Chapter 4.1: Chemical Reactions

Chapter 4.1: Chemical Reactions

Homework

• Problems from page 155: #1-5
• Worksheet

Introduction to Chemical Reactions

• Chemical Change: A transformation of one or more substances into different substances, characterized by distinct properties.
• Chemical Reactions:
  • Involve a change in energy, which can manifest as absorption or release of heat, light, or sound.
  • Occur at varying rates; some reactions are fast, while others are slow.
  • A catalyst can be employed to speed up reactions.

Indicators of a Chemical Change

• Clues to a chemical change include:
  • Change in color.
  • Absorption or release of energy (heat, light, sound).
  • Production of gas.
  • Formation of a precipitate (a solid) from two liquids.
  • Difficulty in reversing the change.
• Important Note: A change in state (e.g., liquid to gas) or the dissolving of a substance is NOT classified as a chemical change.

Chemical Equations

• Chemical Equations: Represent chemical reactions using words or symbols and molecular formulas.
  • Reactants: Starting materials in a reaction.
  • Products: New substances formed in a reaction.
  • Arrow (→): Indicates the direction of the reaction and is read as "produces".
  • States of Matter: Indicated in brackets after chemical formulas:
  • (s) = solid
  • (l) = liquid
  • (g) = gas
  • (aq) = aqueous (dissolved in water)

Word Equations

• Word Equations: Utilize words to represent chemical reactions, following naming rules for substances.
  • Examples:
  • Magnesium reacts with hydrochloric acid:
    $ext{Mg (s) + HCl (aq) → MgCl}<em>2 ext{ (aq) + H}</em>2 ext{(g)}$
  • Iron(II) sulfate reacts with sodium chloride:
    $ext{FeSO}<em>4 ext{(aq) + NaCl → FeCl}</em>2 ext{ (s) + NaSO}_4 ext{(aq)}$

Common Rules for States of Matter

• Common Rules for identifying states of matter in reactions:
  • Ionic Compounds generally exist as (aq) in solutions.
  • Metals typically exist as (s) (solids).
  • Non-metal diatomic molecules exist as (g) (gases).
  • Molecular compounds often exist as (g) (gases).

Skeleton Equations

• Skeleton Equation: An unbalanced equation that shows all reactants and products of a chemical reaction using formulas.
  • How to write skeleton equations:
  • Write the symbol of any metal element that is not already part of a compound, e.g., Fe for iron (e.g., "Iron Nail").
  • For non-metals that belong to HOFBrINCl group, represent them as diatomic molecules (H2, N2, O2, F2, Cl2, Br2, I_2).
  • Example: Reaction of oxygen and fluorine gas:
    $ext{O}<em>2 + ext{F}</em>2$
  • Write the formulas of all other compounds following established rules.

Law of Conservation of Mass

• Law of Conservation of Mass: States that in a chemical reaction, the mass of the products is always equal to the mass of the reactants.
  • Key Principle: Since elements cannot be created or destroyed during a reaction, they can only be rearranged.

Formula Equations

• Formula Equations: Represent chemical reactions using formulas, adhering to formula rules.
  • States of matter are included in the equations.
  • Coefficients indicate the ratios of different substances in the reaction.
  • Example: Calcium reacts with hydrochloric acid to produce aqueous calcium chloride and hydrogen gas:
    $ext{Ca (s) + HCl (aq) → CaCl}<em>2 ext{ (aq) + H}</em>2 ext{(g)}$
• States of matter:
  • Solid (s)
  • Liquid (l)
  • Gas (g)
  • Aqueous solution (aq): means dissolved in water.

Balancing Chemical Equations

• Common Mistakes: Identify errors in equation presentation, such as incorrect stoichiometry.
  • Example of a mistake:
    $ext{Fe + O}<em>2 \ne ext{Fe}</em>2 ext{O}_3$

Balancing Steps for Equations

1. Balance polyatomic ions first if the same ion appears on both sides of the equation.
2. Balance all other elements except H and O.
3. Balance hydrogen (H) next.
4. Balance oxygen (O) last.

Examples of Balancing Chemical Equations

1. Balance the equation:
   $ext{Fe (s) + H}<em>2 ext{SO}</em>4 ext{(aq) → Fe}<em>2 ext{(SO}</em>4)<em>3 ext{(aq) + H}</em>2 ext{(g)}$
2. Balance the equation:
   $ext{C}<em>2 ext{H}</em>6 ext{(g) + O}<em>2 ext{(g) → H}</em>2 ext{O (l) + CO}_2 ext{(g)}$
3. Balance the equation:
   $ext{KOH (aq) + H}<em>3 ext{PO}</em>4 ext{(aq) → K}<em>3 ext{PO}</em>4 ext{(aq) + H}_2 ext{O (l)}$
4. Balance the equation:
   $ext{SnO}<em>2 ext{(aq) + H}</em>2 ext{(g) → Sn (s) + H}_2 ext{O (l)}$

Additional Homework

• Problems from page 155: #1-5
• Complete worksheet accordingly.