CHEMICAL RXNS IN AQUEOUS SOLNS (4.4)

1. CHEMICAL RXNS IN AQUEOUS SOLNS (4.4) • 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).

2. PRECIPITATION (4.5, 4.7) • The formation of a solid (precipitate) when two aqueous solutions are mixed; we say that the product cmp is insoluble or only slightly soluble in water. • Sol. salt I (aq) + Sol. salt II (aq) Solid cmp (s) • Table 4.1 Solubility Rules for Salts in Water (memorize) • Ion interchange or metathesis (switch cation/anion partners)

3. T4.1 SOLUBILITY RULES • Most nitrates are soluble • Most salts with Grp 1A ions and NH4+ are soluble. • Most salts with Cl-, Br-, I- are soluble EXCEPT those with Ag+, Pb2+, Hg22+ • Most sulfates are soluble EXCEPT those with Ba2+, Pb2+, Hg22+, Ca2+. • Most hydroxides are slightly soluble EXCEPT the strong bases. • Most sulfides, carbonates, chromates and phosphates are slightly soluble.

4. CHEMICAL EQUATION (4.6) • 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.

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

6. SOLUTION STOIOCHIOMETRY • Typical stoichiometric calculation for reactions taking place in aq soln. • Write balanced net ionic eqn to identify the chem reaction • Calculate mols of known A from VA and MA • Calculate mols of unknown B, then VB • VA, MA  #mol A  #mol B  VB if MA and MB are known • Determine LR after Step 1 if appropriate.

7. STOICHIOMETRIC PROBLEMSProb 48, 50

8. ACID + BASE RXNS • Acids donate protons, i.e. provide H+(aq) or H3O+ (aq, hydronium) ions in water (Arrhenius). • Polyprotic acids: sulfuric, phosphoric. • Bases accept protons.

9. 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) • Net ionic: HF(aq) + OH-(aq) → F-(aq) H2O(ℓ) • SA + WB: Problem 4.57c

10. ACID-BASE TITRATION (volumetric analysis) • Exptal technique for determining quantity of an unknown substance (analyte in beaker) by reacting a measured volume of it with another reactant (titrant in buret) of known concentration. • This method works when the rxn is 100% complete (reaches equivalence pt) and that there is an indicator (color change, pH) that signals the rxn completion (endpoint).

11. NEUTRALIZATION TITRATION • Write the balanced acid (assume to be analyte) + base (use strong base) rxn for the titration. • Use an indicator (e.g. phenolphthalein) that signals the equivalence point. • The molarity and volume of the titrant (SB) must be known accurately. • Fig 4.18

12. NEUTRALIZATION TITRATION Prob 4.66, 64

13. 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 states or numbers are assigned to atoms and they change in a redox rxn. • Both oxidation and reduction must occur simultaneously. 2

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

15. DETERMINING OX# (T4.2) • OX# of an atom in an element is 0 [Fe, O2] • If the species is neutral, sum of OX# is 0 [NaCl, SnO4] • If the species is charged, sum of OX# is value of charge (NH4+; SO42-) • OX# of a monatomic ions is its charge: 1A atoms have OX# = +1; 2A atoms have OX# = +2; 7A atoms have OX# = -1, etc

16. 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

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

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

19. ACTIVITY SERIES (Expt 7) • 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.

20. Activity Series of Metals in Aqueous Solution

21. INTERPRETATION OF ACTIVITY SERIES • The strongest RA is at the top (Li) meaning that Li loses electrons and is oxidized. Therefore, the other metal must be reduced. • When Lithium and Calcium react, • 2Li(s) + Ca2+(aq)  2Li+(aq) + Ca(s) • We say that Li displaces calcium ion from soln. Li(s) dissolves and Ca(s) forms

22. 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+

23. 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 (i.e. #e- gained by by atom Y = #e- lost by atom X) • Add two half-rxns and cancel identical species. • Check for atom and charge balance

24. The Half-Reaction Method (Acidic Solution)

25. 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. Make sure there are no H+ ions remaining.