Transcript Working With Chemical Reactions

Working With Chemical Reactions
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When we discuss chemical reactions we break them
down into categories:
- Combination (Synthesis, Addition)
- Single Replacement
- Metathesis (Double Replacement)
- Combustion
- Decomposition
- Organic
- Redox
- Acid Base Reactions
Additional Categories
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Lewis Acid Base Reactions – Adduct formation
Reactions of Anhydrides – Substances that do not
contain water, but react with water to produce acids
and bases, usually in the form of a gas bubbled through
water or a solid. May also have acidic and basic
reactants.
Non-trivial redox – slightly unpredictable
Identification Problems
It is difficult to identify a reaction using only the
reactants. For example…
Solid Calcium Oxide is exposed to a stream of Carbon
Dioxide Gas.
You are asked to write the equation, in net ionic form, if
possible and to answer the following question:
What type of reaction has occurred?
Which Presents a Problem
Presented with the complete reaction
Solid Calcium Oxide is exposed to a stream of Carbon
Dioxide Gas.
CaO + CO2 → CaCO3
It would be easy to determine that this is a combination
reaction... but how can the behavior of CaO be
predicted?
Strategies to Help
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Focus on gathering information about the
reactants so we can determine the kinds of
reactions they will undergo and better predict
products.
Learn strategies that can help to incorporate large
amounts of information into our existing knowledge
base.
First… Rethink Memorization
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Rote memorization has distinct disadvantages.
It doesn’t allow you to apply knowledge in a variety of
situations.
When thinking of ways to study, it’s essential that
facts not remain isolated but that there is an
understanding the same principles may apply in
more that one case.
Memorization –
Acquiring a Knowledge Base
•The Purpose of Memorization, is to allow you to acquire
a working knowledge base that can be readily recalled so
that you can use your mental energy to focus on higher
level problem solving.
•In order to readily recall and remember information
IT MUST BE ORGANIZED IN A WAY THAT IS
• Associated with your existing knowledge
• Grouped so that it can be readily used
To Help You to Learn How to Do This
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Reactants can be organized into broad categories,
and breaking them down according to
element/compound recognition (when looking at the
reactants).
Key tools you will need mainly: solubility guidelines,
acid/base lists, the reactivity series, must also be
memorized.
Reactions By Reactant
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Starting from the reactions that are most numerous,
and working towards narrower categories.
1) Simple Reactions Involving Metals
2) Non-metal Reactions – not organic
3) Acid Base Reactions
4) Complex Ions
5) Non-trivial Redox
REMEMBER!!!
There are 3 things that drive reactions forward:
1) Formation of a Gaseous product
2) Formation of water
3) Formation of an insoluble precipitate
Reactions of Elemental Metals
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Active metals react with cold water, steam, and
non-oxidizing metals to varying degrees.
These are single replacement reactions
The table on page 153 in your text clearly shows
this trend
Reactions of Elemental Metals
 The most active metals (K, Na, Li, & Ca), will react
with cold water to produce a metal oxide and
hydrogen gas.
Small chunks of Sodium are added to cold water.
2Na + 2H2O → 2NaOH + H2
Net Ionic
2Na + 2H2O → 2Na+ + 2OH- + H2
Describe a test to confirm the gaseous product in
your reaction: Hold a match to a flask containing
a small sample and listen for a pop.
Sodium and Water
Source 1
The flame is Hydrogen Gas which has
ignited due to the heat produced by the
reaction.
Reactions of Elemental Metals
 The active metals from Al to Iron in the reactivity
series will react with steam to produce the metal oxide
and hydrogen gas.
Steam is passed over solid magnesium.
Mg + H2O  MgO + H2
Reactions of Elemental Metals
 Active to less active metals, (Li to Pb on reactivity
series), will react with non-oxidizing acids to produce
the salt of the metal and Hydrogen gas.
Calcium Metal is added to a dilute solution of
Hydrochloric Acid.
Ca + HCl → CaCl2 + H2
Net Ionic: Ca + 2H+ → Ca+2 + H2
Give at least one visual observation for this reaction:
The release of hydrogen gas would cause bubbling.
Reactions of Active Metals with O2
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SLPMg ='s 4 active metals: Sodium, Lithium,
Potassium and Magnesium (only magnesium is
stable in air and has to be ignited)
Lithium Metal is burned in air.
Besides Combustion, what type of reaction could this be
classified as?
4Li + O2 → 2Li2O
This is a synthesis, or combination reaction.
Reactions of Active Metals with O2
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SLPMg will burn in Oxygen to produce metal oxides
that will all become strong bases when dissolved in
water.
These are combustion reactions and Synthesis
reactions (combination), and redox reactions
Mg being
ignited
Mg Burning
(Source 1)
Less Reactive Metals + O2
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Less reactive metals will also react with Oxygen to
form oxides, however they do so without combustion.
Six commonly react with unheated oxygen, each
more slowly from Al, Zn, Fe, Sn, Pb, Cu.
Rust - Iron Oxide
Oxidized Copper
Less Reactive Metals + O2
Iron is heated and allowed to react in the presence of
excess oxygen.
3Fe + 2O2  Fe3O2
The word excess in the question indicates that you should give Iron the
higher oxidation state.
What is the oxidation state of the Iron in the resulting
compound.
The Iron has adopted a 3+ charge.
Least Active Metals
Hg, Ag, Pd, Pt and Au are the least reactive metals.
 If they occur as oxides, they will readily decompose if
heated.
 Gold and Platinum are
the least reactive, and
won’t form oxides.
Since gold does not
Oxidize these gold
Artifacts from Egypt to
Survive thousands of
years.
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Reactions of Group 1 + 2 Metals
With Nitrogen
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You may remember that Nitrogen as
generally nonreactive due to its triple
bond.
Nitrogen does react with certain main
group elements, but may require intense
heat to provide the energy needed to drive
the reaction.
Reactions of Group 1 + 2 Metals
With Nitrogen
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Group 1 - only Lithium reacts with Nitrogen at
room temperature to yield Li3N.
6Li + N2  2Li3N
Group 2 - Mg, Ca, Ba and Sr will all react with
nitrogen to produce compounds, but must be
strongly heated. In all cases, the Nitrogen will
adopt a -3 oxidation state.
Example Question
Magnesium Metal is added to nitrogen gas.
3Mg + N2 → Mg3N2
What is the oxidation number of Magnesium before and
after the reaction?
No Net Ionic equation is necessary. In this case since the
magnesium is elemental its Oxidation state is initially 0
and goes to a +2.
Decomposition of the Products
6Li + N2 → 2Li3N (salt like solid)
 Decomposes to:
Li3N + 3H2O → 3Li+ + OH- + NH3
 3M + N
2 → M3N2 (s) – where M= Mg,Ca, Ba, Sr
 Decomposes when heated to:
M3N2 (s) → 3M (s) + N2(g)
 Or the Metal Nitride can react with water to
produce ammonia and the metal hydroxide
M3N2 (s) + 6H2O (l) → 3M(OH)2 (aq) + 2NH3 (g)
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Reactions of Metal Oxides
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Metal Oxides that are soluble, will react with water to
form hydroxides.
Calcium metal is heated strongly in the presence of
oxygen.
2Ca + O2 → 2CaO
If the product of this solution is added to water is the
resulting mixture acidic or basic?
2CaO + H2O →2Ca+2 + 2OHThe formation of hydroxide indicates this solution is
basic.
Reactions of Metal Oxides
 Metal Oxides that are soluble, will react with
water to form hydroxides.
Solid Lithium Oxide is added to excess water.
Is the final solution acidic, basic or neutral?
Li2O + H2O  2LiOH
Net ionic: Li2O + H2O  2Li+ + OHThe resulting solution is basic.
Example Questions
Solid Barium Oxide is added to distilled
water.
Is the resulting solution acidic basic or
neutral?
Reactions of Metal Oxides
 Metal oxides will react with non-metal oxides to
form salts.
Solid Calcium Oxide is exposed to a stream of
Carbon Dioxide gas.
CaO + CO2  CaCO3
What type of reaction has occurred?
Synthesis/Combination
** notice that this forms an insoluble salt
Example Questions
Sulfur Dioxide gas is passed over Solid
Calcium Oxide.
Name the product compound.
Example Questions
A mixture of solid Calcium Oxide,
And solid Tetraphosphorus Decaoxide is heated.
Is the product compound soluble in water?
CaO + P4O10 → Ca + 4P + 11O
There is the potential to make phosphate and therefore
get an insoluble product
• 6 calcium oxides and 1 tetraphosphorous
decaoxide make exactly 2 calcium phosphates.
6CaO + P4O10 → 2Ca3(PO4)2
Reverse Reaction
 Carbonates will decompose back into metal
oxides and carbon dioxide when roasted
(heated).
CaO + CO2  CaCO3
CaCO3 + HEAT CaO + CO2
Reactions of Metal Oxides
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Metal Oxides will react with acids to produce water and
a salt of the metal...this is like a version of a
neutralization reaction.
CaO + 2HCl  CaCl2 + H2O
Net Ionic: CaO + 2HCl  Ca+2 + Cl- + H2O
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From your packet: “A metal oxide is, in a real sense, a
basic anhydride, i.e. a base with the water removed.”
So an acid plus a basic anhydride = neutralization
Reactions of Metal Hydrides
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Typical metal hydrides are those formed from the
reaction of a active group one or two metal, and
hydrogen gas.
In a metal hydride, hydrogen will carry a -1 charge,
instead of +1. You will notice that in hydrides H is not
in front, but follows the metal cation, since here it is
an anion.
Reactions of Metal Hydrides
Metal hydrides react with water to produce the metal
hydroxide, which you will recognize as a strong base, and
hydrogen gas will be evolved.
Solid Lithium Hydride is added to distilled water.
LiH + H2O → LiOH + H2
Net Ionic: LiH + H2O → Li+ + OH- + H2
Is the final solution acidic, basic or neutral?
The resulting solution would be basic.
Reaction Reversal
 Metal hydroxides may then decompose
into metal oxides and water
Zn(OH)2 → ZnO + H2O
Other Patterns - Decomposition
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Chlorates decompose into metal chlorides
and oxygen gas
Alkali and alkaline nitrates decompose
into nitrites and oxygen gas
Example Question
Solid Potassium Chlorate is heated in the presence of
manganese dioxide as a catalyst.
How many moles of reaction products can be produced
from one mole of potassium chlorate.
Difficulties
One of the more difficult types of questions are those
that demand recall of a particular reaction in terms
of physical observations.
 For example:
Aluminum Metal is added to a solution of Copper (II)
Chloride.
List at least 2 observations that indicate a chemical
reaction is occurring.
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Working through it
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Single replacement, double replacement, redox??
Well sort of all of the above.
We can get credit for at least half this equation
because we know our reactivity series and we know
Aluminum is more active than copper so:
Al + CuCl2 → AlCl3 + Cu
Balanced: 2Al + 3CuCl2 → 2AlCl3 + 3Cu
Net Ionic: 2Al + 3Cu2+ → 2Al3+ + 3Cu
Now What?
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What if you've never done this reaction?
Some information that can help: If you would like
you can memorize the colors of some transition
metal ions. In water they form complex ions, which
give them colors...
Colors of Complex Ions
That's one clue...
If you know Copper is being reduced to Copper
metal, its complex ion will come out of solution, so
the blue/green color of the solution will fade.
 Now think harder, where is the copper going? Well
it's being converted back to copper metal, which is a
reddish color, so we could take a stab, and say there
will be a red precipitate.
 Third... if you would like to see this for yourself, this
reaction is most definitely exothermic go to:
http://www.youtube.com/watch?v=OQPwPGDQqzs
 So you could also say, heat is given off.
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Basic Comparison of Active
Metals
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If the two metals are in the same group, the one in
the lower position will be more reactive.
If the two metals are in the same period, the one
further left will be more reactive.
Transition metals can be memorized as a list, often
people use a mnemonic device, using the first letters.
Good Old Precipitates
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Knowing your solubility guidelines can really give you
a lot of insight into reactions.
A question that says both compounds are in solution
can be written out in ionic form, where a pair of
suspected precipitators can be identified.
Additionally, a lot of complex ion reactions, involve
the dissolving of an otherwise insoluble compound,
using a strong base or acid, another clue!
Example Question
Solutions of Silver Nitrate and Sodium Chromate are
mixed.
What is the oxidation number of the chromium in the
chromate ion?
• Write your reaction and balance
• 2AgNO3 + Na2CrO4 → Ag2CrO4 + 2NaNO3
• Now you must write is it in net ionic form:
• 2Ag+ + CrO4-2 → Ag2CrO4
• The Oxidation number of chromium in chromate is
always the same, so even if you struggled with reaction,
you should be able to say that this chromium is a +6
Example Question
Solutions of Manganese (II) Sulfate and Ammonium
Sulfide are mixed.
List any precipitate that forms during this reaction.
Example Question
A solution of copper (II) chloride is added to a solution
of sodium sulfide.
Name the spectator ions in this reaction.
Reactions of Non-Metals
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Organic reactions are specific and have their own
section.
Non-metals and oxygen react to form non-metal
oxides, limited oxygen might produce a monoxide
(like Carbon monoxide), excess produces the dioxide.
2 non-metals can undergo Combination/synthesis
and form molecular compounds.
Reactions of Non-metals
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Non-metal oxides and water form ternary acids.
ie. Dinitrogen pentoxide and water form?
Carbon Dioxide and water form?
Reverse Reaction: Ternary acids decompose into
non-metal oxides and water.
Answers
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Nitric Acid
Carbonic Acid
These are synthesis/combination reactions
so all the reactants are in the product, acid
should be written in ionic form because
they are soluble.
Individual Reactions
Unfortunately some characteristic reactions do need
to be recognized and memorized.
 Carbon disulfide has a characteristic reaction with
oxygen to produce Carbon Dioxide and Sulfur
Dioxide.
CS2 + O2 → CO2 + SO2
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Other Patterns - Decomposition
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Some ammonium salts decompose to lose ammonia.
Ammonium salts containing nitrate or dichromate
decompose to produce an oxide, water and nitrogen
gas. The N or Cr undergoes reduction.
Ex.
Solid ammonium carbonate is heated.
Classify the reaction type.
Yields 2 ammonia, Carbon dioxide, and water. Heat is your clue that this is
decomposition. Remove ammonia, and also carbonates decompose to
carbon dioxide, the remaining elements form water.
Other Patterns - Decomposition
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Hydrogen peroxide decomposes in the
presence of a catalyst to water and
oxygen
Ex.
A solution of Hydrogen Peroxide is
catalytically decomposed.
Name the element being reduced and the
element being oxidized.
This needs to be balanced, two hydrogen peroxide yields two
water and one oxygen molecule.
Example Question
A solution of Hydrogen Peroxide is catalytically
decomposed.
Name the element being reduced and the element being
oxidized.
Recommended Practice
 Create your own way to study the reactivity series,
either as a visual or a memory trick. Be prepared to
hand in a copy.
 Create your own way of representing the solubility
rules, you can use the table example given in class, but
make sure it includes all the precipitates you are
responsible for.
 Reaction Schematic – Organize all the reactions
covered so far into a graphic format, like a table or
flow chart. Make connections!!
Homework
 Complete the reaction worksheet
Sources and Links
 1) University of Siegen, Germany – cool chemistry photo
page - http://www2.uni-siegen.de/~pci/versuche/english/v44-1-1.html