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Predicting Reactions

Page history last edited by Brendt Bly 11 years, 9 months ago

Notes

Ap_reactions1.pdf

 

Also, the following notes were taken from chemmybear.com, which has been down recently and I don't know if it will come back.

Classifying Reactions
to help with
Predicting Reactions

(based on a worksheet on Classifiying Reactions from Mr. Groves' AP Chemistry class)


There are five different types of reactions possible in the reactions section in the AP Test:

1. Double Replacement Reactions (Metathesis)

2. Redox (Oxidation - Reduction) Reactions

3. Organic Reactions

4. Complex Ion Formation

5. Lewis Acid-Base Reactions

Other tips and Final Touches.

Some practice problems extracted from The Chem Team internet site.


RedOx (Oxidation - Reduction) Reactions

Memorize the common strong oxidizers, generally ions with lots of oxygen, MnO4-, Cr2O72-, IO3-, etc. (in the "STRONG OXIDIZERS" section on "Stuff I Should Know for the AP TEST But Do Not Know Yet (Acrobat)"), memorize what they turn into, and look for something to oxidize.

Memorize the common strong reducers (on the handout mentioned above), memorize what they turn into, and look for something to reduce.

Memorize the equations for the oxidation and the reduction reactions of water during the electrolysis of water.

Reduction: 4H2O + 4e- --> 2H2 + 4OH-

Oxidation: 2H2O -->O2 + 4H+ + 4e-

Watchout for:

Keywords "acidified solution" or an acid included in the reactants. The H+ ions form H2O with the oxygens.

Anytimes you see a neutral element, Cu°, O2, H2, etc. it must be redox.

Michael's Tips & Tricks:

1. When reactions occur between a metal like Fe°, Cu°, Sn°, etc. (multiple ions possible metals: Fe2+ and Fe3+) reacts with reactive gases like O2, F2, Cl2 with the addition of heat, Fe° will ionize itself to maximum positive charge (-ic metal ions). Example:

2Fe°(s) + 3Cl2(g) + heat --> 2FeCl3(s)

2. When applying the rule of "Free Halogens + Dilute OH-Hypohalites ions," the addition of halide ions (such as Cl-) to the equation are required to obtain full credit on the reaction equations. Example:

Cl2(g) + 2OH--->ClO- + Cl- + H2O

3. When the oxides of an alkali metal (Family 1), Ca, Ba, or Sr dissolve in water, hydroxides will form, but no gases will be released. Example:

K2O(s) + H2O--> 2K+ + 2OH-

4. When the hydrides of an alkali metal (Family 1), Ca, Ba, or Sr dissolve in water, hydroxides will form and H2 gas is released. Example:

LiH(s) + H2O -->Li+ + OH- + H2(g)

5. Look for "battery" reactions (activity series of metals). When you are not sure which one will undergo changes, look at the reduction potential chart given in the AP Test (the metal with greatest potential will reduce). Examples:

Mg°(s) + 2Ag+-->Mg2+ + 2Ag°(s)

6. Be aware of Disproportionation Redox Reactions. These are the reactions when a portion substance is oxidized while the rest is reduced. The same chemical substance undergoes both oxidation and reduction. NO2 and H2O2 are classic chemicals that have this ability. Example:

3NO2(g) + H2O -->2H+(aq) + 2NO3-(aq) + NO(g)

7. (Trick #1) When you see electrolysis such as KI in water, and you are stuck on whether H2 gas will form from which side. The following is the solution that always works. You know potassium is always going to the negative electrode. Let say K° forms at the negative electrode (cathode) and immediately undergoes reaction with water:

2K° + 2H2O --> 2KOH + H2

Since all these happens on the negative electrode. So, H2 gas forms from the negative electrode and that's exactly what happens when water reduces at the cathode.

8. (Trick #2) When CuSO4(aq) is electrolyzed, you know that Cu° metal is going to form because copper's potential is higher than water. So, positive side will attract SO42- ions. Nevertheless, SO42- can't further oxidize (full of oxygen and no more unshared pair of electrons possible for further oxidation). As the result, you should use the other side of the hydrolysis in Trick #1: instead of OH-, put H+; instead of H2, put O2. This makes sense because when you electrolyze H2O, you get H2, and O2; and OH- and H+ from each electrode will neutralize and become water again.

2Cu2+ + 2H2O--> O2 + 4H+ + 2Cu°

9. (Trick #3) When you recognize great oxidizers like Cr2O72-, MnO4-, and MnO2, with acidified solutions in the test, but you may not remember what they turn into. So, remember the definition of an oxidizer: the ability to give off its oxygen components. So, acidified Cr2O72-, turn into Cr3+ and MnO2, and MnO4- will turn to Mn2+. As you can see, the oxygen components are "ripped off" and form water with H+ ions.

 


Organic Reactions

These are especially useful for the older tests. The "acorn book" states that organic compounds may show up as examples, but organic reactions are pretty restricted.

Oxidation - complete combustion of hydrocarbons results in CO2 and H2O. Milder oxidation may change a primary alcohol into an aldehyde, a secondary alcohol into a ketone, etc.

Substitution - example: Br2 + a hydrocarbon results in HBr + a Br substitute for one of the hydorgens on a carbon.

Esterfication - organic acid + alcohol results in an ester and H2O

Addtion - a double bond is borken and atoms are attached to two carbons. If H2O is added, an H and OH add to the carbons. If Br2 is added, a Br atom is added to each of the carbons.

 


Complex Ion Formation

These are usually formed from a transition metal surrounded by ligands (polar molecules or negative ions). As a "rule of thumb" you place twice the number of ligands around an ion as the charge on the ion... example: the dark blue Cu(NH3)42+ (ammonia is used as a test for Cu2+ ions), and Ag(NH3)2+.

Memorize the common ligands.

Ligands Names used in the ion
H2O aqua
NH3 ammine
OH- hydoxy
Cl- chloro
Br- bromo
CN- cyano
SCN- thiocyanato (bonded through sulphur)

isothiocyanato (bonded through nitrogen)

Watchout for:

Alumninum also forms complex ions as do some post transitions metals. Ex: Al(H2O)63+

The names are very impressive, but easy..the ions above are the tetraamminecopper(II) ion, the diamminesilver(I) ion, and hexaaquoaluminum(III) ion. Zn(OH)42- is the tetrahydroxyzinc(II) ion, the charge is the sum of the parts (2+)+4(-1)= -2.

Acid-base reactions may change NH3 into NH4+ (or vice versa) which will alter its ability to act as a ligand.

Visually, a precipitate may go back into solution as a complex ion is formed. For example, Cu2+ + a little NH4OH will form the light blue precipitate, Cu(OH)2. With access ammonia, the complex, Cu(NH3)42+, forms.

Keywords such as "excess" and "concentrated" of any solution may indicate complex ions. AgNO3 + HCl forms the white precipitate, AgCl. With excess, concentrated HCl, the complex ion, AgCl2-, forms and the solution clears.

The odd complex ion, FeSCN2+, shows up once in a while simply because it is commonly used in the CHEMStudy first-year equilibrium lab.

Transitional metals, such as Iron, Zinc and Chromium, can form complex ions. Aluminum can form complex ions as well.

 


Lewis Acid-Base Reactions

"HAVE PAIR WILL SHARE" -- Lewis Base. The formation of a coordinate covalent bond between NH3 and BF3 to form H3N:BF3 is a classic example of a Lewis acid-base reaction.

Oxides of metals and nonmetals are also examples:

CO2 + H2O--> H2CO3

CaO + CO2-->CaCO3

Watchout for:

Some reactions come from industrial pocesses such as the formation of bleach (ClO- ion) from Cl2 and dilute NaOH). Note these as you come across them and add them to your "bag of tricks".

You can write (or think about) chemicals in different ways for different reactions. Ammonia may be NH3 (aq) for complex ions, NH4OH for double replacement or acid-base reactions; water may be H2O or you might think of it as H+ and OH- for hydrolysis or redox reactions; HNO3 may be an acid (donating H+) or it may be an oxidizer (forming NO or NO2 + H2O).

Michael's Tips & Tricks:

1. When you see NO3-, NO2-, CO32-, SO32-, SO42- ions in solids and are heated in vacuum, NO2(g), NO(g), CO2(g), SO2(g), SO3(g) are evolved. Example:

CaCO3(s) + heat CaO(s) + CO2(g)

2. (Trick #4) When you see a compounded gas bubble through any solution, they should be changed to "The Dissolved" state which is actually a Lewis Acid-Base Reaction:

CO2(g) + H2O --> H2CO3

NH3(g) + H2O --> NH4OH

SO2(g) + H2O --> H2SO3

 


Some other tips and final touches:

1. Don't panic.

2. You earn a point for just writing the reactants in chemical form.

3. Don't forget to cancel out spectators.

4. Get familiar with the chemicals: what are they going to do, function as, how will they change.

5. Make sure you are very clear with which type of acid described: Hydrochloric acid (HCl), Chloric acid (HClO3), and Chlorous Acid (HClO2). Misunderstanding of the names will ruin your chance of getting partial credits.

6. Read very carefully.

7. Do more practice problems and be careful.

8. If it were easy, it wouldn't be AP.

9. Be confident and proud! You have made it this far and you are going to make it thru the test.

 


Produced by Michael Lo, Ruth Cusick, Paul Groves

1997-98 updated 2003

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