Problem 25 - Chemistry: Atoms First

Question

Complete and balance the equations for the following acid-base neutralization reactions. If water is used as a solvent, write the reactants and products as aqueous ions. In some cases, there may be more than one correct answer, depending on the amounts of reactants used. (a) \(\mathrm{Mg}(\mathrm{OH})_{2}(s)+\mathrm{HClO}_{4}(a q) \rightarrow\) (b) \(\mathrm{SO}_{3}(g)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow\) (assume an excess of water and that the product dissolves) (c) \(\operatorname{SrO}(s)+\mathrm{H}_{2} \mathrm{SO}_{4}(l) \rightarrow\)

Step-by-Step Solution

Answer

Balanced reactions: (a) \( \text{Mg}(\text{OH})_2(s) + 2\text{HClO}_4(aq) \rightarrow \text{Mg}(\text{ClO}_4)_2(aq) + 2\text{H}_2\text{O}(l) \), (b) \( \text{SO}_3(g) + \text{H}_2\text{O}(l) \rightarrow \text{H}_2\text{SO}_4(aq) \), (c) \( \text{SrO}(s) + \text{H}_2\text{SO}_4(l) \rightarrow \text{SrSO}_4(s) + \text{H}_2\text{O}(l) \).

Step 1: Balance the magnesium hydroxide and perchloric acid reaction

Magnesium hydroxide is a base and perchloric acid is a strong acid. When they react, the products are magnesium perchlorate and water. To balance this reaction, we need to ensure that the number of atoms of each element is the same on both sides of the equation. The balanced reaction in ionic form is: \( \text{Mg}^{2+}(aq) + 2\text{OH}^- (aq) + 2\text{H}^+ (aq) + 2\text{ClO}_4^- (aq) \rightarrow \text{Mg}^{2+}(aq) + 2\text{ClO}_4^- (aq) + 2\text{H}_2\text{O}(l) \).

Step 2: Balance the reaction of sulfur trioxide with water

Sulfur trioxide reacts with water to form sulfuric acid. When there is an excess of water, the water is often not shown in the ionic equation since it acts as the solvent. The balanced reaction for the neutralization is: \( \text{SO}_3(g) + \text{H}_2\text{O}(l) \rightarrow \text{H}_2\text{SO}_4(aq) \).

Step 3: Balance the strontium oxide and sulfuric acid reaction

Strontium oxide is a basic oxide, and it reacts with sulfuric acid to produce strontium sulfate and water. To establish the balanced equation, make sure the number of atoms for each element is the same on both sides: \( \text{SrO}(s) + \text{H}_2\text{SO}_4(l) \rightarrow \text{Sr}^{2+}(aq) + \text{SO}_4^{2-}(aq) + \text{H}_2\text{O}(l) \).

Key Concepts

Chemical Equation Balancing

Balancing chemical equations is a fundamental skill in chemistry that ensures the law of conservation of mass is respected. This law states that matter cannot be created or destroyed in a chemical reaction. Therefore, each element must have the same number of atoms on both the reactant and product sides of the equation.

For example, when balancing the reaction between magnesium hydroxide and perchloric acid, we write down the number of atoms for each element before and after the reaction. The unbalanced equation is: \( \mathrm{Mg}(\mathrm{OH})_{2}(s) + \mathrm{HClO}_{4}(aq) \rightarrow \). To balance it, we observe that two moles of hydroxide ions (\(\text{OH}^-\)) from magnesium hydroxide can neutralize two moles of hydrogen ions (\(\text{H}^+\)) from perchloric acid to form two moles of water (\(\text{H}_2\text{O}\)), resulting in a balanced equation with equal numbers of each type of atom on both sides.

Working systematically by balancing one element at a time and using coefficients to multiply the number of atoms or molecules as needed helps achieve a balanced equation. The skill of effectively balancing equations is crucial in predicting the amounts of reactants and products involved and is foundational for understanding complex chemical reactions.

Aqueous Ions in Reactions

When dealing with reactions in an aqueous solution, it's important to understand the role of ions. Many substances dissociate into ions when dissolved in water, leading to ionic equations that more accurately represent what's happening in solution.

In a neutralization reaction, the acid and base usually produce water and a salt, which in an aqueous solution means ions are formed. The balanced molecular equation might look simple, but the ionic equation shows the actual species present in the solution. For instance, the strontium oxide and sulfuric acid reaction can be written as \( \text{SrO}(s) + \text{H}_2\text{SO}_4(l) \rightarrow \text{Sr}^{2+}(aq) + \text{SO}_4^{2-}(aq) + \text{H}_2\text{O}(l) \), highlighting the ions in the product side.

Understanding ionic equations is crucial for predicting precipitation reactions, acid-base neutralizations, and redox reactions. Recognizing which compounds dissolve to form ions and which remain intact is necessary for writing correct ionic equations. This knowledge also aids in predicting reaction outcomes such as the formation of precipitates or gas evolution.

Neutralization Reactions

Neutralization reactions are a type of chemical reaction where an acid and a base react to form water and a salt. This reaction is incredibly important in various fields, from industrial processes to biological systems.

The general form of a neutralization reaction is \( \text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water} \). In our textbook example, the sulfur trioxide and water reaction is a neutralization process where sulfuric acid is the salt formed: \( \text{SO}_3(g) + \text{H}_2\text{O}(l) \rightarrow \text{H}_2\text{SO}_4(aq) \). It's important to note that even though we typically think of neutralization as involving strong acids and bases, weak acids and weak bases can also participate in these reactions.

The strength of an acid or base in water is related to the degree to which it dissociates into ions. Strong acids and bases dissociate completely, making their reactions go to completion. Weak acids and bases do not fully dissociate, making their neutralization reactions more complex to balance and predict the outcome.