1. Law of Conservation of Mass Balancing Equations
Chemical Reactions
Law of Conservation of Mass
Balancing Equations

2. Law of Conservation of Mass
Mass of Reactants: g
Mass of Products: g
Mass of Products: ??? g

3. The Law of Conservation of mass states that mass cannot be created or destroyed in a reaction.
This applies to MASS
This also applies to EACH TYPE OF ATOM

4. Law of Conservation of Mass
16 g x +
64 g  +
36 g
= 44g
16 grams of methane gas reacts with 64 grams oxygen gas to produce carbon dioxide and 36 grams of water. How much carbon dioxide is produced in this closed system?

5. Law of Conservation of Mass word equations
Ex. 1 When 72 grams of water is decomposed, 8 grams of hydrogen gas is made along with some oxygen gas. What mass of oxygen gas is made? Ex. 2 Some carbon was reacted with 4 grams of hydrogen gas to make 16 grams of methane. What mass of carbon was used?

6. Counting Atoms in Compounds
The subscript tells you how many atoms are in a compound.
C3H5 = 3 Carbons, 5 Hydrogens
When there is a polyatomic ion, multiply each element by the subscript!
Pb(SO4)2 = Pb – S – O – 8

7. Counting Atoms with Coefficients
When you add a coefficient in front of a compound, you multiply EACH atom by that number.
2 AlCl3
2 Pb(NO3)2
3 (NH4)2(CO3)3
Al = 2*1 = 2
Cl = 2*3 = 6
N = 3*2*1 = 6
H = 3*2*4 = 24
C = 3*3*1 = 9
O = 3*3*3 = 27
Pb = 2*1 = 2
N = 2*2*1 = 4
O = 2*2*3 = 12

8. What are word equations?
Equations that are written with common English wording and chemical names.
You must be able to translate between the two.
For example:
When sodium metal reacts with aqueous iron (II) chloride, iron metal and sodium chloride are formed.
Would be written as:
Na (s) + FeCl2 (aq)  Fe (s) + NaCl (aq)

9. Step 1: Write the unbalanced equation by translating the written names into chemical formulas
Put a “+” when it says something like:
and, reacts with, is mixed with, is combined with, burns in, is put in contact with, etc.
Put an “” when it says something like:
Yields, forms, reacts to form, makes, are produced, results in, is created, is made, gives off, bubbles out, etc.
Pay close attention to the order of:
REACTANTS (the before)
PRODUCTS (the after)

10. Step 2: Figure out the states of each of the chemicals in the equation
(s) means the chemical is solid
(g) means gas
(aq) means aqueous (dissolved in H2O)
(l) means a true liquid:

11. Ca(OH)2 (aq) + HCl (aq)  CaCl (aq) + H2O (l)
For Example:
When calcium hydroxide solution reacts with hydrochloric acid, dissolved calcium chloride and water are formed.
Would be written as:
Ca(OH)2 (aq) + HCl (aq)  CaCl (aq) + H2O (l)

12. Balancing Equations with Coefficients
You are making sure the same number of each type of atom is present BEFORE (reactants) and AFTER (products).
You cannot change SUBSCRIPTS
You can change COEFFICIENTS
Coefficients
These you can change
Subscripts
DO NOT CHANGE

13. Steps To Balance a Chemical Reaction
Balance elements except H and O.
Balance H.
Balance O 2 1
___ H2 + ___ O2  __ H2O 2 H O 4
2 =
2 =
= 2
= 2 4
2 =
= 1
= 1 2

14. The Polyatomic Ion Shortcut
It it looks the same on both sides, count as one thing instead of balancing elements individually.
If it changes or splits apart in any way, you can’t use this.
___ HNO3 + ___ Ca  __ Ca(NO3)2 + __ H2 2 1 1 1 H
(NO3) Ca 2
1 =
1 =
= 2 2
1 =
1 =
= 2
1 =
= 1

15. Balance and Classify the following equations in your notebooks.
Practice Problems-
_____C + _____O2  _____CO2
___Cu + ____AgNO3  ____Cu(NO3)2 + ____Ag
_____CH4 + _____O2  _____CO2 + _____H2O
___Ba(NO3)2 + __Na2SO4  __NaNO3 +__BaSO4
____ CaCl2  ____ Ca + ____ Cl2

16. Balance and Classify the following equations in your notebooks.
Practice Problems-
__1__C + __1__O2  __1__CO2
_1_Cu + _2__AgNO3  _1_Cu(NO3)2 + _2_Ag
__1__CH4 + __2__O2  __1__CO2 + __2__H2O
_1_Ba(NO3)2 + _1_Na2SO4  _2_NaNO3 +_1_BaSO4
__1__ CaCl2  __1__ Ca + __1__ Cl2