CHE130 L11

Important Reminders

  • Complete practice worksheet
  • Read sections 4.1 – 4.3
  • Homework (HW 5) due Monday, October 6

Chapter 4.1: Writing and Balancing Chemical Equations

Writing and Balancing Chemical Reactions

  • The previous chapters introduced element symbols to represent various atoms, molecules, and compounds.
  • A balanced chemical equation utilizes symbolism to represent:
    • The identities of the substances
    • The relative quantities of substances undergoing chemical (or physical) transformations.

Balancing Reactions

Example of a Reaction

  • Consider the example:
    • Reactants:
    • C₄H₄ + 4 O₂
    • Balanced Product:
    • 1 CH₄ + 2 O₂ → 1 CO₂ + 2 H₂O
  • Coefficients are key in depicting the amounts and identities of the substances.

Coefficients in Chemical Reactions

  • The previously described chemical equation is balanced.
  • To find the number of atoms for a specific element:
    • Multiply the coefficient of any formula containing that element by the element's subscript.

Principles for Balancing Reactions

  • An unbalanced reaction can be balanced visually (by inspection).
  • The coefficients can be adjusted as needed to achieve balance.
  • Subscripts define the identity of substances and cannot be altered without changing the meaning of the equation.

Practice Problem

  • Balance the following reaction:
    • KOH + H₃PO₄ → K₃PO₄ + H₂O
  • Completed Balanced Reaction:
    • 3 KOH + H₃PO₄ → K₃PO₄ + 3 H₂O

Using Fractions in Balancing

  • It can be helpful sometimes to introduce fractions as intermediate coefficients to achieve balance.
    • Example: C₂H₆ + O₂ → H₂O + CO₂

Additional Information from Chemical Equations

  • Physical states of reactants/products are indicated with symbols near formulas:
    • (g): gas
    • (l): liquid
    • (s): solid
    • (aq): aqueous solution
  • Example reaction:
    • 2Na(s) + 2H₂O(l) ⟶ 2NaOH(aq) + H₂(g)

Special Conditions in Reactions

  • Special conditions required for reactions may be indicated by words/symbols above or below the equation’s arrow.
    • Δ indicates heat is needed:
    • CaCO₃(s) ⟶ CaO(s) + CO₂(g)

Chemical Equations for Ionic Reactions

Overview

  • Many chemical reactions occur in aqueous environments.
  • When ions are involved, chemical equations can be expressed at different detail levels.
    • Molecular Equation:
    • CaCl₂(aq) + 2AgNO₃(aq) ⟶ Ca(NO₃)₂(aq) + 2AgCl(s)
    • Molecular equations do not explicitly show the ionic species in solution.

Dissociation in Water

  • Ionic compounds can dissociate into ions when they dissolve in water.
  • Example:
    • AgCl does not dissolve significantly in water.

Complete and Net Ionic Equations

  • Complete ionic equation representation includes all dissolved ions in the chemical reaction.
  • Example:
    • Starting with:
    • CaCl₂(aq) + 2AgNO₃(aq) ⟶
    • Complete ionic:
      • Ca²⁺(aq) + 2Cl⁻(aq) + 2Ag⁺(aq) + 2NO₃⁻(aq) ⟶ Ca²⁺(aq) + 2NO₃⁻(aq) + 2AgCl(s)
    • Net Ionic Equation:
      • Ag⁺(aq) + Cl⁻(aq) ⟶ AgCl(s)
  • Spectator Ions:
    • Ions that remain unchanged and do not participate in the reaction, e.g., Ca²⁺(aq) and NO₃⁻(aq) in the example.

Extra Practice

To write balanced molecular, complete ionic, and net ionic equations for the following:

  • NaCl(aq) + H₂O(l) → NaOH(aq) + H₂(g) + Cl₂(g)

Chapter 4.2: Classifying Chemical Reactions

Precipitation Reactions

  • A precipitation reaction occurs when dissolved substances react to form one or more solid products.
  • This type is also referred to as double displacement, double replacement, or metathesis.
  • Common in nature and industry; examples include coral reefs and animal kidney stones.

Solubility and Precipitation Reactions

  • Solubility: Maximum concentration of a substance that can exist under specific conditions, i.e., how much can dissolve in a solvent (like water).
  • Soluble substances dissolve well in water.
  • A substance precipitates when its concentration exceeds its solubility.
    • Insoluble substances precipitate readily from solution.

Solubility Rules

  • Memorization of solubility rules is essential:
    1. Nitrate, chlorate, perchlorate, and acetate salts are soluble (with exceptions).
    2. Salts with alkali metal cations and ammonium ions are soluble.
    3. Chloride, bromide, and iodide salts are generally soluble (with exceptions).
    4. Fluoride salts are soluble except with certain ions.
    5. Sulfate salts are soluble, with specific exceptions.
    6. Sulfide salts are typically insoluble, with exceptions.
    7. Carbonate, phosphate, and chromate salts are generally insoluble, with exceptions.
    8. Hydroxide and oxide salts tend to be insoluble, with exceptions.

Example of a Precipitation Reaction

  • Molecular Equation: KI(aq) + Pb(NO₃)₂(aq) ⟶
    • Detailed Ionic Reaction: [Full details needed]
    • Net Ionic Reaction: [Full details needed]

Acid-Base Reactions

Definition

  • An acid-base reaction occurs when an H⁺ ion is transferred between chemical species.
  • An acid creates hydronium ions (H₃O⁺) when dissolved in water.
    • Example: HCl
    • H⁺ transfer process depicted:
    • HCl(g) + H₂O(l) ⟶ H₃O⁺(aq) + Cl⁻(aq)

Strong Acids

  • Strong acids fully react in water.
    • Examples include:
    • HCl, HBr, HI, HNO₃, HClO₄, H₂SO₄
  • These acids completely dissociate in aqueous solutions.

Weak Acids

  • Many compounds act as weak acids, partially reacting with water.
  • Most molecules remain intact, leading to a lower number of hydronium ions produced.
    • Common Example: Acetic acid (CH₃COOH)
    • Others include HF, H₃PO₄, H₂CO₃, H₂SO₃, HClO, and HClO₂.

Examples of Weak Acids

  • Acetic Acid in Water: The following interactions take place, depicting equilibrium behavior.

Bases and Strong Bases

Definition

  • A base produces hydroxide ions (OH⁻) when dissolved in water.
  • Most common bases are ionic compounds of alkali or alkaline earth metal cations with hydroxide ions.
    • Examples include: NaOH, KOH, Ca(OH)₂, Ba(OH)₂
  • Strong bases fully dissociate when dissolved in water.

Weak Bases

  • Some bases generate hydroxide ions by partially reacting with water.
  • Example: Ammonia (NH₃)
    • This reaction involves transfer of H⁺ ions from water to ammonia.

Neutralization Reactions

Definition

  • A neutralization reaction is a form of acid-base reaction where the reactants comprise an acid and a base.
  • The typical products are a salt and water, neither being water itself.
    • Example: Reaction of HCl and NaOH results in NaCl and water.

Neutralization Reactions: Complete and Net Ionic

  • The presentation of neutralization reactions in complete and net ionic forms is necessary for clarity and understanding of participant species.