Chemical Reactions Study Notes

Unit 7: Chemical Reactions

Overview

  • Focuses on understanding chemical reactions, their classifications, and stoichiometry.

Key Themes

  • Physical vs. Chemical Processes   - Physical Changes: No change in identity of the substance (e.g., crumpled, evaporated, smashed, tied, sublimed, stretched, melted).   - Chemical Changes: New substance formed (e.g., combusts, decomposes, rusts, explodes).

Evidence of a Chemical Change

  • Color Changes: Appearance or disappearance of colors in reaction.
  • Temperature Changes: Measurement indicating endothermic or exothermic reactions.
  • Gas Production: Formation of gas bubbles during reactions.
  • Fire or Explosion: Indication of an energetic chemical change.
  • Precipitate Formation: A solid forms when two liquids are mixed (e.g., extAgClext{AgCl} from mixing extNaClext{NaCl} with extAgNO3ext{AgNO}_3).

Word and Chemical Equations

  • Example of a Word Equation: At room temperature, acetic acid reacts with sodium bicarbonate producing sodium acetate, carbon dioxide, and water.
  • Chemical Equation:   - ext{NaHCO}_3(s) + ext{HC}_2 ext{H}_3 ext{O}_2(aq) ightarrow ext{NaC}_2 ext{H}_3 ext{O}_2(aq) + ext{CO}_2(g) + ext{H}_2 ext{O}(l)

Coefficients and Symbols in Chemical Equations

  • Reactants and Products:   - Reactants are on the left; products are on the right.
  • Example: 2 ext{H}_2(g) + ext{O}_2(g) ightarrow 2 ext{H}_2 ext{O}(g)
Symbols Explanation
  • +: Separates reactants/products.
  • →: Reads as yields (reactants to products).
  • States of matter: (s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous solutions.
  • Δ: Indicates heat is supplied to the reaction.
  • Pt: A catalyst like platinum, noted above the yield sign.

Examples of Describing Chemical Reactions

  • Example Reaction: ext{Ca}(s) + ext{Br}_2(g) ightarrow ext{CaBr}_2(s)   - Description: Solid calcium is heated with bromine gas to produce solid calcium bromide.

Law of Conservation of Mass

  • Mass of reactants equals mass of products.   - Example: If 100 g of coal is burned, the final mass remains the same.
  • In a chemical reaction, matter is transformed, not created or destroyed.

Balancing Chemical Equations

Steps to Balance
  1. Write formulas of substances involved.
  2. Count atoms of each element on both sides.
  3. Adjust coefficients as needed for balance.
  4. Verify atom counts are equal on both sides.
Practice Balancing Chemical Equations
  1. Sample Problem: ext{P}_4 + ext{O}_2 ightarrow ext{P}_2 ext{O}_5
  2. Sample Problem: ext{Al} + ext{Fe}_2 ext{O}_3 ightarrow ext{Fe} + ext{Al}_2 ext{O}_3
  3. Sample Problem: ext{C}_3 ext{H}_8 + ext{O}_2 ightarrow ext{CO}_2 + ext{H}_2 ext{O}

Types of Chemical Reactions

  1. Combination/Synthesis Reaction: A + B → AB
  2. Decomposition Reaction: AB → A + B
  3. Single Replacement Reaction: A + BC → AC + B
  4. Double Replacement Reaction: AB + CD → AD + CB
  5. Combustion Reaction: Hydrocarbon + O₂ → CO₂ + H₂O

Solutions and Solubility

  • Solutions: A mixture of solute dissolved in solvent.
  • Solubility: Soluble materials dissolve (e.g., coffee in water); insoluble materials do not (e.g., sand in water).
  • Familiarize with solubility rules for predicting precipitate formation.

Net Ionic Equations

  • Represent only those species that participate in the reaction.
  • Example: For the reaction extPb(NO3)2(aq)+2extNaI(aq)→extPbI2(s)+2extNaNO3(aq)ext{Pb(NO}_3)_2(aq) + 2 ext{NaI}(aq) → ext{PbI}_2(s) + 2 ext{NaNO}_3(aq),   - Complete Ionic Equation: extPb2+(aq)+2extNO3−(aq)+2extNa+(aq)+2extI−(aq)→extPbI2(s)+2extNa+(aq)+2extNO3−(aq)ext{Pb}^{2+}(aq) + 2 ext{NO}_3^{-}(aq) + 2 ext{Na}^{+}(aq) + 2 ext{I}^{-}(aq) → ext{PbI}_2(s) + 2 ext{Na}^{+}(aq) + 2 ext{NO}_3^{-}(aq)   - Net Ionic Equation: extPb2+(aq)+2extI−(aq)→extPbI2(s)ext{Pb}^{2+}(aq) + 2 ext{I}^{-}(aq) → ext{PbI}_2(s)

Stoichiometry

  • Definition: Mathematics of balancing chemical equations and predicting quantities of products/reactants.
  • Requires a balanced equation to calculate mole ratios and yields.
Theoretical vs. Actual Yield
  • Theoretical Yield: Maximum possible product calculated from balanced equation.
  • Actual Yield: Amount obtained during the reaction.
  • Percent Yield Equation: extPercentYield=extActualYield(g)extTheoreticalYield(g)imes100ext{Percent Yield} = \frac{ ext{Actual Yield (g)}}{ ext{Theoretical Yield (g)}} imes 100
Example Calculations
  1. For the combustion reaction of methanol: 2 ext{CH}_3 ext{OH} + 3 ext{O}_2 ightarrow 2 ext{CO}_2 + 4 ext{H}_2 ext{O}
  • Given data: ΔH = -726 kJ
  • If 1.34 mol of extH2extOext{H}_2 ext{O} formed, relate to oxygen production in calculations.

Enthalpy (ΔH)

  • Heat transfer during a reaction.
  • Enthalpy change: extΔH=extH<em>extproducts−extH</em>extreactantsext{ΔH} = ext{H}<em>{ ext{products}} - ext{H}</em>{ ext{reactants}}
  • Endothermic Reaction: Heat absorbed (ΔH > 0).
  • Exothermic Reaction: Heat released (ΔH < 0).
Hess's Law
  • Total enthalpy change is the same regardless of route taken.
  • Total ΔH for multi-step reactions equals the sum of steps.
Application of Hess's Law
  • Example of using Hess's law to determine ΔH from given steps in reactions.

Unit 7 Test Overview

  • Topics included:   - Understanding chemical reactions and equations.   - Law of conservation of mass.   - Balancing equations.   - Exploring different types of reactions (synthesis, decomposition, combustion, single & double replacement).   - Net ionic equations and solubility.   - Stoichiometry (mole ratio).   - Limiting & excess reactants.   - Theoretical yield vs. actual yield.   - Enthalpy concepts and Hess’s law.