4.2 Synthesis and Decomposition Reactions
5.3 Elements and their Oxides
Synthesis
Two reactants combine to form a larger more complex product
A + B --> AB
a) Metals + Non-metal --> ionic compound
Mg (s) + Cl2 (g) --> MgCl2 (s)
* Purple Haze Demo: http://www.youtube.com/watch?v=N5h5ohd8298
What has happened? Aluminum + iodine --> aluminum iodide solid (water is the catalyst)
Oxide - a compound of any element combined with oxygen (usually from air)
Special case: Metal + oxygen --> Metal oxide
Ex. sodium + oxygen -->sodium oxide (aka basic oxide why???)
Demo: Sodium oxide + water --> sodium hydroxide (place sodium into water under a blast shield then
test the products with red litmus and pH paper)
NOTE: anytime a metal oxide is placed in water, a BASE is made!
b) Non-metal + Non-metal --> molecular compound
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involving hydrogen
H2 (g) + F2 (g) --> HF (g)
easy to predict the products
not involving hydrogen are difficult to predict and require chemical tests to
verify the product produced (ex. carbon + oxygen can make CO or CO2
depending on amt of O)
Special Case: Non metal + oxygen --> non-metal oxide
Ex. C + O2 --> CO2 (aka acidic oxide why?)
Carbon dioxide + water --> Carbonic acid
sulfur trioxide + water --> sulfuric acid
nitrogen dioxide + water --> nitric acid
NOTE: anytime a non-metal oxide is placed in water, an ACID is made!
Decomposition
A larger more complex compound breaks down to form two or more simpler products
They usually need energy to get started (requires heat, electricity or a catalyst)
AB --> A + B
a) ionic compound --> element + element
2 KCl --> 2 K(s) + Cl2 (g)
DEMO: Hoffman apparatus - electricity split water into hydrogen and oxygen gas (do gas tests)
DEMO: Elephant toothpaste: 30% hydrogen peroxide (50mL) + 2g KI (catalyst) --> oxygen gas and water
(do on top of a piece of aluminum foil in a 500 mL erlenmyer flask . test water with cobalt chloride
paper --> pink, test oxygen with glowing splint --> relights)
****very hot, leave to cool*****
DEMO: Genie in a Bottle: 30% hydrogen peroxide (20 ml) + MnO2 (catalyst) --> oxygen + water (complete
the reaction in a 500 mL erlenmyer flask on top of a piece of aluminum foil)
****very hot, leave to cool****
b) compounds with polyatomic ions or molecular compounds are difficult to predict
2 KClO3 --> 2 KCl + 3 O2 (chlorates will release oxygen and make a metal chloride)
CaCO3 --> CaO + CO2
(carbonates will release carbon dioxide and a metal oxide)
DEMO: magnesium carbonate heated in a test tube over a Bunsen burner (2 min) test for carbon dioxide
(flaming splint dies out)
Homework: p. 161 #1-9
p. 204 #1-7