_{(aq)} \rightarrow Mg^{2+}_{(aq)} + H_{2}O_{(l)} + CO_{2(g)}\).
• Magnesium oxide reacting showcases the core interaction with hydrogen ions: \(MgO_{(s)} + 2H^+_{(aq)} \rightarrow Mg^{2+}_{(aq)} + H_{2}O_{(l)}\).
• When magnesium hydroxide reacts, the formation of water is the key change: \(Mg(OH)_{2(s)} + 2H^+_{(aq)} \rightarrow Mg^{2+}_{(aq)} + 2H_{2}O_{(l)}\).

Net ionic equations are critical for understanding the essence of the reactions, focusing solely on the ions playing an active role, thereby simplifying complex reactions into more digestible parts.

Acid-Base Reactions

Acid-base reactions involve the transfer of protons (H+) from an acid to a base. The reaction produces a salt and often water, which is what's observed in the reactions of magnesium compounds with hydrochloric acid.

In these reactions:

• Hydrochloric acid (HCl), a strong acid, donates protons which react with the magnesium compounds.
• Magnesium carbonate leads to the formation of magnesium chloride and releases carbon dioxide gas, a notable characteristic.
• Magnesium oxide and magnesium hydroxide reactions both result in the formation of magnesium chloride and water.

All these reactions are typical of acid-base interactions where the strong acid provides hydrogen ions (H+) that react with basic or amphoteric magnesium compounds. This is why the presence of magnesium chloride and water is common across all the reactions. Only magnesium carbonate produces a gas, which can help distinguish it from the others.