Problem 95 - General Chemistry

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.

Step-by-Step Solution

Answer

Molecular Equation: \( \text{Mg}(s) + 2\text{HBr}(aq) \rightarrow \text{MgBr}_2(aq) + \text{H}_2(g) \) Net Ionic Equation: \( \text{Mg}(s) + 2\text{H}^+(aq) \rightarrow \text{Mg}^{2+}(aq) + \text{H}_2(g) \)

Step 1: Identify the Reactants and Products

Magnesium (\(\text{Mg}\)) reacts with hydrobromic acid (\(\text{HBr}\)) to form hydrogen gas (\(\text{H}_2\)) and magnesium bromide (\(\text{MgBr}_2\)). These substances are identified as the reactants and products of the reaction.

Step 2: Write the Molecular Equation

The molecular equation for the reaction is:\[\text{Mg}(s) + 2\text{HBr}(aq) \rightarrow \text{MgBr}_2(aq) + \text{H}_2(g)\]This equation shows the reactants (magnesium and hydrobromic acid) and the products (magnesium bromide and hydrogen gas) without splitting them into ions.

Step 3: Dissociate Aqueous Compounds into Ions

Write the ionic equation, considering that \(\text{HBr}\) is a strong acid and \(\text{MgBr}_2\) is soluble in water:\[\text{Mg}(s) + 2\text{H}^+(aq) + 2\text{Br}^-(aq) \rightarrow \text{Mg}^{2+}(aq) + 2\text{Br}^-(aq) + \text{H}_2(g)\]Separate the aqueous compounds (\(\text{HBr}\) and \(\text{MgBr}_2\)) into their ionic forms.

Step 4: Write the Net Ionic Equation

Cancel out spectator ions (ions that appear on both sides of the equation): Here, \(\text{Br}^-\) ions are spectator ions. The net ionic equation is:\[\text{Mg}(s) + 2\text{H}^+(aq) \rightarrow \text{Mg}^{2+}(aq) + \text{H}_2(g)\]This equation shows only the ions that participate in the reaction.

Key Concepts

Molecular Equation

A molecular equation is a way to represent a chemical reaction using the chemical formulas of the reactants and products. It shows the complete formula of each compound without breaking them down into ions. This type of equation gives a clear picture of what substances are involved in the reaction. For example, in the reaction between magnesium metal and hydrobromic acid, the molecular equation is: \[\text{Mg}(s) + 2\text{HBr}(aq) \rightarrow \text{MgBr}_2(aq) + \text{H}_2(g) \] This equation tells us that magnesium (solid) reacts with hydrobromic acid in an aqueous solution to produce magnesium bromide in an aqueous solution and hydrogen gas. This kind of equation is helpful when starting to learn about reactions because it provides a straightforward look at the overall process.

Spectator Ions

In chemical reactions, especially those involving ionic compounds, not all ions present in the reaction mixture participate in the chemical change. These non-participating ions are called spectator ions because they "watch" the reaction happen but do not get involved. For our example, the ions in hydrobromic acid, which disassociates into \(\text{H}^+\) and \(\text{Br}^-\), have different roles. The \(\text{Br}^-\) ions are spectator ions. When writing the net ionic equation, we exclude the spectator ions to focus on the actual chemical change. Thus, the spectator ions remain unchanged on both sides of the equation. Recognizing these ions is important for simplifying reactions and understanding which species are actually reacting.

Aqueous Solution

An aqueous solution is a solution in which the solvent is water. This is a common state for many reactions, particularly in chemistry labs. When a compound dissolves in water, it may dissociate into ions if it is soluble. This is particularly relevant for ionic compounds and strong acids. For instance, in our exercise, hydrobromic acid (\(\text{HBr}\)) is dissolved in water, making an aqueous solution of hydrobromic acid. The term *(aq)* signifies that a substance is dissolved in water, as with \(2\text{HBr}(aq)\). Understanding solutions in this context is crucial for recognizing how substances behave and interact in water.

Dissociation in Water

Dissociation in water occurs when an ionic compound or acid dissolves in water, breaking into its constituent ions. This process is common in aqueous solutions and is essential for understanding reactions involving ionic compounds and acids. In our specific example, when hydrobromic acid (an acid) dissolves in water, it dissociates completely to form \(\text{H}^+\) and \(\text{Br}^-\) ions. Similarly, substances like magnesium bromide would dissociate into \(\text{Mg}^{2+}\) and \(\text{Br}^-\) ions when dissolved in water. Dissociation is fundamental to forming the ionic equations and is pivotal in predicting the outcome of reactions in solution. It is a key concept in physical chemistry and explains a wide range of reaction behaviors.