Chemical Reactions and Solution Stoichiometry Zumdahl’s Chapter 4 Chemical Reactions and Solution Stoichiometry

Chapter Contents Water Aqueous Solutions Solution Composition Electrolytes Strong and Weak Nonelectrolytes Solution Composition Chemical Reactions Precipitations Describing Reactions Stoichiometry Acid-Base Reactions Titration Oxidation-Reduction Oxidation States Balancing Redox Half Reaction Method

Water, H2O, Universal Solvent Polar (covalent) Molecule ( = 1.4) Hydration The tendency for salt to “fall apart” when strong positive and negative charges are replaced by water-ion interactions Hydration spheres cradle ions Hydrogen Bonding ( –H•••O– ) O lone pair binds neighbor H (at ~ 15%) “Like dissolves like.”

Electrolytes Ions conduct electricity in proportion to their number (& mobility) Strong Electrolytes are fully ionized. NaCl, HNO3, Ca(OH)2, MgSO4, soap, etc. Weak Electrolytes are mostly molecular. Tap water, CH3CO2H, (NH4)OH, etc. Nonelectrolytes do not ionize. Pure water, alcohols, sugars, etc.

Solution Composition Concentration as mol L–1 or Molarity I.e., moles solute per Liter of final solution Alt., molality, moles solute per 1 kg solvent. Dilution conserves number of moles C1 V1 = C2 V2 solves dilution problems. Molarity ideal for dispensing solutions. Controlled volume = controlled moles solute Convert to moles by CV, then apply rxn. stoich.

Types of Chemical Reactions Categorized by motivational factors! Le Châtlier: “Rxn. favors missing components.” Gas Evolution (gas leaves the solution ) Precipitation (solid leaves the solution ) Weak electrolyte (ions leave the solution) E.g., acid+base makes water! Redox (electrons find happiness)

MEMORIZE THE SOLUBILITY TABLE N choice.

Description of Solution Reactions Molecular Equations: HCl + KOH  KCl + H2O Complete Ionic Equations: H+ + Cl– + K+ + OH–  K+ + Cl– + H2O Net Ionic Equation: H+(aq) + OH–(aq)  H2O(l)

Precipitation Stoichiometry Write balanced net ionic reaction. Determine limiting reactant. Use Concentration  Volume to get moles. Calculate product moles. If required, calculate leftover reactants. Use moles divided by Final Volume to get concentration of leftovers.

Acid – Base Titrations If at least one is “strong,” neutralization will be complete because H2O is very “weak!” Choose indicator for strong visual signal at completion. For titrant, CV dispensed gives moles. Stoichiometry determines moles sample Sample moles / sample vol = original M

Oxidation – Reduction Reactions: REDOX Oxidation: loss of electrons (e.g., metals) Reduction: gain of electrons (e.g., F2) BOTH MUST OCCUR (because electrons conserved) Oxidizing Agent gets Reduced (and converse) Oxidation States (imagine everything ionic) Add up to charge on species Always zero for neutral elements

Solution Redox Stoichiometry Determine redox agents Use Half Reaction Methods Balance red- and ox- separately with e– Balance excess O with H2O Balance excess H with H+ Scale each for equal number of e– transfer Add & cancel (esp. e–) equally left & right “Titrate” equation algebraically if OH–