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 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.

•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) + 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)

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 H2O 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 (pp144-145).