Chapter 4: RXN TYPES and SOLN STOICHIOMETRY (2)

1. Chapter 4: RXN TYPES and SOLN STOICHIOMETRY (2) Aqueous Solutions and Electrolytes Precipitation, Acid-Base, Oxidation-Reduction (Redox) Reactions

2. CHEMICAL RXNS IN AQUEOUS SOLNS (4.4) • Precipitation is the formation of solid from two aqueous solutions. • Acid-Base neutralization is the formation of water. • Oxidation-Reduction or redox is the transfer of electrons from one reactant atom to another reactant atom.

3. 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. • Soluble salt I (aq) + Soluble salt II (aq) Solid cmp (s) + Soluble salt III • Table 4.1 Solubility Rules for Salts in Water (memorize) • Ion interchange or metathesis (switch cation/anion partners)

4. T4.1 SOLUBILITY RULES Practice Prob 35, 37, 4, 43 • 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 (Ca, Sr, Ba). • Most sulfides, carbonates, chromates and phosphates are slightly soluble.

5. Fig 4.13 K2CrO4(aq) + Ba(NO3)2(aq)  KNO3(aq) + BaCrO4(s)

6. CHEMICAL EQUATION (4.6) • Identify reactants, products, states of matter [g, s, aq, ℓ]. • Balance equation to conserve mass (and charge). • Calculate quantitative or stoichiometric relationships (mol ratios) between rxn participants (R or P) based on balanced chemical rxn.

7. CHEMICAL EQUATIONS (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. This eqn expresses the basic chemical rxn.

8. Figure 4.15 a&b The Reaction of K2CrO4 and Ba(NO3)2

9. Chapter 4 Problems • 36, 38, 44, 46

10. SOLUTION STOIOCHIOMETRY (Ch 4 + Ch 3 + Ch 2) • 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.

11. STOICHIOMETRIC PROBLEMS • 48, 50

12. 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. • Review Ch 2 nomenclature and Ch 4 strong acids (SA) and bases (SB).

13. 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

14. ACID + BASE NEUTRALIZATION • The key in a neutralization reaction is that one H+ ion reacts with one OH- ion. • It is NOT that one mol acid reacts with one mol base. • Be careful with acids that provide 2 or 3 H+ ions per mol acid (sulfuric or phosphoric). • Similarly for bases like barium hydroxide.

15. 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).

16. 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

17. NEUTRALIZATION TITRATION • Prob 4.64 and 4.66 (This is what you will do in lab)

18. OXIDATION-REDUCTION REACTIONS • A redox reaction involves the transfer of electrons between atoms in the reactants. • Electrons gained by one atom must equal electrons lost by another. (conservation of e-s) • Oxidation states or numbers are assigned to atoms and they change in a redox rxn. • Both oxidation and reduction must occur simultaneously. (or e-s would not be conserved) 2

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

20. 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, MnO2] • If the species is charged, sum of OX# is value of overall 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

21. 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

22. 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.

23. 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.

24. Figure 4.20 A Summary of Oxidation-Reduction Process

25. Redox Basics • Prob 68, 72

26. 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.

27. Activity Series of Metals in Aqueous Solution

28. INTERPRETATION OF ACTIVITY SERIES • What rxn will occur between lithium and calcium? The choices are • Li(s) + Ca(s)  Li+(aq) + Ca2+(aq) • 2Li(s) + Ca2+(aq)  2Li+(aq) + Ca(s) • 2Li+(aq) + Ca(s)  Ca2+(aq) + 2Li(s) • 2Li+(aq) + Ca2+(aq)  2Li(s) + Ca(s) • The strongest RA is at the top (Li) meaning that Li loses e-s and is oxidized. And Ca2+ gains e-s and must be reduced.

29. INTERPRETATION OF ACTIVITY SERIES • Therefore 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

30. BALANCING REDOX EQNS Half-Rxn Method (acid) • Write half chem eqn for reduction • This requires determining what atom is reduced; use OX#s • Write half chem eqn for oxidation • same • Balance all atoms except H and O • Balance O with H2O and H with H+

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

32. The Half-Reaction Method (Acidic Solution)

33. Half-Reaction Method (base) • Follow steps for balancing in acid • 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.

34. Balancing Redox Equations • Prob 74, 76