Problem 21 - Chemistry: Atoms First

Question

Complete and balance the following acid-base equations: (a) HCl gas reacts with solid Ca(OH) \(_{2}(s)\) (b) A solution of \(\operatorname{Sr}(\mathrm{OH})_{2}\) is added to a solution of \(\mathrm{HNO}_{3}\)

Step-by-Step Solution

Answer

The balanced equations are: (a) \(2\text{HCl}(g) + \text{Ca(OH)}_2(s) \rightarrow \text{CaCl}_2 + 2\text{H}_2\text{O}(l)\) and (b) \(\text{Sr(OH)}_2(aq) + 2\text{HNO}_3(aq) \rightarrow \text{Sr(NO}_3)_2(aq) + 2\text{H}_2\text{O}(l)\).

Step 1: Analyze the Reaction for HCl and Ca(OH)2

Since HCl is an acid and Ca(OH)2 is a base, this is an acid-base reaction. Write the reactants and realize that the products will be a salt and water.

Step 2: Write the Unbalanced Reaction for HCl and Ca(OH)2

Write the chemical equation with the reactants on the left side and the products (which are CaCl2 and water) on the right side: \( \text{HCl}(g) + \text{Ca(OH)}_2(s) \rightarrow \text{CaCl}_2 + \text{H}_2\text{O}(l) \).

Step 3: Balance the Equation for HCl and Ca(OH)2

Balance the number of atoms of each element on both sides of the equation. For this reaction, we need 2 moles of HCl to react with 1 mole of Ca(OH)2 to produce 1 mole of CaCl2 and 2 moles of water: \( 2\text{HCl}(g) + \text{Ca(OH)}_2(s) \rightarrow \text{CaCl}_2 + 2\text{H}_2\text{O}(l) \).

Step 4: Analyze the Reaction for Sr(OH)2 and HNO3

Recognize that this is also an acid-base reaction. Strontium hydroxide is a base and nitric acid is an acid; they react to form a salt (strontium nitrate) and water.

Step 5: Write the Unbalanced Reaction for Sr(OH)2 and HNO3

Write out the reaction with Sr(OH)2 and HNO3 as reactants, and Sr(NO3)2 and water as products: \( \text{Sr(OH)}_2(aq) + \text{HNO}_3(aq) \rightarrow \text{Sr(NO}_3)_2(aq) + \text{H}_2\text{O}(l) \).

Step 6: Balance the Equation for Sr(OH)2 and HNO3

To balance this equation, you'll need two moles of nitric acid for every mole of strontium hydroxide. The balanced reaction will be: \( \text{Sr(OH)}_2(aq) + 2\text{HNO}_3(aq) \rightarrow \text{Sr(NO}_3)_2(aq) + 2\text{H}_2\text{O}(l) \).

Key Concepts

Chemical Equations

Understanding chemical equations is crucial for anyone studying chemistry. A chemical equation represents a chemical reaction where the reactants are shown on the left and the products on the right. The arrow in between signifies the direction of the reaction. For example, when HCl reacts with Ca(OH)₂, the equation is initially written as HCl + Ca(OH)₂ → CaCl₂ + H₂O.

This equation tells us that hydrochloric acid reacts with calcium hydroxide to produce calcium chloride and water. However, it's vital to balance the equation so that the number of atoms for each element is the same on both sides, respecting the Law of Conservation of Mass. This ensures that the chemical equation is accurate and portrays the stoichiometry of the reaction. A balanced equation for this reaction is 2HCl + Ca(OH)₂ → CaCl₂ + 2H₂O, indicating that two moles of HCl are required for each mole of Ca(OH)₂.

Reaction Stoichiometry

Reaction stoichiometry is all about the quantitative relationship between reactants and products in a chemical reaction. It stems from the coefficients found in a balanced chemical equation, which indicate the ratio of molecules or moles of each substance involved. For instance, in the reaction between HCl and Ca(OH)₂, the stoichiometric coefficients are '2' for HCl and '1' for Ca(OH)₂, meaning 2 moles of HCl react with every 1 mole of Ca(OH)₂.

Stoichiometry is not only vital for understanding the proportions of chemicals required in a reaction but also for calculating how much product will form or how much of a reactant is necessary to produce a given amount of product. Therefore, mastering reaction stoichiometry is fundamental for anyone involved in practical chemistry, allowing accurate prediction and measurement of chemical consumption and production.

Neutralization Reactions

Neutralization reactions are a specific type of chemical reaction where an acid and a base react to form water and a salt. This reaction is extremely important in various fields, including environmental science, medicine, and industrial chemistry. In the provided example of Sr(OH)₂ reacting with HNO₃, strontium hydroxide (the base) neutralizes nitric acid (the acid) to form strontium nitrate (the salt) and water.

The balanced chemical equation Sr(OH)₂ + 2HNO₃ → Sr(NO₃)₂ + 2H₂O reflects the stoichiometry of the neutralization reaction. Each neutralization reaction must be balanced to determine the precise amounts of reactants needed. This concept can even extend to real-world applications like titrations, which are used to determine the concentration of an unknown acid or base solution. Grasping the principles behind neutralization reactions is key to many processes where pH control is necessary.