REACTIONS IN AQUEOUS SOLUTION

1. REACTIONS IN AQUEOUS SOLUTION Aqueous Solutions and Electrolytes Net Ionic Equations Reactions in Solutions (Precipitation, Acid-Base, Oxidation-Reduction (Redox))

2. AQUEOUS SOLUTIONS • Water is the solvent, other species (present in small amounts) are the solutes.. • Water is polar (there is a charge separation between the O and H atoms) and has a very high capacity to dissolve many compounds. • When ionic compounds dissolve in water, an aqueous solution of cations and anions (hydration) is created. Ions dissolved in water are designated by (aq).

3. AQUEOUS SOLUTIONS (2) • Cmps that form ions in water conduct electricity; these cmps are electrolytes. • The extent of ionization varies from one cmp to another leading to strong (>70% ionization) and weak (<5%) electrolytes • The extent of ionization (% ionization) determines how much current can be conducted.

4. AQUEOUS SOLUTIONS (3) • Water can also dissolve nonionic cmps, especially those that are polar. (HCl) • Finally, many nonpolar molecules do not dissolve in water; these are non-electrolytes (sugar, pure water). • Table 4.1

5. CHEMICAL RXNS IN AQUEOUS SOLNS • Chem. Rxns are driven by energetic forces. • Precipitation (formation of solid is the driving force). • Acid-Base neutralization (formation of water is the driving force). • Oxidation-Redox (redox; transfer of electrons to reduce electrical potential is the driving force).

6. PRECIPITATION (ppt) • The formation of a solid (precipitate) in an aqueous solution • Occurs when the compound formed is slightly soluble or insoluble (Sec. 4.4) • Ion interchange or metathesis (switch cation/anion partners) • Qualitative Analysis • Quantitative Analysis - Stoichiometry

7. CHEMICAL EQUATION • Identify reactants, products, states of matter [g, s, aq, l]. • Balance equation to conserve mass. • Calculate quantitative or stoichiometric relationships between rxn participants (R or P) based on balanced chemical rxn.

8. CHEMICAL EQUATION (2) • Molecular Equation: write all reactants and products as “molecules”, show state of each • Complete Ionic Equation: write strong electrolytes as ions • Net Ionic Equation: cancel out spectator ions

9. ACIDS (T4.2) • Acids provide H+(aq) or H3O+ (aq, hydronium) ions in water (Arrhenius). • Strong acids dissociate and ionize nearly completely in water (approaching 100% extent of rxn) to give H+(aq) and an anion. • Weak acids are in equilibrium with ions. • Polyprotic acids: sulfuric, phosphoric.

10. BASES (T4.2) • Bases provide OH- ions in water (Arrhenius). Strong bases dissociate and ionize nearly completely in water to give OH-(aq) and cations. vs weak bases

11. ACID + BASE RXN: NEUTRALIZATION • Acid + Base → Salt + Water • SA + SB: HCl (aq) + NaOH(aq) → NaCl(aq) + H2O(l) • Net ionic: H+(aq) + OH-(aq) → H2O(l) • WA + SB: HF(aq) + KOH(aq) → KF(aq) + H2O(l) • HF(aq) + OH-(aq) → F-(aq) H2O(l)

12. OXIDATION-REDUCTION REACTIONS • A redox reaction involves the transfer of electrons between reactants • Electrons gained by one species must equal electrons lost by another • Oxidation numbers change in a redox rxn. • Both oxidation and reduction must occur simultaneously.

13. OXIDATION STATES OR NUMBERS (OX#) • Actual or imaginary charge on atom: single atom, atom in molecule or atom in polyatomic ion • We will study rules for assigning OX# and then use this information to balance redox equations

14. DETERMINING OX# (p127-8) • OX# of an atom in an element is 0. • If the species is neutral, sum of OX# is 0 • If the species is charged, sum of OX# is value of charge • OX# of a monatomic ions is its charge: 1A atoms have OX# = +1; 2A atoms have OX# = +2; 7A atoms have OX# = -1, etc

15. OX# (2) • In molecular (covalent) cmps O has OX# = -2; sometimes -1 (with metal) • In molecular (covalent) cmps H has OX# = +1; sometimes -1 (peroxide) • F always has OX# = -1; other halides can have other OX#s • There are exceptions

16. OXIDATION • If atom X in compound A loses electrons and becomes more positive (OX# increases), we say X is oxidized. • Also, we say that A is the reducing agent (RA) or is the electron donor.

17. REDUCTION • If atom Y in compound B gains electrons and becomes more negative (OX# decreases), we say Y is reduced. • Also, we say that B is the oxidizing agent (OA) or is the electron acceptor.

18. ACTIVITY SERIES (T4.3) • Redox participants have varying capacities to gain or lose electrons. • The Activity Series lists metal elements in order of decreasing strength as a reducing agent; ie. ability to lose electrons and undergo oxidation. • A particular rxn in the list will cause the reduction of any rxn below it.

19. BALANCING REDOX RXNS: Oxidation Number Method • Balance chem eqn except for H and O • Assign OX# to all atoms • Sum OX#s for atoms undergoing oxidation • Sum OX#s for atoms undergoing reduction • These sums must be equal, so multiply each by appropriate factor to equate #e- lost = #e- gained. • Add water and then H+ to balance O and H. • Check for atom and charge balance

20. BALANCING REDOX EQNS Half-Rxn Method (acid) • Write half chem eqn for reduction • Write half chem eqn for oxidation • Balance all atoms except H and O • Balance O with H2O and H with H+

21. Half-Reaction Method (acid, 2) • Add electrons to balance charge (I.e. show loss or gain of electrons) • Balance the number of electrons between the two half-rxns by multipying by appropriate factor • Add two half-rxns and cancel identical species. • Check for atom and charge balance

22. Half-Reaction Method (base) • Follow steps for acidic solution • Add OH- ions to cancel out the H+ ions, thus forming water. • Cancel out water molecules • Check for atom and charge balance

23. REDOX TITRATIONS • Titration - technique for determining quantity/concentration of an unknown analyte by reacting a measured volume of it with another reactant (titrant) of a known concentration. • This method works when the redox rxn is 100% complete and that there is an indicator that signals the end of the rxn.

24. TITRATION • Start with a balanced chem eqn between the titrant (known [T]) and the analyte (unknown [A]). • Select an indicator that changes color when the redox rxn is 100% complete. • Add a known volume of T from a buret to a known volume of A with indicator added until all of T has reacted (indicator has turned color).

25. TITRATIONS (2) • The goal is to stop adding T when the rxn is 100% complete. • A stoichiometric calculation yields the quantity/concentration of A.

26. REDOX RXNS • Redox rxns are very common and take place in many applications (pp144-145).