Transcript Pre- AP & NET IONIC EQUATIONS

Frustrated student
Frustrated Teacher
• How do I fit it in my curriculum through out
the year?
• Why do the kids just not get it?
• How can they learn so many reactions?
Your approach may depend largely on the type of
daily schedule your school follows.
Block Schedule &
Double Periods
• Warm ups were net ionic
equations
• Time to discuss
• Students have time to
practice in class
• Quiz students often… twice
per week
Single Periods &
Modified Block
• Warm ups took up to much
time with discussion
• Students do not have time
in class to practice
• Quizzes not as often once
every week.
I cannot change the school schedule. What do I
do?
Spend more time in pre-AP teaching reaction
chemistry.
• Nomenclature should be mastered in pre-AP.
• This means knowing oxidation numbers.
• Learning the most common polyatomic ions.
POLYATOMIC ION CHARTS
Quiz for weeks 3 & 4
Nitrate
NO3-
Carbonate
CO32-
Sulfate
SO4 2-
Chromate
CrO42-
Phosphate
PO4 3--
Dichromate
Cr2O72-
Acetate
Hydrogen sulfate
HSO4-
Hydroxide
C2H3O2 – or
CH3COOOH-
Ammonium
NH4+
Arsenate
AsO 4
Oxalate
C2O42-
Permanganate
MnO4-
Hg22+
Peroxide
O22-
Di-mercury
(mercury I)
Bromate
3-
BrO3-
Quiz for weeks 5 & 6 will include all ions from this chart and previous
chart
Thiocyanate
SCN-
Hypochlorite
ClO-
Cyanide
CN-
Chlorite
ClO2-
Hydrogen
phosphate
Nitrite
H 2PO4-
Chlorate
ClO3-
NO2-
Per chlorate
ClO4--
Sulfite
SO32-
Arsenite
AsO33-
Phosphite
PO32-
Hydrogen
Carbonate
(bicarbonate
HCO3-
Monatomic Ions - Quizzes for week 7 – 18 will
include all tables
All group 1 metals
1+
Tin
Sn2+ or Sn4+
All group 2 metals
2+
Zinc
Zn2+
All group3 metals
3+
Silver
Ag+
All group 16 (6)
nonmetals
All group 17 (7)
nonmetals
All group 15 (5)
nonmetals except
bismuth
Iron
2-
Copper
Cu+ or Cu2+
1-
Cobalt
Co2+ , Co3+, Co6+
3Bi3+
Scandium
Sc3+
Fe2+ or Fe3+
Nickel
Ni2+ or Ni3+
Cr3+ , Cr2+ , or Cr6+
Manganese*
Mn2+ or Mn4+
Chromium
(there are more charges)
Mercury II
Hg2+
Mercury I
Hg22+
• Students should also know how to calculate oxidation
numbers elements (ions and compounds)
• Identify the following for a redox reaction
-element or ion oxidized
-element or ion reduced
-reducing agent
-oxidizing agent
- element or ion that loses electrons
- element or ion that gains electrons
If on the period system, you should reach
nomenclature at the end of the 18th week.
Students then take a nomenclature quiz every
week.
Work on reaction chemistry the last 9 weeks.
Which one(s)?
• Anhydrides
• Combustion (Hydrocarbons)
• Decomposition (simple)
• Double replacement
• Redox (Single replacement)
Anhydrides can be taught
with types of reactions as well as double, single replacement, and
combustion. Taught as molecular equations
The net ionic equations for double and single replacement
can be taught after solutions.
Of course, students will have to be introduced to solubility rules.
This needs to be done in pre-AP (especially if not on a double or
block period).
Don’t re-invent the wheel…use the NMSI
website for reactions or other printed resources.
This is a great resource it also
comes with a teachers edition
with answers.
The Ultimate Chemical Equations
Handbook by George R Hague, Jr. and Jane D.
Smith
• Anydrides
-Nonmetallic oxides plus water
MgO + H2O → Mg(OH)2
-Metallic oxides plus water
CO2 + H2O → H2CO3
-Metallic hydrides plus water
NaH + H2O → Na+ + OH- + H2
- Group 1 plus water
Na + H2 O → Na + + OH- + H2
• Anydrides
-Nonmetallic oxides plus water
CO2 + H2O → H2CO3
Solid magnesium oxide is added to water
MgO + H2O → Mg(OH)2
-Metallic oxides plus water
Gaseous carbon dioxide is bubbled through water
CO2 + H2O → H2CO3
• Anydrides
-Group I in water
solid sodium is added to water
Na + H2O → Na+ + OH- + H2
• Addition Reactions
- Simple synthesis
sodium + chlorine
2 Na + Cl2 * → 2NaCl
- Nonmetallic oxide and water
CO2 + H2O→ H2CO3
- Metallic oxides in water
MgO + SO2 → MgSO3
- Metallic oxides and nonmetallic oxides
Na2O + CO2 → Na2CO3
• Combustion of hydrocarbons
- Hexane is burned in air
2 C6 H14 +19 O2 → 12 CO2 + 14 H2O
• Single replacement (net ionic)
• Double replacement (net ionic)
* Teach molecular first and net ionic after
solutions.
 Nomenclature (binary ionic & covalent)
 Writing and balancing equations
single, double, synthesis, decomposition,
combustion of hydrocarbons
 Anhydrides taught with synthesis and decomposition
reactions
 Calculation of oxidation numbers
 Oxidation, reduction, oxidizing agents, and reducing
agents.
 Net ionic equations for single and double replacement
(taught with solutions)
 Solubility rules
The first two weeks start with memorization quizzes
- solubility rules
- strong acids/strong bases
- diatomic molecules
*Nomenclature should be reviewed as a summer
assignment.
Whether this will remain the case is not known, but if it does, a default
“fair” question about such a (redox) reaction might have to do with a
change in oxidation numbers which can be determined without having
any idea what the reaction behavior is like. Below is a simplified set of
rules which should look familiar.
1. the oxidation number of an element is 0
2. the oxidation number of monoatomic ion is equal to its charge
3. in compounds:
the oxidation number of hydrogen is +1
(except in metal hydrides where it is -1)
the oxidation number of oxygen is -2
(except in peroxides where it is -1)
the oxidation number of alkali metals is +1
the oxidation number of alkaline earth metals is +2
the oxidation number of terminal halogens is -1
4. the sum of all the oxidation numbers in a molecule or ion is equal to the charge
Students are quizzed every week starting in the
following order:
Anhydrides (2) weeks
Additions Reactions & Decomposition (2) weeks
Single & Double (ppt) (2 weeks)
Redox (2 weeks)
Combustion (2weeks)
Complex Ions (2 weeks)
*I teach in this order because I will have covered the
topics in pre-AP or AP.
Over the past 10 years the number of truly difficult non-trival
reactions has dwindled. There have only been 2 and those were
in years when you could easily have avoided them by choosing
other options. Now that there are no options and reactions
must be balanced it is questionable whether these kinds of
reactions will ever appear again. It is clear that monoatomic ion
redox will continue to be considered fair game. Here is an
example from the 2007 exam:
“A solution containing silver(I) ion (an oxidizing agent) is mixed
with a solution containing iron(II) ion (a reducing agent)”
Such reactions can be completed by following simple
rules. The non-trivial sort require knowledge of
common oxidizer/reducer pairs or at the very least
some common-sense elimination of unlikely products
followed by inspired guessing. Memorizing the
"common" pairs may help but you will probably get
farther by trying to reason through the process since
there is no guarantee that the pairs you memorize will
be used on the exam.
Some essential principles to keep in mind:
• elements in their highest positive oxidation state (same as group #,
whether A or B) can ONLY be reduced
• elements in their lowest oxidation state (0 for metals, negative for
non-metals, corresponding to distance from noble gases) can ONLY
be oxidized
• intermediate oxidation states can go either way!!!
• if the mixture is acidic, H+ should be included as a reactant; water is
one product
• if the mixture is basic, OH- should be included as a reactant; water
is one product
• occasionally the acid anion or base cation may precipitate with a
product ion
***The last three items on the list may be irrelevant if truly complex non-trivial
processes simply die a quiet death on future exams.
oxidizers [remember, oxidizers will become reduced]
MnO4- (in acid) → Mn2+
MnO4- (in neutral or basic) → MnO2
MnO2 (in acid) → Mn2+
Cr2O72- (in acid) → Cr3+
HNO3 (conc.) → NO2
HNO3 (dilute) → NO
H2SO4 (hot, concentrated) → SO2 [if not hot and conc., this acts like HCl or other normal acids]
metal cations → lower charge cations or (rarely) free metals
free halogens → halide ions
reducers [remember, reducers will become oxidized]
halide ions → free halogens
free metals → metal cations
SO32- (or SO2) → SO42NO2- → NO3free halogens (dil. basic) → hypohalite ions [like XO-]
free halogens (conc. basic) → halate ions [like XO3-]
metal cations → higher charge cations
Below are some reactions from past tests.
1. a solution containing tin(II) ions is added to acidified potassium dichromate solution
--there is a color change during this reaction; which atom is most likely responsible? explain
2. powdered iron is added to a solution of iron(III) sulfate
--which species are spectators?
3. solutions of tin(II) chloride and iron(III) chloride are mixed
--both the reactant and product mixtures are colored; which ions account for the colors?
1. 3 Sn2+ + 14 H+ + Cr2O72- → 3 Sn4+ + 2 Cr3+ + 7 H2O
--the chromium changes in oxidation state and is a transition metal; most transition metal
compounds
are colored and the color changes with oxidation state
2. Fe + 2 Fe3+ → 3 Fe2+
--the sulfate ion is a spectator
3. Sn2+ + 2 Fe3+ → Sn4+ + 2 Fe2+
--since iron is a transition metal it is likely that the colors come from Fe2+ and Fe3
OK! How much time do I spend on non-trivial
redox?
That depends on how much time you have.
If on a modified block or period schedule, I suggest
you spend very little time. It is worth the gamble
with the new format. There are other topics that
are much more important.
The morning I
introduce complex
ions to my AP chem
students.
Reactions of coordination compounds and ions are not
covered in depth on the exam but you will sometimes see
them in the reaction-writing section and they are easy
enough to complete with a few basic principles in mind. Most
can be recognized by the choice of reactants: generally a
transition metal ion or compound (also occasionally the
amphoteric species from Group 3A such as Al) and a source of
ligands. The most common ligands involved in questions are
ammonia, the hydroxide ion, and the cyanide ion. Key to
recognizing such is often the word "excess", indicating that
enough of the complexation agent has been added to
eliminate the possibility of precipitation of lesser-coordinated
species. Occasionally this word will not appear and instead
“concentrated” is used to descibe the added complexing
agent (in that case usually an acid or base)
One of the hurdles to get over is some
knowledge of the likely coordination numbers
for metal ions. Unfortunately there is no simple
way to remember all of them. Some you may
recognize from work done in the lab or
something you read. In a pinch, it may be
helpful to know that often the coordination
number is twice the cation charge. In any case,
you will seldom lose points just because you
used a coordination number of 4 instead of 6.
Historically, reactions involving complex ions on the
exam fall into three broad categories:
1. complexation of a soluble salt e.g. a concentrated solution of ammonia is
added to a solution of copper(II) chloride
4 NH3 + Cu2+ → [Cu(NH3)4]2+
2. complexation of an insoluble salt e.g. excess concentrated potassium
hydroxide solution is added to a precipitate of zinc hydroxide
2 OH- + Zn(OH)2 → [Zn(OH)4] 23. destruction of a complex by acid/base neutralization e.g. dilute hydrochloric
acid is added to a solution of diamminesilver nitrate
2 H+ + Cl- + [Ag(NH3)2] + → AgCl + 2 NH4+
Examples of actual reactions from past A.P. exams along with added
questions.
1. excess dilute nitric acid is added to a solution of tetramminecadmium(II) ion
--what is the coordination number of the complex ion?
4 H+ + [Cd(NH3)4] 2+ → Cd2+ + 4 NH4+
--the coordination number of [Cd(NH3)4] 2+ is 4
2. pellets of aluminum metal are added to a solution containing an excess of sodium hydroxide
--which reactant acts as a Lewis acid? explain
2. Al + 4 OH- → [Al(OH)4] --the Al accepts electron pairs from the hydroxide ions and is thus a Lewis acid
3. an excess of ammonia gas is bubbled through a solution saturated with silver chloride
--which reactant acts as a Lewis base? explain
2 NH3 + AgCl → [Ag(NH3)2] + + Cl--the NH3 donates electron pairs to the silver ion and is thus a Lewis base
4. a concentrated solution of ammonia is added to a suspension of zinc hydroxide
--what visual change occurs in the reaction mixture?
. 4 NH3 + Zn(OH)2 → [Zn(NH3)4] 2+ + 2 OH--the suspension starts out slightly cloudy and ends up clear
5. a solution of ammonium thiocyanate is added to a solution of iron(III) chloride
--describe the color changes that occur during the reaction
SCN- + Fe3+ → [FeSCN] 2+ (other species up to CN6 accepted)
--the original solutions are nearly colorless while the product mixture is orange to blood red
After teaching complex ions.
Was this ever taught in my college chemistry
courses, especially if I have a comprehensive
degree?
Do you remember the qualitative analysis of
cations lab? (Chem I)
Again, do not re-invent the wheel.
Quizzes are on the NMSI site.
NMSI has great a resource by Kristen Henry.
Organic is not included in her handout.
Chemical Equations Handbook by George R.
Hague and Jane O. Smith
All quizzes should be cumulative. Don’t forget to add at least 3 to 5 questions
about the reactions:
-oxidizing agents
-reducing agents
- color
- what is oxidized
-what is reduced
- how to test for gases- what gas is produced
- what solid is produced
- what ion stay in solution
- what is losing electrons
-what is gaining electrons
- stoichiometry
(There are not a lot of old AP test with this format. The format changed
In 2007).
Test format:
3 equations
One question per equation
Students must balance equation including
charges.
 1 point for products, 2 points for reactants
1 point for balancing, and 1 point for
answering question correctly.
 This section is included in part B of free response
(40 minutes)
Students should attack problems by identifying
type of reaction in this order.
Synthesis
Decompositions
Single Replacement
Double (ppt)
Complex Ions
Non-Trivial Redox
Example: 2006 Question 4
Clue students in on wording
- solutions
- solids
- the must know reacting species for strong
acids and bases
- concentrated/dilute
- catalysis
- two ions ( positive and negative) precipitation or addition
- two ions (positive) redox
- the last resort…look at reduction potential chart.
• Have students start a list of common mistakes
and clues. They should keep them in their
notebook and add to the list after the quizzes
are returned.
• Make the list of common mistakes and
reminders as you grade the quiz
•Labs and demonstration are an excellent way of
imbedding reactions into the curriculum.
•Students remember what they see rather than
symbols and formulas on a sheet of paper.
•Also important due to the format change in 2007.
This could be a lab question also.