Question

Magnesium metal reacts with hydrobromic acid to produce hydrogen gas and a solution of magnesium bromide. Write the molecular equation for this reaction. Then write the corresponding net ionic equation.

Short answer

Molecular: \( \text{Mg} + 2 \text{HBr} \rightarrow \text{H}_2 + \text{MgBr}_2 \); Net Ionic: \( \text{Mg} + 2\text{H}^+ \rightarrow \text{H}_2 + \text{Mg}^{2+} \).

Step-by-step solution

Identifying the Reactants and Products

In this reaction, magnesium (Mg) reacts with hydrobromic acid (HBr) to produce hydrogen gas (H₂) and magnesium bromide (MgBr₂). We start by identifying all the reactants: Mg and HBr, and products: H₂ and MgBr₂.

Writing the Molecular Equation

Write the balanced molecular equation for the reaction. The molecular equation is written with all the compounds in their undissociated form:\[ \text{Mg}(s) + 2 \text{HBr}(aq) \rightarrow \text{H}_2(g) + \text{MgBr}_2(aq) \] This shows the reaction between magnesium metal and hydrobromic acid to form hydrogen gas and magnesium bromide.

Dissociating the Aqueous Compounds

In the molecular equation, identify the aqueous compounds and dissociate them into ions. Hydrobromic acid (HBr) dissociates into \(2H^+\) and \(2Br^-\), while magnesium bromide (MgBr₂) becomes \(Mg^{2+}\) and \(2Br^-\).\[ \text{Mg}(s) + 2\text{H}^+(aq) + 2\text{Br}^-(aq) \rightarrow \text{H}_2(g) + \text{Mg}^{2+}(aq) + 2\text{Br}^-(aq) \]

Identifying and Removing Spectator Ions

Identify ions that appear on both sides of the equation unchanged; these are the spectator ions. In this reaction, \(Br^-\) ions are spectators. They appear on both the reactant and product sides without undergoing any change.

Writing the Net Ionic Equation

After removing the spectator ions from the dissociated equation, write the net ionic equation, which shows only the species that actually participate in the reaction:\[ \text{Mg}(s) + 2\text{H}^+(aq) \rightarrow \text{H}_2(g) + \text{Mg}^{2+}(aq) \] This represents the essential chemical change, the reaction of magnesium metal with hydronium ions to produce hydrogen gas and magnesium ions.

Key concepts

Molecular Equations

When we express a chemical reaction in its entirety, we create a molecular equation. This type of equation presents all the reactants and products in their complete, undissociated forms. It serves as a broad overview of the chemical changes occurring in the reaction.

For example, consider the reaction between magnesium metal and hydrobromic acid. The molecular equation is:\[ \text{Mg}(s) + 2 \text{HBr}(aq) \rightarrow \text{H}_2(g) + \text{MgBr}_2(aq) \]This equation highlights how the solid magnesium and the aqueous hydrobromic acid interact to yield hydrogen gas and magnesium bromide in solution. The elements and compounds are presented as they appear in the real physical world, without considering ionic dissociation. This clarity makes it easy to see the overall transformation from reactants to products.

A key point to remember is that the molecular equation provides you with the starting point for more detailed analyses, like writing net ionic equations.

Net Ionic Equations

Net ionic equations focus on the core of chemical reactions, showing only the species that directly participate in the chemical change. These equations exclude ions that do not undergo any transformation during the reaction. Such ions are known as spectator ions.

To derive a net ionic equation, we need to do a few things:

• Dissociate all soluble ionic compounds in the molecular equation into their respective ions.
• Identify the ions that appear on both the reactant and product sides unchanged (these are the spectator ions).
• Omit the spectator ions to concentrate only on the substances involved in the chemical change.

Using our example reaction, the net ionic equation is:\[ \text{Mg}(s) + 2\text{H}^+(aq) \rightarrow \text{H}_2(g) + \text{Mg}^{2+}(aq) \]Here, we see that it's the reaction between magnesium metal and hydrogen ions that leads to the production of hydrogen gas and magnesium ions. The simplicity of net ionic equations highlights the essential chemistry taking place and helps us focus on the meaningful changes.

Spectator Ions

Spectator ions are like observers in a sports arena; they are present but do not participate in the main event. In chemical equations, these ions appear in identical forms on both sides of a reaction. They do not change or get involved in the chemical transformations happening.

Identifying spectator ions is crucial for writing net ionic equations, as their absence helps clarify the true nature of the chemistry at play. In our reaction of magnesium with hydrobromic acid, the \(Br^-\) ions are spectators:

• They appear as \(Br^-\) in the reactants sides.
• They remain as \(Br^-\) in the products side.

Removing these from the equation allows us to focus on the ions that actually change, giving us the net ionic equation.
Remember, while spectator ions don't participate directly, they help balance the charge in solution and maintain electrical neutrality throughout the reaction. Understanding their role provides deeper insight into the dynamics of chemical reactions.