1. Chemistry /10/14
“Mystery creates wonder and wonder is the basis of man’s desire to understand.” –Neil Armstrong
Do Now:
How many chorine atoms are in the following compound?:
4BaCl2
Agenda:
Students will be able to balance a chemical equation unassisted by taking notes & working in groups
Announcements:
Science club is meeting tomorrow after school!

2. Introduction
Chemical reactions occur when bonds between the outermost parts of atoms are formed or broken

3. Chemical Equations
Their Job: Depict the kind of reactants and products and their relative amounts in a reaction.
4 Al (s) O2 (g) ---> 2 Al2O3 (s)
The numbers in the front are called
Coefficients

4. Chemical Equations
Lavoisier, 1788
Because of the principle of the conservation of matter,
an equation must be balanced.
It must have the same number of atoms of the same kind on both sides.

5. Symbols Used in Equations
Solid (s)
Liquid (l)
Gas (g)
Aqueous solution (aq)
+ sign (combining)
→ ( yields or makes )
Reactants - left of arrow
Products- right of arrow

6. How molecules are symbolized
Cl Cl Cl2
Molecules may also have parenthesis to indicate numbers of atoms. E.g. Ca(OH)2
Notice that the OH is a group
The 2 refers to both H and O
How many of each atom are in the following?
a) NaOH
b) Ca(OH)2
c) 3Ca(OH)2 O H Ca
Na = 1, O = 1, H = 1
Ca = 1, O = 2, H = 2
Ca = 3, O = 6, H = 6

7. Balancing Equations
When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may not change the subscripts.
Changing the subscripts changes the compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent)

8. Subscripts vs. Coefficients
The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

9. Balancing equations: MgO
The law of conservation of mass states that matter can neither be created or destroyed
Thus, atoms are neither created or destroyed, only rearranged in a chemical reaction
Thus, the number of a particular atom is the same on both sides of a chemical equation
Example: Magnesium + Oxygen
Mg + O2  MgO O Mg +  Mg O
However, this is not balanced
Left: Mg = 1, O = 2
Right: Mg = 1, O = 1

10. Balance equations by “inspection”
From Mg + O2  MgO
2Mg + O2  2MgO is correct
Mg + ½O2  MgO is incorrect
Mg2 + O2  2MgO is incorrect
4Mg + 2 O2  4MgO is incorrect
Hints: start with elements that occur in one compound on each side. Treat polyatomic ions that repeat as if they were a single entity.
a) P4 + O2  P4O10
b) Li + H2O  H2 + LiOH
c) Bi(NO3) K2S  Bi2S KNO3
d) C2H O2  CO H2O 5 2 2 2 2 3 6
3.5 2 3
C2H O2  CO H2O 2 7 4 6

11. 1 1 1 2 1 2 2 2 1 1 2 2 4 4 3 2 1 2 1 K + H2O  H2 + KOH Reactants K K
Products K H O 2 1 1 2 2 4 4 3 2 1 2 1

12. Steps to Balancing Equations
There are four basic steps to balancing a chemical equation.
Write the correct formula for the reactants and the products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS!
Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side.
Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation.
Check your answer to see if:
The numbers of atoms on both sides of the equation are now balanced.
The coefficients are in the lowest possible whole number ratios. (reduced)

13. Some Suggestions to Help You
Some Helpful Hints for balancing equations:
Take one element at a time, working left to right except for H and O. Save H for next to last, and O until last.
IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an element)

14. Balance these skeleton equations:
a) Mg + HCl  MgCl2 + H2
b) Ca + N2  Ca3N2
c) NH4NO3  N2O + H2O
d) BiCl3 + H2S  Bi2S3 + HCl
e) C4H10 + O2  CO2 + H2O
f) O2 + C6H12O6  CO2 + H2O
g) NO2 + H2O  HNO3 + NO
h) Cr2(SO4)3+ NaOH  Cr(OH)3+ Na2SO4
i) Al4C3 + H2O  CH4 + Al(OH)3

15. Balance these skeleton equations:
a) Mg + 2HCl  MgCl2 + H2
b) 3Ca + N2  Ca3N2
c) NH4NO3  N2O + 2H2O
d) 2BiCl3 + 3H2S  Bi2S3 + 6HCl
e) 2C4H O2  8CO2 + 10H2O
f) 6O2 + C6H12O6  6CO2 + 6H2O
g) 3NO2 + H2O  2HNO3 + NO
h) Cr2(SO4)3+ 6NaOH  2Cr(OH)3+ 3Na2SO4
i) Al4C3 + 12H2O  3CH4 + 4Al(OH)3

16. a) Li + H2O ® H2 + LiOH
b) P4 + O2 ® P4O10
c) C2H6 + O2 ® CO2 + H2O
d) CS2 + O2 ® CO2 + SO2
e) AsCl3 + H2S ® As2S3 + HCl
f) AgNO3 + FeCl3 ® AgCl + Fe(NO3)3
g) KClO3 ®KCl + O2
h) SO2 + O2 ® SO3

17. a) 2Li + 2H2O ® H2 + 2LiOH
b) P4 + 5O2 ® P4O10
c) 2C2H6 + 7O2 ® 4CO2 + 6H2O
d) CS2 + 3O2 ® CO2 + 2SO2
e) 2AsCl3 + 3H2S ® As2S3 + 6HCl
f) 3AgNO3 + FeCl3 ® 3AgCl + Fe(NO3)3
g) 2KClO3 ® 2KCl + 3O2
h) 2SO2 + O2 ® 2SO3