Problem 39 - Chemistry The Central Science In Si Units

Question

Complete and balance the following molecular equations, and then write the net ionic equation for each: (a) \(\mathrm{HBr}(a q)+\mathrm{Ca}(\mathrm{OH})_{2}(a q) \longrightarrow\) (b) \(\mathrm{Cu}(\mathrm{OH})_{2}(s)+\mathrm{HClO}_{4}(a q) \longrightarrow\) (c) \(\mathrm{Al}(\mathrm{OH})_{3}(s)+\mathrm{HNO}_{3}(a q) \longrightarrow\)

Step-by-Step Solution

Answer

(a) Net ionic: \( 2 \text{H}^+(aq) + \text{2 OH}^-(aq) \rightarrow 2 \text{H}_2O(l) \); (b) Net ionic: \( \text{Cu(OH)}_2(s) + 2 \text{H}^+(aq) \rightarrow \text{Cu}^{2+}(aq) + 2 \text{H}_2O(l) \); (c) Net ionic: \( \text{Al(OH)}_3(s) + 3 \text{H}^+(aq) \rightarrow \text{Al}^{3+}(aq) + 3 \text{H}_2O(l) \).

Step 1: Write Complete Molecular Equation for (a)

Identify and combine the reactants \( \text{HBr}(aq) \) and \( \text{Ca(OH)}_2(aq) \) to form products. The expected products will be \( \text{CaBr}_2(aq) \) and \( \text{H}_2O(l) \). Thus, the complete molecular equation is: \[ \text{2 HBr}(aq) + \text{Ca(OH)}_2(aq) \rightarrow \text{CaBr}_2(aq) + 2 \text{H}_2O(l) \]

Step 2: Write Net Ionic Equation for (a)

Identify and remove spectator ions. In this reaction, \( \text{Br}^- \) and \( \text{Ca}^{2+} \) are spectator ions. The net ionic equation involves the following: \[ 2 \text{H}^+(aq) + \text{2 OH}^-(aq) \rightarrow 2 \text{H}_2O(l) \]

Step 3: Write Complete Molecular Equation for (b)

Combine the reactants \( \text{Cu(OH)}_2(s) \) and \( \text{HClO}_4(aq) \). The expected products are \( \text{Cu(ClO}_4)_2(aq) \) and \( \text{H}_2O(l) \): \[ \text{Cu(OH)}_2(s) + 2 \text{HClO}_4(aq) \rightarrow \text{Cu(ClO}_4)_2(aq) + 2 \text{H}_2O(l) \]

Step 4: Write Net Ionic Equation for (b)

Since \( \text{Cu(OH)}_2(s) \) does not dissociate, the net ionic equation for this reaction is: \[ \text{Cu(OH)}_2(s) + 2 \text{H}^+(aq) \rightarrow \text{Cu}^{2+}(aq) + 2 \text{H}_2O(l) \]

Step 5: Write Complete Molecular Equation for (c)

For the reaction \( \text{Al(OH)}_3(s) \) with \( \text{HNO}_3(aq) \), the products formed are \( \text{Al(NO}_3)_3(aq) \) and \( \text{H}_2O(l) \). The balanced molecular equation is: \[ \text{Al(OH)}_3(s) + 3 \text{HNO}_3(aq) \rightarrow \text{Al(NO}_3)_3(aq) + 3 \text{H}_2O(l) \]

Step 6: Write Net Ionic Equation for (c)

With \( \text{Al(OH)}_3(s) \) not dissociating in solution, the net ionic equation is: \[ \text{Al(OH)}_3(s) + 3 \text{H}^+(aq) \rightarrow \text{Al}^{3+}(aq) + 3 \text{H}_2O(l) \]

Key Concepts

Balancing Chemical Equations

Balancing chemical equations is a crucial skill to ensure that the number of atoms for each element is the same on both sides of the equation. This process respects the Law of Conservation of Mass, which states that mass cannot be created or destroyed in a chemical reaction. When balancing equations:

• First, write the correct formulas for all reactants and products.
• Count the number of atoms of each element present in the reactants and products.
• Adjust coefficients—the numbers in front of the formulas—to get the same number of each type of atom on both sides.
• Start with the most complex substance, adjusting coefficients, not subscripts.
• Finally, check to ensure that all atoms are balanced and that coefficients are in the smallest possible ratio.

For example, in the equation between hydrogen bromide and calcium hydroxide, the balanced equation ensures that both hydrogen and oxygen are accounted for on both sides ensuring no atoms are lost in the process.

Spectator Ions

Spectator ions are ions that appear in the same form on both the reactant and product sides of a chemical equation. They do not participate in the actual chemical change but are present to balance the charge in solution. Identifying and removing spectator ions is an essential step to derive the net ionic equation. These ions are often the ones that remain dissolved and unchanged as the reaction takes place, commonly seen in aqueous solutions. For instance, in a typical reaction with acids and bases:

• The ions that make up soluble products remain as spectator ions.
• They help in maintaining electrical neutrality in the solution.
• When forming net ionic equations, spectators are left out to highlight the actual chemical change.

Recognizing spectator ions helps simplify complex reactions and focus on the core changes happening chemically.

Chemical Reactions

Chemical reactions involve the process where reactants transform into products, showcasing new substances with different properties. There are various types of reactions, including synthesis, decomposition, single-replacement, double-replacement, and combustion. Key points about chemical reactions:

• They involve breaking existing bonds and forming new ones to create new materials.
• Conservation of energy applies; the energy required to break bonds is offset by the energy released when new bonds form.
• Indicators such as temperature change, gas formation, and color change can suggest a chemical reaction has occurred.

When writing net ionic equations for chemical reactions, it’s essential to focus on the actual chemical changes. For example, in the reaction between aluminum hydroxide and nitric acid, the transformation involves creating soluble aluminum ions and water, illustrating the reaction’s core event. This understanding lays the foundation for predicting products and understanding overall reaction mechanics.