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 netionic equation for the reaction that occurs when each substance reacts with a hydrochloric acid solution. (b) By observing the reactions in part (a), how could you distinguish any of the three magnesium substances from the other two?

Step-by-Step Solution

Answer

(a) 1. MgCO3 + 2HCl -> MgCl2 + H2O + CO2; MgCO3 + 2H^+ -> Mg^2+ + H2O + CO2 2. MgO + 2HCl -> MgCl2 + H2O; MgO + 2H^+ -> Mg^2+ + H2O 3. Mg(OH)2 + 2HCl -> MgCl2 + 2H2O; Mg(OH)2 + 2H^+ -> Mg^2+ + 2H2O (b) Observe gas formation (MgCO3) and solubility in water (Mg(OH)2 slightly soluble, MgO almost insoluble).

Step 1: (a) Balanced Molecular Equations and Net Ionic Equations

1. Reaction of magnesium carbonate (MgCO3) with hydrochloric acid (HCl): Balanced molecular equation: MgCO3 (s) + 2HCl (aq) -> MgCl2 (aq) + H2O (l) + CO2 (g) Net ionic equation: MgCO3 (s) + 2H^+ (aq) -> Mg^2+ (aq) + H2O (l) + CO2 (g) 2. Reaction of magnesium oxide (MgO) with hydrochloric acid (HCl): Balanced molecular equation: MgO (s) + 2HCl (aq) -> MgCl2 (aq) + H2O (l) Net ionic equation: MgO (s) + 2H^+ (aq) -> Mg^2+ (aq) + H2O (l) 3. Reaction of magnesium hydroxide (Mg(OH)2) with hydrochloric acid (HCl): Balanced molecular equation: Mg(OH)2 (s) + 2HCl (aq) -> MgCl2 (aq) + 2H2O (l) Net ionic equation: Mg(OH)2 (s) + 2H^+ (aq) -> Mg^2+ (aq) + 2H2O (l)

Step 2: (b) Distinguishing the Magnesium Substances

In each of these reactions, we can identify some differences that would help us distinguish one magnesium substance from the others: 1. Magnesium carbonate (MgCO3) - This substance produces carbon dioxide (CO2) gas when reacting with hydrochloric acid. The formation of gas can be observed as bubbles or fizzing during the reaction. 2. Magnesium oxide (MgO) - This substance does not produce any gas when reacting with hydrochloric acid, making it distinct from the magnesium carbonate. However, it is similar to magnesium hydroxide in this aspect. 3. Magnesium hydroxide (Mg(OH)2) - To distinguish magnesium hydroxide from magnesium oxide, we can observe the solubility in water. Magnesium hydroxide is slightly soluble in water, forming a milky solution when mixed. On the other hand, magnesium oxide is nearly insoluble in water. By looking at the solubility of the substances in water, we can differentiate magnesium hydroxide from magnesium oxide. In summary, to distinguish the three magnesium substances from each other, we can observe the formation of gas during the reaction with hydrochloric acid (which indicates the presence of magnesium carbonate), and the solubility of the substances in water to differentiate magnesium hydroxide from magnesium oxide.

Key Concepts

Balanced Molecular Equations

Balanced molecular equations are fundamental in chemistry as they show the complete and exact details of a chemical reaction. When magnesium compounds react with hydrochloric acid (HCl), we aim to balance each equation. For magnesium carbonate (MgCO3), the equation is:

• \[ \text{MgCO}_3 (s) + 2\text{HCl} (aq) \rightarrow \text{MgCl}_2 (aq) + \text{H}_2\text{O} (l) + \text{CO}_2 (g) \]

Magnesium oxide (MgO) and hydrochloric acid result in:

• \[ \text{MgO} (s) + 2\text{HCl} (aq) \rightarrow \text{MgCl}_2 (aq) + \text{H}_2\text{O} (l) \]

Lastly, magnesium hydroxide (Mg(OH)2) reacts similarly:

• \[ \text{Mg(OH)}_2 (s) + 2\text{HCl} (aq) \rightarrow \text{MgCl}_2 (aq) + 2\text{H}_2\text{O} (l) \]

Balancing these equations ensures that the number of atoms for each element is the same on both sides of the equation. This shows how reactants transform into products in a reaction.

Net Ionic Equations

Net ionic equations simplify the chemical reactions by focusing on the core elements involved, excluding spectator ions. These equations are particularly useful for predicting the outcome of reactions in aqueous solutions. For instance, with magnesium carbonate (MgCO3), the net ionic equation becomes:

• \[ \text{MgCO}_3 (s) + 2\text{H}^+ (aq) \rightarrow \text{Mg}^{2+} (aq) + \text{H}_2\text{O} (l) + \text{CO}_2 (g) \]

For magnesium oxide (MgO):

• \[ \text{MgO} (s) + 2\text{H}^+ (aq) \rightarrow \text{Mg}^{2+} (aq) + \text{H}_2\text{O} (l) \]

And magnesium hydroxide (Mg(OH)2):

• \[ \text{Mg(OH)}_2 (s) + 2\text{H}^+ (aq) \rightarrow \text{Mg}^{2+} (aq) + 2\text{H}_2\text{O} (l) \]

These net ionic equations highlight the transformation of hydrogen ions (H+) with magnesium-containing compounds. It helps students better understand the core interactions in a chemical reaction.

Identifying Gas Formation

Identifying gas formation in a chemical reaction often serves as a valuable indicator of specific substances' presence. This is particularly observable in reactions involving magnesium carbonate (MgCO3) and hydrochloric acid (HCl), where carbon dioxide (CO2) gas forms. The formation of gas can be seen as:

• Bubbles or fizzing during the reaction process.
• A visible effervescence, which is unmistakable.

Gas formation not only helps in identifying the production of carbonates but also aids students in understanding case-specific chemical reactions practically.

Solubility in Water

Solubility in water is a crucial characteristic in chemistry, defining how a substance dissolves in water. For magnesium compounds reacting with hydrochloric acid, solubility can help distinguish them. Magnesium hydroxide (Mg(OH)2):

• Is slightly soluble in water, yielding a milky solution.

In contrast, magnesium oxide (MgO):

• Is nearly insoluble, remaining as a solid when placed in water.

Understanding solubility helps differentiate between magnesium hydroxide and magnesium oxide when observing reactions.

Chemistry Education

Chemistry education empowers students to explore and understand chemical reactions deeply. Learning through exercises such as magnesium reactions with hydrochloric acid allows students to grasp:

• The methodology in writing both balanced molecular and net ionic equations.
• The ability to observe and identify changes such as gas formation and solubility characteristics.

By engaging with these concepts, students improve their analytical skills and expand their knowledge of chemical reactions. Such educational exercises prepare learners for more complex topics and experiments in chemistry.