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The Question (Learning Goals) What is the Law of
Conservation of Mass (or atoms)?
What is a balanced Chemical Reaction?
How do we balance a chemical reaction?? (A Level)
Review: Chemical Reactions
Chemical Reactions Rearrange atoms and molecules. They do not create or destroy them!!
New Product atoms and old Reactant atoms are not created or destroyed. ATOMS ARE ALWAYS CONSERVED!!!
Reactants Products
to produce
Review: Chemical EquationsWhen a chemical reaction is written using chemical formulas and symbols, it is called a chemical equation.
All chemical equations have yields arrows!!
Conservation of Mass or Atoms
When chemicals are reacted in a closed system (container), you can show that the mass before and after the reaction is the same.
There must always, always, always be the same number of Reactant atoms and Product atoms for each type of atom!!
This concept is THE KEY CONCEPT!!!!
Conservation of Mass or Atoms
Lavoisier showed that a closed system must be used when studying chemical reactions.
Closed System: Is a “system” where nothing can come in or go out.
Conservation of Mass or Atoms
When chemicals are reacted in a closed system (container), you can show that the mass before and after the reaction is the same.
There must always, always, always be the same number of Reactant atoms and Product atoms for each type of atom!!
This concept is the KEY CONCEPT!!!!!
http://www.lghs.net/ourpages/users/dburns/scienceonsimpsons/Clips_files/ConservationofMass.m4v
Numbers in equations
2
CoefficientsTells you how many
of each type of reactant or product
molecule in the reaction.
2
Balancing EquationsThe combined mass of the burning wood and
oxygen is converted into carbon dioxide and water.
The law of Conservation of Atoms (or Mass) tells us that there must be the same number of atoms on each side of the chemical equation!!!!
Rules for Balancing Equations
1. Atoms of Reactants must equal atoms of Products.
2. Coefficients are used to balance chemical equations.
3. Never change a submarine-script.
Write down all the atoms under the “yields” arrow.
Write the number of each atom on the Reactant and Product side of the equation.
Does this obey the law of conservation of mass
If ALL of the numbers match there is no VIOLATION of the Law of Conservation of mass?
Does this obey the law of conservation of mass?
If not use Coefficients to balance the numbers.
Steps for Balancing Equations
2 Na + O2 → 2 Na2O
Na
O
Steps for Balancing EquationsWrite down all the atoms under the “yields” arrow.Write the number of each atom on the Reactant and
Product side of the equation.If ALL of the numbers match there is no VIOLATION of the Law of Conservation of mass?
Does this obey the law of conservation of mass?
If not use Coefficients to balance the numbers.
2 CH4 + O2 → 2 CO2 + H2O
C
H
O
Steps for Balancing EquationsWrite down all the atoms under the “yields” arrow.Write the number of each atom on the Reactant and
Product side of the equation.If ALL of the numbers match there is no VIOLATION of the Law of Conservation of mass?
Does this obey the law of conservation of mass?
If not use Coefficients to balance the numbers.
Al + 3 O2 → 2 Al2O3
Al
O
Steps for Balancing EquationsWrite down all the atoms under the “yields” arrow.Write the number of each atom on the Reactant and
Product side of the equation.If ALL of the numbers match there is no VIOLATION of the Law of Conservation of mass?
Does this obey the law of conservation of mass?
If not use Coefficients to balance the numbers.
2 NaClO3 → 2 NaCl + 3 O2
Na
Cl
O
Steps for Balancing EquationsWrite down all the atoms under the “yields” arrow.Write the number of each atom on the Reactant and
Product side of the equation.If ALL of the numbers match there is no VIOLATION of the Law of Conservation of mass?
Does this obey the law of conservation of mass?
If not use Coefficients to balance the numbers.
The Answer
What is a balanced Chemical Reaction?
One with equal numbers of atoms on each side of the “yields” arrow. (Matter is never created nor destroyed!!)
How do we balance a chemical reaction??
With coefficients!!
NaHCO3(s) + HCl (l) → NaCl (aq) + H2O (l) + CO2
(g)
1 Na 1
2 H 2
1 C 1
3 O 3
1 Cl 1
Steps for Balancing Equations
CH4 + 2O2 → CO2 + 2H2O
1 C 1
4 H 4
4 O 4
Steps for Balancing Equations
4 Al + 3 O2 → 2 Al2O3
4 Al 4
6 O 6
Steps for Balancing Equations
CH4 + O2 → CO2 + H2O
1 C 1
4 H 2
2 O 3
Steps for Balancing Equations
Al + O2 → Al2O3
1 Al 2
2 O 3
Steps for Balancing Equations
C7H17 + O2 → H2O + CO2
C
H
O
Steps for Balancing Equations
NaBr + Li3PO4 → LiBr + Na3PO4
NaBrPOLi
Steps for Balancing Equations
C7H17 + O2 → H2O + CO2
C
H
O
Steps for Balancing EquationsWrite down all the atoms under the “yields” arrow.Write the number of each atom on the Reactant and Product side of the equation.Do they match? If not use Coefficients to balance the numbers.
NaBr + Li3PO4 → LiBr + Na3PO4
NaBrPOLi
Steps for Balancing EquationsWrite down all the atoms under the “yields” arrow.Write the number of each atom on the Reactant and Product side of the equation.Do they match? If not use Coefficients to balance the numbers.
NaHCO3(s) + HCl (l) → NaCl (aq) + H2O (l) + CO2 (g)Na
H
C
O
Cl
Steps for Balancing EquationsWrite down all the atoms under the “yields” arrow.Write the number of each atom on the Reactant and Product side of the equation.Do they match? If not use Coefficients to balance the numbers.
