Problem 69 - Chemistry and Chemical Reactivity

Question

The mineral dolomite contains magnesium carbonate. This reacts with hydrochloric acid. $$\mathrm{MgCO}_{3}(\mathrm{s})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{CO}_{2}(\mathrm{g})+\mathrm{MgCl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell)$$ (a) Write the net ionic equation for this reaction and identify the spectator ions. (b) What type of reaction is this?

Step-by-Step Solution

Answer

(a) Net ionic equation: \(\mathrm{MgCO}_3(\mathrm{s}) + 2\mathrm{H^+}(\mathrm{aq}) \rightarrow \mathrm{CO}_2(\mathrm{g}) + \mathrm{Mg^{2+}}(\mathrm{aq}) + \mathrm{H}_2\mathrm{O}(\ell)\); Spectator ion: \(\mathrm{Cl^-}\). (b) Gas evolution and acid-base reaction.

Step 1: Identify the Ionic Compounds

In the balanced equation, identify the compounds that will dissociate into ions. Hydrochloric acid (\(\mathrm{HCl}\)) and magnesium chloride (\(\mathrm{MgCl_2}\)) are soluble and dissociate into ions.

Step 2: Write the Dissociation of Compounds

When dissolved, \(\mathrm{HCl}\) dissociates into \(\mathrm{H^+}\) and \(\mathrm{Cl^-}\), whereas \(\mathrm{MgCl_2}\) dissociates into \(\mathrm{Mg^{2+}}\) and \(\mathrm{2Cl^-}\).

Step 3: Write the Complete Ionic Equation

Substitute the dissociated ions back into the equation: \(\mathrm{MgCO}_{3}(\mathrm{s}) + 2\mathrm{H^+}(\mathrm{aq}) + 2\mathrm{Cl^-}(\mathrm{aq}) \rightarrow \mathrm{CO}_{2}(\mathrm{g}) + \mathrm{Mg^{2+}}(\mathrm{aq}) + 2\mathrm{Cl^-}(\mathrm{aq}) + \mathrm{H}_{2} \mathrm{O}(\ell)\).

Step 4: Identify and Remove Spectator Ions

Compare the ions on both sides of the equation to find spectator ions, which appear unchanged on both sides. \(\mathrm{Cl^-}\) appears unchanged and is a spectator ion, so remove it.

Step 5: Write the Net Ionic Equation

Remove the spectator ions from the complete ionic equation. The net ionic equation is: \(\mathrm{MgCO}_{3}(\mathrm{s}) + 2\mathrm{H^+}(\mathrm{aq}) \rightarrow \mathrm{CO}_{2}(\mathrm{g}) + \mathrm{Mg^{2+}}(\mathrm{aq}) + \mathrm{H}_{2}\mathrm{O}(\ell)\).

Step 6: Determine the Reaction Type

The equation represents a gas evolution reaction where carbon dioxide gas is released. It is also a type of acid-base reaction, where the acid reacts with a carbonate compound.

Key Concepts

Spectator Ions

In a chemical equation, not all ions participate in the reaction. Spectator ions are those that appear on both sides of the equation without undergoing any change. They simply "watch" the action happen without getting involved.

For example, in our reaction:

• Hydrochloric acid (HCl) dissociates into hydrogen ions (\(\mathrm{H^+}\)) and chloride ions (\(\mathrm{Cl^-}\)).
• Magnesium chloride (\(\mathrm{MgCl_2}\)) dissociates into magnesium ions (\(\mathrm{Mg^{2+}}\)) and chloride ions.

After writing the complete ionic equation, you can see that chloride ions (\(\mathrm{Cl^-}\)) appear unchanged on both sides. That's why they're called spectator ions.

To find the net ionic equation, you remove these spectator ions, simplifying the equation to show only the species that actively participate in the reaction.

Acid-Base Reactions

An acid-base reaction occurs when an acid reacts with a base, leading to the formation of water and sometimes other products. In our case, the focus is on hydrochloric acid (\(\mathrm{HCl}\)) reacting with magnesium carbonate (\(\mathrm{MgCO_3}\)).

This reaction involves:

• The \(\mathrm{H^+}\) ions from the acid combine with anions from the carbonate compound.
• The acid donates protons, initiating the formation of water and carbon dioxide.

This specific reaction illustrates how acids can react with metal carbonates, showcasing the fundamentals of acid-base chemistry. The formation of water and carbon dioxide is a common characteristic of these reactions.

Gas Evolution Reactions

Gas evolution reactions are fascinating because they result in the production of a gas. This visibly shows that a chemical change is occurring.

In this particular reaction, carbon dioxide (\(\mathrm{CO_2}\)) is the gas evolved. Such reactions:

• Occur when an acid reacts with a carbonate, leading to the release of carbon dioxide gas.
• Are notable for producing bubbles or fizzing, which is the gas leaving the solution.

These reactions are a type of acid-base reaction but are specifically characterized by the release of a gas. They help in identifying the presence of carbonate compounds and understanding the behavior of acids with different substances.