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CP Chem. Overview Unit 4
Balancing Chemical Equations
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The first reaction we did can be shown in the following way in words:
Zinc plus hydrochloric acid form hydrogen gas and dissolved zinc chloride
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In symbols this would be:
Zn(s) + HCl(aq)  H2(g)
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+ ZnCl2(aq)
The second reaction(rxn) we did would be this:
Hydrogen plus oxygen in the presence of a spark forms water
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In symbols this would be:
H2(g) + O2(g)
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
H2O(g)
Several things of note:
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Diatomic molecules
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Reactants
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Products
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State symbols
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Reaction arrow
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Catalyst
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The thing of real importance is the fact neither of these reactions are balanced
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How do we balance equations so they follow the Law of Conservation of Mass
We balance equations by changing coefficients………
NEVER by changing subscripts !!!!!
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So now we can correctly balance the above two equations…
Summary on balancing:
1)
2)
3)
4)
5)
EX:
Determine the correct formulas for all the reactants and products
Write a skeleton equation
Count atoms on both sides (polyatomics can be counted as one if they are on both sides)
Balance using coefficients (save H & O for last if they’re present)
Check balancing and make sure coefficients are in the lowest ratio
Types of reactions
I.
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Combination
Here two or more reactants come together to make a single product
EX:
II.
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Decomposition
Here one reactant “breaks apart” to form two or more products
EX:
III.
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Single Replacement
Here an element replaces a element that is part of a compound
“element plus compound forming an element and a compound”
Whether this reaction takes place is determined by the activity series (pg 217)
EX:
IV.
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Double Replacement
Here atoms or ions from different compound replace each other
“compound plus compound forms a compound plus a compound”
EX:
IV.
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Combustion
an element or compound reacts with oxygen
complete combustion of a hydrocarbon produces carbon dioxide and water
incomplete (low oxygen present) produces carbon monoxide and water
EX:
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