1. Balancing Chemical Equations
Chapter 8
Balancing Chemical Equations

2. 8.1 Chemical Equations
Chemical equations represent, with symbols and formulas, the reactants and products in a chemical reaction.
reactants  products
Requirements for all chemical equations:
must show all reactants and products
formulas must be correct
Law of Conservation of Mass must be satisfied (equation must be balanced)

3. Chemical Equations Balanced Equations – use coefficients
H Cl2  2 HCl
Hydrogen reacts with chlorine to yield hydrochloric acid
Symbols used in equations
2NaCl(s)  2Na(s) + Cl2(g)
NaCl(aq) - solution of sodium chloride in water
H2O(l) – liquid water
g or ↑ = gas

4. Chemical Equations More symbols  = one way reactions
↔ = reversible reaction
ΔH = heat
cat = catalyst (a substance that speeds up a reaction without being used up in the reaction)
S or ppt or ↓ = precipitate (solid - only found on products side)

5. Chemical Equations Significance of chemical equations
H2(g) Cl2(g)  2 HCl (g) means…
Atoms: 2 atoms of hydrogen gas react with 2 atoms of chlorine gas and yields 2 atoms of hydrogen, 2 atoms of chlorine
Molecules: 1 molecule of hydrogen gas reacts with 1 molecule of chlorine gas and yields 2 molecules of hydrochloric acid
Molar mass: H = 2.0; Cl = 71.0; HCl = 73.0

6. Balancing Chemical Equations
Helpful hints to balancing….
1 atom at a time
Balance atoms that appear only 1X per side first
Balance polyatomic ions as whole units
Balance diatomic elements last
Save H + O for last

7. Balancing Chemical Equations
Examples
__H2O(l)  __ H2(g) +__O2(g)
__(NH4)2CO3 (aq)+__CaCl2(s)  __CaCO3(s) +__NH4Cl(aq)

8. Balancing Chemical Reactions
__Al(s) + __Br2(g)  __AlBr3(s)
__C2H5OH(g) + __O2(g)  __CO2(g) +__H2O(l)

9. Balancing Chemical Reactions
NOT IN PACKET!!
_C3H6 + _ O2  _ CO2 + _H2O

10. Balancing Chemical Equations
Tin (IV) oxide + Carbon  Tin + Carbon monoxide
SnO2(aq) + C(s)  Sn(s) + CO(g)
SnO2(aq) + 2C(s)  Sn(s) + 2CO(g)

11. Balancing Chemical Equations
Aqueous Iron (III) Chlorate reacts with solid calcium to yield calcium chlorate and solid iron
Fe(ClO3)3(aq) + Ca (s)  Ca(ClO3)2(aq) + Fe(s)
2Fe(ClO3)3(aq) + 3Ca (s)  3Ca(ClO3)2(aq) + 2Fe(s)

12. 8.2 Types of Chemical Reactions
synthesis
decomposition
combustion
single replacement
double replacement
Remember: You must first find the correct products, then balance the equation!

13. Types of Chemical Reactions
Synthesis (direct combination) - needs energy to happen (usually heat)
General formula
A B > AB
2H O2 → H2O

14. Types of Chemical Reactions
Synthesis Examples
Ba + S 
Mg + Cl2 
Al + Cl2 
Na + O2 
REMEMBER TO BALANCE!!!!!!!!!!
BaS
MgCl2
AlCl3
Na2O

15. Types of Chemical Reactions
Decomposition (analysis) - needs energy to happen (usually heat or electricity)
general formula
AB > A + B
2H2O → 2H O2

16. Types of Chemical Reactions
Examples
FeCl3 
HgO 
CuSO4 · 5 H2O 
Fe + Cl2
Hg + O2
CuSO4(s) + H2O(g)

17. Types of Chemical Reactions
Combustion - the reaction of hydrocarbons and oxygen
General Formula
CxH y O2  CO H2O

18. Types of Chemical Reactions
CH O2 → CO H2O
C3H O2 → CO H2O
C4H O2 → CO H2O
C2H5OH + O2 → CO H2O

19. Types of Chemical Reactions
Single replacement - take place in aqueous solutions - need very little energy to happen
Two Types
Positive Ions Switch
AB + M  MB A
Negative Ions Switch
MB + X  MX B

20. Types of Chemical Reactions
Positive Ions Switch
HI(aq) + Mg(s) 
AlCl3(aq) + Ca(s) 
Ca(s) + HOH(l) 
MgI2(aq) I2(s)
CaCl2(aq) + Al(s)
Ca(OH)2(aq) + H2(g)

21. Types of Chemical Reactions
Negative Ions Switch
NaCl + F2 
BaS + O2 
NaF + Cl2
BaO S

22. Types of Chemical Reactions
Double Replacement - aqueous solution - little energy - usually forms one soluble ionic product (aka - aqueous) and either a ppt, water, or a gas that bubbles out of water
General Formula (molecule + molecule)
AB + CD  CB + AD
AgNO3 + NaCl → AgCl NaNO3

23. Types of Chemical Reactions
FeCl3 + NaOH 
H2SO4 + NaOH 
NH4Cl + NaOH 
Fe(OH)3 + NaCl
HOH + Na2SO4
NH4OH + NaCl

24. 8.3 Activity Series of the Elements
Another Lie!! : )
Some reactions happen and some don’t!
Assume all synthesis, decomposition, and combustions happen
Not all single or double displacement occur
Single Replacement
Use activity series
Double Replacement
Use solubility table

25. Activity Series of the Elements
Rules for the single replacement activity series:
Any single element above an element in a compound will replace it.
The top 5 elements react with water.
Metals above H react with acids (molecules that start with H – not water).
The nonmetal reactivity series is F> Cl > Br...

26. Activity Series of the Elements
Ca + H2O →
Al + H2O →
Al + HI →
Ca(OH) H2
No Rxn
AlI H2

27. Activity Series of the Elements
Cu + HI 
NaCl + F2 
NaF + Cl2 →
No Rxn
NaF + Cl2
No Rxn

28. Solubility Table of the Elements
Rules for double replacement reactions using a solubility table:
If one of the products formed is water, the reaction happens.
If a gas is formed, the reaction happens.
If an insoluble product forms (I or Ss), the reaction happens

29. Solubility Table of the Elements
Na2CrO4 + KCl →
FeCl KOH →
HCl + NaOH →
No Rxn
Fe(OH)3(ppt) + KCl(aq)
H2O(l) + NaCl(s)

30. Solubility Trends Cations Anions - General trend -
very soluble - Na, K, ammonium
very insoluble- Ag, Pb, Hg, transitions
Anions -
very soluble – nitrate
for monatomics- F>Cl>Br ....
very insoluble - carbonate, hydroxide, phosphate, sulfate
sulfides - decompose
General trend -
As size decreases, solubility increases OR
Heavier stuff doesn’t dissolve well

31. Showing Energy Changes in Equations
endothermic -
A + B + heat  C
ΔH is positive
exothermic -
A + B  C + heat
ΔH is negative

32. Some helpful notes on writing phases in chemical reactions
Metals are solids (except Hg, Br)
SR and DR reactions, reactants that are compounds are always aqueous.
SR and DR reactions, products that are compounds should have their phases identified using a solubility chart (aqueous vs. precipitate)
S and D reactions, ionic compounds are solids.
In C reactions, the water, CO2, and O2 are gases. The hydrocarbon is hard to tell, but is usually a liquid after C=6 or higher.
Most other covalent compounds are gases.
Acids (chemicals starting with hydrogen) are always aqueous