Problem 77 - 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: \(\mathrm{MgCO}_{3}(\mathrm{s}) + 2 \mathrm{H}^{+}(\mathrm{aq}) \rightarrow \mathrm{CO}_{2}(\mathrm{g}) + \mathrm{Mg}^{2+}(\mathrm{aq}) + \mathrm{H}_{2}\mathrm{O}(\ell)\); spectator ions: \(\mathrm{Cl}^{-}\). (b) Double displacement reaction.

Step 1: Write the Molecular Equation

The given chemical reaction is already in molecular form:\[ \mathrm{MgCO}_{3}(\mathrm{s}) + 2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{CO}_{2}(\mathrm{g}) + \mathrm{MgCl}_{2}(\mathrm{aq}) + \mathrm{H}_{2}\mathrm{O}(\ell) \]

Step 2: Identify Strong Electrolytes and Break Them into Ions

In the given reaction, \(\mathrm{HCl}\) is a strong acid and is dissociated in water into ions. Similarly, \(\mathrm{MgCl}_{2}\) is soluble and dissociates into ions. Therefore, the ionic equation is:\[ \mathrm{MgCO}_{3}(\mathrm{s}) + 2 \mathrm{H}^{+}(\mathrm{aq}) + 2 \mathrm{Cl}^{-}(\mathrm{aq}) \rightarrow \mathrm{CO}_{2}(\mathrm{g}) + \mathrm{Mg}^{2+}(\mathrm{aq}) + 2 \mathrm{Cl}^{-}(\mathrm{aq}) + \mathrm{H}_{2}\mathrm{O}(\ell) \]

Step 3: Identify and Cancel Spectator Ions

Spectator ions do not participate in the actual reaction. In this equation, \(2 \mathrm{Cl}^{-}(\mathrm{aq})\) appears unchanged on both sides and is thus the spectator ion. Cancel them out to find the net ionic equation:\[ \mathrm{MgCO}_{3}(\mathrm{s}) + 2 \mathrm{H}^{+}(\mathrm{aq}) \rightarrow \mathrm{CO}_{2}(\mathrm{g}) + \mathrm{Mg}^{2+}(\mathrm{aq}) + \mathrm{H}_{2}\mathrm{O}(\ell) \]

Step 4: Identify the Reaction Type

The reaction involves an acid \(\mathrm{HCl}\) reacting with a carbonate \(\mathrm{MgCO}_{3}\), yielding carbon dioxide \(\mathrm{CO}_{2}\), water \(\mathrm{H}_{2}\mathrm{O}\), and a salt \(\mathrm{MgCl}_{2}\). This is characteristic of an acid-carbonate reaction, which is a type of double displacement reaction.

Key Concepts

Net Ionic Equation

In chemistry, a net ionic equation simplifies a chemical reaction by showing only the particles that participate in the transformation. The net ionic equation emphasizes the core of the reaction by removing the spectator ions, which do not alter during the reaction. To formulate a net ionic equation, first identify the strong electrolytes in the reaction and break them into their constituent ions. For instance, in the given reaction, hydrochloric acid (HCl) dissociates into \(\mathrm{H}^{+}\) and \(\mathrm{Cl}^{-}\) ions, while magnesium chloride (MgCl\(_2\)) separates into \(\mathrm{Mg}^{2+}\) and two \(\mathrm{Cl}^{-}\) ions.

Once separated, observe which ions appear on both sides of the equation without changing. These are the spectator ions, which we'll discuss further in the next section. Cancel out these spectator ions to reveal the net ionic equation: \(\mathrm{MgCO}_{3}(\mathrm{s}) + 2 \mathrm{H}^{+}(\mathrm{aq}) \rightarrow \mathrm{CO}_{2}(\mathrm{g}) + \mathrm{Mg}^{2+}(\mathrm{aq}) + \mathrm{H}_{2}\mathrm{O}(\ell)\). This represents the essence of the chemical change occurring in the reaction.

Spectator Ions

Spectator ions are ions in a chemical equation that appear in the same form on both sides of the equation. They remain in the solution as unchanged ions and do not participate directly in the chemical change. Identifying these ions is crucial because they help simplify the equation to only show the substances involved in the reaction.

In our reaction between magnesium carbonate and hydrochloric acid, the chloride ions (\(\mathrm{Cl}^{-}\)) are present in the same state among both the reactants and products. Since they do not change or participate in the actual chemical reaction process, \(\mathrm{Cl}^{-}\) ions are considered spectator ions. Once we identify these, we can remove them from both sides of the full ionic equation to simplify it to the net ionic equation. This process allows us to easily focus on the actual substances undergoing change, delivering clearer insights into the reaction mechanics.

Double Displacement Reaction

A double displacement reaction, in chemistry, involves the exchange of ions between two compounds to form two different compounds. It's often characterized by the swapping of positive ions between reactant compounds. This type of reaction can also be identified as an acid-base reaction, precipitation reaction, or gas formation reaction.

In the example with dolomite rock allowing magnesium carbonate to react with hydrochloric acid, this reaction can be seen as a double displacement reaction. Here, the \(\mathrm{H}^{+}\) ions from hydrochloric acid switch with the magnesium ions (\(\mathrm{Mg}^{2+}\)) in magnesium carbonate. As a result, carbon dioxide gaseous byproduct forms, alongside water and magnesium chloride, the solid product.

• The reaction showcases a classic acid-carbonate reaction, which fits under the umbrella of double displacement because of its ionic exchanges and product formation.
• Such reactions are key in many industrial processes and educational models, providing foundational insight into chemical interactions.

Double displacement reactions are significant as they demonstrate how ions and compounds can interconnect in chemical processes, paving the way for practical applications and further scientific exploration.