1. Balancing Chemical Equations

2. Evidence of a chemical reaction
The temperature may change (but could be from adding or removing energy)
Different coloured materials may form (precipitate)
New phases may be formed (precipitate/gas)

3. Conservation Laws In a closed chemical system, the reaction follows
Conversation of mass
Total mass does not change
Conversation of electrical charge
Total charge does not change
Conversation of energy
Total energy does not change
Conversation of atoms
Total number of atoms does not change
What we use to balance chemical equations

4. Some terms to know
Chemical reaction equation – equation shown during a chemical reaction
Reactants – starting chemicals
Product – resulting chemicals
Coefficients – refer to the number of molecules in each species in the reaction
System – The part of the universe being studied
Closed – Nothing can enter or leave system
Open – things can enter or leave system

5. Some terms to know
Conserved – The quantity does not change during reaction
Only possible in a closed system
Why do we a closed system?
We must our reaction follow the conservation laws
Or else we will have skewed results

6. Coefficients
We put coefficients in front of chemical formulas to state how many molecules of each species are present
We put coefficients to balance our atoms
Reactant atoms must equal product atoms
You have done some balancing in grade 10

7. Strategy - 1
Scan the equation and balance ELEMENTS that only occur ONCE on both sides of our equation.
Metal atoms usually show up once on both sides, so these should usually be your starting point
If there are no metal atoms, such as an organic reaction, you should balance anything that is not H or O first.
We usually balance H and O last as they show up multiple times.

8. Strategy - 2
After balancing the first element, there will usually be another element attached to the element you balanced
Balance that other element
Repeat until you have balanced everything except for H and O
Balance H and O LAST!

9. Strategy - 3 Assume that if there are no coefficients, it is ZERO.
If you get comfortable, you can forget this step.
This step is used so that you have something to balance with if you write 1.

10. Strategy - 4 Balance polyatomic ions as one single group of atoms.
Not always the case, but 95% of the time.
Do not balance it as individual atoms.
So assume PO4, SO4, NH4, NO3, OH etc etc are all a single element and balance them accordingly.

11. Strategy - 5
You can use fractions/decimals when you are nearing the end and something cannot be balanced with whole numbers.
So you can put ½ or any fraction/decimal in front of a chemical species
This is usually done for diatomics as you may have 3O on one side and 1O2 on the other side. You put 3/2 or 1.5 in front of the O2 to balance it for now.

12. Strategy - 6
If you do put fractions/decimals, you must multiply everything to get whole numbers.
Multiply by the LCD to get rid of denominators
So if it is ½, and the LCD is 2. Multiply by 2 and the fraction becomes 1.

13. Strategy - 7 You can remove the 1’s at the very end to make it neat.
At this point, we can ASSUME that any blanks are 1
Remember, if you get comfortable, you can skip strategy 3 and not assume it is ZERO step.

14. Strategy - 8 We need to add the STATE into our chemical equation
The state will tell us if something is solid, liquid, or gas.
We show this by subscripts with parentheses.
(s) for solid, (l) for liquid and (g) for gas
2H2(g) + O2(g) → 2H2O(l)

15. Strategy - 8
Metals are usually solid (except mercury which is a liquid)
Non-metals can be solid, liquid, or gases
Bromine is the only liquid
Column are usually solid
Diatomic are gases except for bromine (liquid) and iodine (solid)
Non-diatomics are solids
Noble gases are gases

16. Diatomics
Diatomics are molecules that exist as two atoms of the same element join together in nature
There are 7 diatomics
H, N, O, F, Cl, Br, I
Two ways to remember
7 + H
HOFBrINCl

17. What about aqueous?
Aqueous means something is a solution and dissolved in the solvent water.
NaOH, HCl, HCl are aqueous as they are dissolved in water
Liquids are only for pure substances, aqueous is not a pure substance
NH3 and H2O are pure substances and are liquids
We use (aq) to represent this state
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

18. Example - 1 Balance the following H2S + PbCl2 → PbS + HCl
(NH4)3PO4 + NaOH → Na3PO4 + NH3 + H2O
C19H17NO3 + O2 → CO2 + H2O + N2
Cr2(SO4)3 + KI + KIO3 + H2O → Cr(OH)3 + K2SO4 + I2
MoCl3 + O2 + AgCl → MoCl4 + Ag2O

19. Practice - 1
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