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Synthesis, Decomposition & Combustion
Mr. Shields
Regents Chemistry
U11 L05
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Reaction Types
Chemical reactions occur all around us
everyday both in industry and in the home.
Unimaginable numbers of these unique and
specific reactions exist.
With so many reactions occurring in our lives the
Chemist needs a way to help organize them into some
sort of manageable scheme.
There are 5 general types of chemical reactions that
Make up this scheme
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Reaction Types
Combustion
A+BAB
ABA+B
ABC+D
CH4+O2CO2+H2O
A+BCAB+C
General reaction types
AX+BYAY+BX
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Synthesis
Synthesis is also known as direct combination; it is the
combining together of several elements and/or
compounds into one new compound.
Synthesis takes the general format:
+
A+BAB
One product
There can also be more
+ than 2 elements and/or compounds
That combine together. For example….
A+B+CABC
One product
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Synthesis
Here’s some examples of synthesis:
2Fe + 3O2  2Fe2O3 (red Ochre; rust)
2Mg + F2  2MgF2
C + Cl2 + F2  CCl2F2
(Freon)
2H2O + Cl2  2H2CLO (hypochlorus acid)
2H2 + O2  2H20
In many of these reactions heat and light
may also be released.
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Decomposition
What is a Decomposition reaction?
A breaking apart of a larger molecule into
several smaller molecules and/or elements
ABC+D
ABA+B
Ex: H2CO3  CO2 + H20
Ex: HgO  Hg + 02
Can you think of an example where
Decomposition reactions are
extensively utilized?
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Decomposition Examples
1. Explosions (the rapid evolution of gases and heat)
2. H2O2 Decomposition when poured on a wound
- The bubbles are O2
2H2O2  2H20 + O2 (catalyzed by the Fe in
hemoglobin)
3. Another example is the decomposition of limestone
when heated
CaCO3 + heat  CaO + CO2 (g)
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Decomposition Examples
4. Another example of a decomposition reaction is one
Involving Mercury (II) sulfide. It is used to produce
liquid mercury.
2HgS + heat  2Hg + S
(cinnabar)
So what’s common in all these decomposition reactions?
A more complicated molecule breaks down into several
Simpler molecules and/or elements
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COMBUSTION
CH4+O2  CO2+ H2O
+ HEAT & LIGHT
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Combustion (definition):
A chemical process that involves the reaction of an
Organic Compound with OXYGEN producing several
Specific substances as well as heat and light
AN UNCONTROLLED COMBUSTION REACTION
Bitterroot Montana - 2000
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Complete combustion reaction takes the form
C2H6 + O2  CO2 + H2O
Examples of reactions that looks like
Combustion but is not…
Reactions such as these
Will often release heat
2Mg + O2  2MgO
And light
4Fe + 3O2  2Fe2O3
4Na + O2  2Na2O
Mg
H2CO3  CO2 + H2O
+
O2
The 1st three are direct synthesis
Because there is just one product
Formed. The last is decomposition.
Heat + Light
NOT COMBUSTION
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Typically, COMBUSTION is used to describe the
Reaction of O2 and some Organic compound.
When there is Sufficient O2 and the organic compound is a
simple hydrocarbon there will be COMPLETE COMBUSTION
The reaction products will be just H20 and CO2
If there is insufficient Oxygen present then
there is INCOMPLETE COMBUSTION and
the products are more varied.
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Ex:
* Incomplete combustion:
CH4 + O2  H2O + CO2 + CO + C
(not balanced)
* Complete combustion:
What’s this ?
2C6H14 + 19O2  14H20 + 12CO2
This is not synthesis; more than
1 product is formed
Always 2 products for complete combustion:
Water and Carbon Dioxide
1 2
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Problems
What are the following reaction types?
2Na + S  Na2S
H2CO3  H20 + CO2
Na2O + H2O  2NaOH
2C2H6 + 7O2  4CO2 + 6H2O + Heat
4Na + O2  2Na2O + Heat
3C02  C + 2CO + 2O2
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