Transcript Conservation of Mass in Chemical Changes

The Law of Conservation of Mass:
The total mass of the reactants must equal
the total mass of the products
Total mass of reactants = Total mass of products
This makes sense because atoms
cannot be created or destroyed
Atoms present at the beginning of
the reaction must still be present
after the reaction has taken place
How??
Atoms rearrange themselves,
bonding to new atoms to make a
different product
So when you write a chemical equation…
The number of atoms of each type must be the
same on each side
Equations must be balanced!
reactants
products
Writing Chemical Equations
• A CHEMICAL EQUATION is
used to represent a
chemical reaction
• E.g. oxygen and hydrogen in
a balloon react to produce
water
• There are 3 forms of
chemical equations that we
can use to represent a
reaction
1. Word Equations
reactant
reactant
hydrogen + oxygen
and
product
water
reacts to produce
2. Skeleton Equations
H2 + O2
H2O
3. Balanced Chemical Equations
2H2 + O2
2H2O
A COEFFICIENT is a number that is placed in
front of a chemical formula in a balanced
chemical equation
The only way to balance a
chemical equation is to change
the coefficients.
If you change a subscript, you
will change the identity of the
substance
H2 + O 2
H2O2
hydrogen peroxide
Tips for Balancing Equations
 Remember the diatomic molecules: H2,
N2, F2, Cl2, Br2, I2 and O2 (Remember
“HOFBrINCl”)
 Make sure your chemical formulas are
correct
 Do a final check by counting atoms of
each element
Example
• A Bunsen burner works
when methane gas
burns in oxygen to
produce carbon dioxide
and water.
• Write the balanced
chemical equation for
this reaction
Step 1: Write the word equation
products
Methane + oxygen  carbon dioxide + water
reactants
Step 2: Write the skeleton equation
• Replace each name with its formula
Methane + oxygen  carbon dioxide + water
CH4 + O2  CO2 + H2O
Step 3: Count atoms
• Count the number of atoms
of each type in reactants
(left side) and products
(right side)
CH4 + O2  CO2 + H2O
Type of Atom
Reactants
Products
Balanced?
C
1
1

H
4
2

O
2
(2+1) = 3

Step 4: Balancing
• Multiply each compound by the appropriate
coefficients to balance the number of atoms
(do NOT change subscripts)
 Balance compounds first and elements last
 Balance hydrogen and oxygen last
 If a polyatomic ion appears in both a
reactant and a product, think of it as a single
unit
• Trial and error (be patient )
Step 4: Balancing
CH4 + O2 -- CO2 + 2H2O
Type of Atom
Reactants
Products
Balanced?
C
1
1

H
4
4

O
2
(2+2) = 4

Remember when you put a coefficient in front of
a compound, ALL atoms are multiplied!!
Step 4: Balance
CH4 + 2O2  CO2 + 2H2O
Type of Atom
Reactants
Products
Balanced?
C
1
1

H
4
4

O
4
4

Human Balancing Act
Sodium
+
Chlorine Gas

Table Salt
Na (s) + Cl2 (g)  NaCl (s)
2 Na (s) + Cl2 (g)  2 NaCl (s)