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), how could you distinguish any of the three magnesium substances from the other two?

Step-by-Step Solution

Answer

The balanced molecular and net ionic equations for the reaction of magnesium carbonate (MgCO3), magnesium oxide (MgO), and magnesium hydroxide (Mg(OH)2) with hydrochloric acid (HCl) are as follows: (a) MgCO3: Molecular - MgCO3(s) + 2HCl(aq) → MgCl2(aq) + H2O(l) + CO2(g), Net Ionic - MgCO3(s) + 2H^+(aq) → Mg^2+(aq) + H2O(l) + CO2(g) (b) MgO: Molecular - MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l), Net Ionic - MgO(s) + 2H^+(aq) → Mg^2+(aq) + H2O(l) (c) Mg(OH)2: Molecular - Mg(OH)2(s) + 2HCl(aq) → MgCl2(aq) + 2H2O(l), Net Ionic - Mg(OH)2(s) + 2H^+(aq) → Mg^2+(aq) + 2H2O(l) To distinguish the magnesium substances, observe the reactions: MgCO3 produces CO2 gas, enabling its identification. To differentiate between MgO and Mg(OH)2, look for a higher amount of water being formed; if more water is produced, it is likely Mg(OH)2.

Step 1: Write balanced molecular equations for each reaction

We must first write the balanced molecular equations for the reaction of each magnesium substance with hydrochloric acid (HCl): (a) Reaction of magnesium carbonate (MgCO3) with HCl: MgCO3(s) + 2HCl(aq) → MgCl2(aq) + H2O(l) + CO2(g) (b) Reaction of magnesium oxide (MgO) with HCl: MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l) (c) Reaction of magnesium hydroxide (Mg(OH)2) with HCl: Mg(OH)2(s) + 2HCl(aq) → MgCl2(aq) + 2H2O(l)

Step 2: Write net ionic equations for each reaction

Next, we write the net ionic equations for each reaction, considering the solubility rules and taking into account that the magnesium substances are solid and not able to be dissolved in water. (a) Net ionic equation for the reaction of MgCO3 with HCl: MgCO3(s) + 2H^+(aq) → Mg^2+(aq) + H2O(l) + CO2(g) (b) Net ionic equation for the reaction of MgO with HCl: MgO(s) + 2H^+(aq) → Mg^2+(aq) + H2O(l) (c) Net ionic equation for the reaction of Mg(OH)2 with HCl: Mg(OH)2(s) + 2H^+(aq) → Mg^2+(aq) + 2H2O(l)

Step 3: Distinguish the magnesium substances based on their reactions

We are now going to observe the reactions and determine how to distinguish the magnesium substances from one another: Looking closely, the reaction involving magnesium carbonate (MgCO3) produces carbon dioxide (CO2) gas, while the other two reactions do not. Therefore, in order to distinguish magnesium carbonate from the other two magnesium substances, you could look for the release of CO2 gas during the reaction. To distinguish between magnesium oxide (MgO) and magnesium hydroxide (Mg(OH)2), we can observe that the magnesium hydroxide produces two molecules of water (2H2O) when it reacts with HCl, while magnesium oxide only produces one molecule of water (H2O). Therefore, if you see a higher amount of water being formed after the reaction, it is more likely to be magnesium hydroxide.

Key Concepts

Molecular Equations

Molecular equations illustrate the overall chemical reaction by showing all compounds involved using their complete formulas. This provides a clear picture of the reactants and products in a chemical process. For example, when magnesium carbonate

• reacts with hydrochloric acid, the equation is: \[ \text{MgCO}_3(s) + 2\text{HCl}(aq) \rightarrow \text{MgCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g) \]
• The balanced equation for magnesium oxide and hydrochloric acid is: \[ \text{MgO}(s) + 2\text{HCl}(aq) \rightarrow \text{MgCl}_2(aq) + \text{H}_2\text{O}(l) \]
• For magnesium hydroxide: \[ \text{Mg(OH)}_2(s) + 2\text{HCl}(aq) \rightarrow \text{MgCl}_2(aq) + 2\text{H}_2\text{O}(l) \]

Molecular equations are vital as they provide an initial comprehensive depiction of the reaction, which helps set the stage before delving into detailed ionic analysis.

Net Ionic Equations

Net ionic equations describe the specific ions that participate in a chemical reaction. They exclude spectator ions and show only the species that undergo a chemical change. This makes net ionic equations a powerful tool for understanding the core interactions in a reaction.

• For the reaction between magnesium carbonate and HCl: \[ \text{MgCO}_3(s) + 2\text{H}^+(aq) \rightarrow \text{Mg}^{2+}(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g) \]
• Magnesium oxide reacts with HCl as follows: \[ \text{MgO}(s) + 2\text{H}^+(aq) \rightarrow \text{Mg}^{2+}(aq) + \text{H}_2\text{O}(l) \]
• The interaction between magnesium hydroxide and HCl is depicted by: \[ \text{Mg(OH)}_2(s) + 2\text{H}^+(aq) \rightarrow \text{Mg}^{2+}(aq) + 2\text{H}_2\text{O}(l) \]

Understanding net ionic equations assists in focusing on the ions involved in the reaction, simplifying the complexity of the full molecular equations.

Magnesium Compounds

Magnesium compounds like magnesium carbonate, oxide, and hydroxide are important in various chemical reactions, especially acid-base reactions. Each of these compounds exhibits unique properties when reacting:

• Magnesium Carbonate (MgCO₃): Notable for releasing carbon dioxide gas when reacting with acids, making it distinguishable from other magnesium compounds.
• Magnesium Oxide (MgO): Reacts to form water and soluble magnesium chloride, relevant in many neutralization processes.
• Magnesium Hydroxide (Mg(OH)₂): Known as milk of magnesia, it behaves like a base, producing twice the amount of water compared to magnesium oxide during acid reaction.

These peculiar characteristics help in identifying and utilizing magnesium compounds effectively in laboratory and industry.

Chemical Reactions in Chemistry

Chemical reactions involve the transformation of substances through breaking and forming bonds, an essential concept in chemistry. Understanding these changes helps grasp how substances interact and form new products.

• Acid-base reactions, such as those involving magnesium compounds and HCl, are typical reaction types.
  
• These reactions often produce water and salt, alongside other possible products like carbon dioxide.
• This exercise illustrates the integral nature of stoichiometry – the calculation of reactants and products in a chemical equation.
  
• Observations in reactions, such as gas production or water formation, are key for identifying substances and confirming chemical equations.

Overall, delving deep into chemical reactions provides insight into matter's behavior and the principles governing chemical changes.