Chemical Reactions and Aqueous Solutions

Chemical Reactions

Investigation 6: Chemical Reactions

This investigation covers three experiences:

• Experience 1: Modeling Chemical Reactions
• Experience 2: Predicting Outcomes of Chemical Reactions
• Experience 3: Reactions in Aqueous Solution

Learning Objectives

• Identify the five general types of chemical reactions and describe reactants and products.
• Predict reaction outcomes based on components and justify why reactants react and products form.
• Investigate and explain precipitate formation using solubility and intermolecular forces.
• Use evidence to claim whether a reaction will form a precipitate.

Types of Chemical Reactions

1. Synthesis: Multiple reactants (elements or compounds) combine to form a single compound.

   • Example: $N<em>2 + 3H</em>2 \rightarrow 2NH_3$

2. Decomposition: A compound breaks down into two or more simpler substances. Classified as thermal, electrolytic, or photo.

3. Single-Replacement: One element is substituted for another in a compound, generating a new compound and a pure element.

   • Example: $Zn + 2AgCl \rightarrow 2Ag + ZnCl_2$

4. Double-Replacement: Two ionic compounds exchange cations or anions to form two new compounds. Forming a precipitate can drive the reaction to the right.

   • Example: $CaBr<em>2 + 2NaOH \rightarrow Ca(OH)</em>2 + 2NaBr$

5. Combustion: A fuel (typically a hydrocarbon) reacts with oxygen gas to form carbon dioxide and water, generating heat and light.

   • Example: $CH<em>4 + 2O</em>2 \rightarrow CO<em>2 + 2H</em>2O$

Inquiry Question

• An alkaline fuel cell uses aqueous potassium hydroxide (KOH) as an electrolyte. A new colleague produces a batch of electrolytes, but the fuel cells malfunction due to calcium hydroxide buildup on the electrodes. What might have happened?

Warm-up Questions

1. What type of reaction is represented by $Al + O_2 \rightarrow ?$

   • Answer: Synthesis

2. Create a balanced chemical equation for the complete combustion of $C<em>3H</em>4$ and predict the products.

   • Answer: $C<em>3H</em>4 + 4O<em>2 \rightarrow 3CO</em>2 + 2H<em>2O$. The expected products are carbon dioxide ($CO</em>2$) and water ($H_2O$).

Reactions in Aqueous Solution

• Predict the role of solubility in determining the products of a double replacement reaction.
• Explain how to write a complete ionic equation.
• Define spectator ions.
• Define net ionic equation.

Example Reaction

$AgNO<em>3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO</em>3(aq)$

• Full ionic equation:
  $Ag^+(aq) + NO<em>3^−(aq) + Na^+(aq) + Cl^−(aq) \rightarrow AgCl(s) + Na^+(aq) + NO</em>3^−(aq)$
• Net ionic equation:
  $Ag^+(aq) + Cl^−(aq) \rightarrow AgCl(s)$
• Ions in the aqueous solution: $Ag^+, NO_3^−, Na^+, Cl^−$
• Precipitate: $AgCl(s)$

Key Definitions

• Complete Ionic Equation: Shows dissolved ionic compounds as dissociated free ions.
  • Example: $Ag^+(aq) + NO<em>3^-(aq) + Na^+(aq) + Cl^-(aq) \rightarrow AgCl(s) + Na^+(aq) + NO</em>3^-(aq)$
• Spectator Ion: An ion not directly involved in a chemical reaction; it does not change oxidation number or composition.
• Net Ionic Equation: Shows only the particles directly involved in the chemical change.
  • Example: $Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)$

Steps for Writing Ionic Equations

1. Write the balanced chemical equation.
2. Break down everything into its ions EXCEPT solids, gases, and water (complete ionic equation).
3. Cross out everything that is the same on both sides (spectator ions).
4. Write what is left (net ionic equation).

Solubility Guidelines

Soluble Ionic Compounds

1. All sodium, potassium, and ammonium salts are soluble.
2. All nitrate, acetate, chlorate, and perchlorate salts are soluble.
3. All chloride, bromide, and iodide salts are soluble EXCEPT those containing lead, silver, or mercury(I) ($Hg_2^{2+}$).
4. All fluoride salts are soluble EXCEPT those containing magnesium, calcium, strontium, barium, or lead.
5. All sulfate salts are soluble EXCEPT those containing calcium, silver, mercury(I), strontium, barium, or lead.

Not Soluble Ionic Compounds

1. All hydroxide and oxide salts are not soluble EXCEPT those containing sodium, potassium, ammonium, or barium.
2. All sulfide salts are not soluble EXCEPT those containing sodium, potassium, or ammonium.
3. All carbonate, phosphate, chromate, and oxide salts are not soluble EXCEPT those containing sodium, potassium, or ammonium.

Solubility Rules for Ionic Compounds

Conclusion: Precipitation Reactions

• Precipitate: A solid formed from a chemical reaction in aqueous solution.
• Precipitation Reaction: A reaction in which a solid forms from a solution.
• Solid = precipitate = insoluble
• Example: In the reaction $Pb(NO<em>3)</em>2(aq) + 2NaI(aq) \rightarrow PbI<em>2(s) + 2NaNO</em>3(aq)$, the precipitate is $PbI_2(s)$.

Example: Predicting Precipitate Formation

Will a reaction between solutions of $NaCl$ and $AgNO<em>3$ form a precipitate? $NaCl(aq) + AgNO</em>3(aq) \rightarrow ?$

The reactants separate into ions. Using solubility rules, $NaNO<em>3$ is soluble, but $AgCl$ is insoluble. Therefore, a chemical reaction occurs: $NaCl(aq) + AgNO</em>3(aq) \rightarrow NaNO_3(aq) + AgCl(s)$

Sample Problem: Net Ionic Equation

Write the net ionic equation for the reaction:
$Na<em>3PO</em>4(aq) + FeCl<em>3(aq) \rightarrow NaCl(aq) + FePO</em>4(s)$

1. Complete ionic equation:
   $3Na^+ + PO<em>4^{3-} + Fe^{3+} + 3Cl^- \rightarrow 3Na^+ + 3Cl^- + FePO</em>4$
2. Net ionic equation:
   $PO<em>4^{3-} + Fe^{3+} \rightarrow FePO</em>4$

Group Work Example

Write the net ionic equation for the reaction:
$Pb(NO<em>3)</em>2(aq) + 2KI(aq) \rightarrow PbI<em>2(s) + 2KNO</em>3(aq)$

1. Complete ionic equation:
   $Pb^{2+} + 2NO<em>3^- + 2K^+ + 2I^- \rightarrow PbI</em>2(s) + 2K^+ + 2NO_3^-$
2. Net ionic equation:
   $Pb^{2+} + 2I^- \rightarrow PbI_2(s)$

Individual Work Example

Write the net ionic equation for the reaction:
$CaCl<em>2(aq) + Na</em>2CO<em>3(aq) \rightarrow CaCO</em>3(s) + 2NaCl(aq)$

1. Complete ionic equation:
   $Ca^{2+} + 2Cl^- + 2Na^+ + CO<em>3^{2-} \rightarrow CaCO</em>3(s) + 2Na^+ + 2Cl^-$
2. Net Ionic Equation:
   $Ca^{2+} + CO<em>3^{2-} \rightarrow CaCO</em>3(s)$