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

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).

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.

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

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).

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

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.

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

ACIDS (T4.2) Acids provide H + (aq) or H 3 O + (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.

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

ACID + BASE RXN: NEUTRALIZATION Acid + Base → Salt + Water SA + SB: HCl (aq) + NaOH(aq) → NaCl(aq) + H 2 O(l) –Net ionic:H + (aq) + OH - (aq) → H 2 O(l) WA + SB: HF(aq) + KOH(aq) → KF(aq) + H 2 O(l) –HF(aq) + OH - (aq) → F - (aq) H 2 O(l)

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.

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

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

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

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.

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.

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.

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

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 H 2 O and H with H +

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

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

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.

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).

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

REDOX RXNS Redox rxns are very common and take place in many applications (pp ).