chem 127

Review Notes, Exam 3

 

Chapters 15 and 16 are chemical equilibrium. Although much of this was covered on exam 2, we have extended our use of equilibrium in  variety of applications. Anything in Chapter 13 (equilibrium) is a tool that you can be expected to use on this exam. 

            [The exam also includes chapter 20]

 

Obviously, all equilibrium work begins with (a) a balanced equation (chemical equation)

                                                                        (b) a formal expression of K  (Kc, Kp)

We have studied a number of applications where the chemical equation is implied and we adopt special subscripts for K (Ka, Kb, Ka1, Ka2, Kw, Ksp, Kstability (transition metal complexes), Pio <equilibrium vapor pressure of a liquid> ...)

However, the process is the same: balanced equation, formal expression for K

 

There aren't that many different questions to ask-- just different ways to express the question..

 

1. If the conditions of a mixture are ...(conc, pressures,...)  will get more or get less of the species we call the reactants. (Will reaction shift to right or to left?) Would a salt precipitate ? Would the sample absorb water vapor or give off moisture?    .... The process is to put conditions into the equilibrium expression to create Q or Ion Product.. and compare with K value.

2. If the system is not at equilibrium, what will be concentrations (pressures) when it reaches equilibrium. How much weak acid will dissociate? What is the pH of a Ammonium ion (weak acid) solution? What is the pH of an acetic acid-sodium acetate buffer?  .... The process is to assign "x" as the change in one species. Then

            determine the change in all ther species using the variable "x." Express final concentration as initial + change and substitute into the equilibrium expression. Solve for x.   .... often we can simplify if x<< terms it is added to or subtracted from.

3. A system is at equilibrium and here are the conditions.... Determine the value of K ... Of course the data might be one or two steps removed... A 0.10 M solution of a weak acid has pH= 2.89; what is Ka for that acid?

4. We have K for a reaction expressed in one style-- find K for a closely related process. ..... If Kp is ___, what is the value of Kc for that reaction? If ammonia has Kb= ____, what is Ka for the ammonium ion.  If the pH  is 6.55 (or [H3O+] has a specified value)  what is the [H3O+] and [OH-] (or wht is pH and pOH)

5. We have now included some problems where two different equilibrium reaction must be met. The simplest is a mixture of strong + weak acid, which is a general case of a common ion effect. This applies equally well to a diprotic acid.  How much more a salt of low solubility will dissolve if we added H+ (tying up the Anion as a weak acid) ? Likewise, could we dissolve a salt (of low solubility) by tying up the cation with a ligand like ammonia?

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That's about all we can do or expect to show up as problems. To operate efficiently you need to understand (know) a set of basic reactions

            weak acid HA + H2O --> H3O+ + A-

            weak base  B  + H2O ---> OH-  + HB+

            acid base indicators (different colors for acid and base form)

                        the language is that of conjugate acids and bases (Brontead-Lowrey definition)

            solubility of  salt (example)  PbCl2 ---> Pb2+ + 2 Cl-   

                        form varies for AgCl or Cr2S3 but the concept is the same

            stability of a transition metal complex:   (M2+ ) +  n Lig --->   M (Lig)n charge

                        these are often difficult equilibrium calculations

                                     unless we can neglect one or more "x" terms.

             you should be able to relate ion charge and oxidation states

                                    usually, M2+ is actually M (H2O)62+

            Don't overlook the simpler equilibrium examples:  H2O (liq) ---> H2O (g)  Kp = PH2O

                        Hydrated salts:    CaCl2 (s) + 6 H2O(g) --> CaCl2.6H2O (s)   Kp = 1 / P6(H2O)

                        Gas dissolving in water   O2 (g)  -----(water)---> O2 (aq)

 

LeChatlier's Law

            changing amount of material on either side of equation ...

            changing volume of systems (changes pressures)

            changing temperature

                        basic rule-- plot of ln(Kequil) vs  1/T (K) yields straight line

                                                slope is - delta Hreaction / R

                                                exothermic reaction: K decreases as temperature rises

Acids-- Need to know that HCl (aq), HNO3 are strong acids; NaOH, KOH and Ca(OH)2 are strong bases. Know acetic acid/acetate ion  well enough, since it is one of the most common weak acid/base pair. Likewise, know NH3/NH4+

 

Neutralization -- acids and bases, when mixed, react. The "fiction" is that the reaction take place first, then the products adjust as dictated by equilibrium.

The nature of acid base titration curves and how to calculate. Obviously an exam question won't ask you to compute 10-20 points on such a curve, but it may ask you to compute one point on such a curve. (Four regions; pH = pka at 1/2 equiv. point)

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Chapter 20 Transition Metal compounds

            What are the transition metals?

            Why are the ions basically +2 (or +3)

                        What is the ion electron configuration?  [core] d1 - d10

            Ligands -- Lewis bases (electron rich, electron donors)

                        coordinate covalent bonds -- real bonds, a bit weak

                        often 6 coordinated metal ions

                                    octahedral geometry

                        often very stable

            naming Complex Ions -- (a) complex cation

                                                            when ligands are ions (--o) or when ligands are neutral

                                                    (b) when complex ion is an anion

                                                            because ligands are anions

                                                            ------- [ate]

            why are transition metal ions colored and most other ions are not?

                        if you really understand, you'll see why Zn+2 complexes are colorless

                        crystal field theory (I'll supply exam ground rules on Monday)

            finding oxidation state from the formula of an ion

                        what is an oxidation state anyway?

            isomers -- geometric and enantiomers (handedness)