1. Chemical Equations
4.3

2. Conservation of Mass
A Chemical change means new compounds are created.
Law of Conservation of Mass: matter is neither created nor destroyed; atoms are just rearranged.
mass of reactants = mass of products
number of each = number of each
atom in reactants = atom in products
[If you could collect and measure all of the exhaust from this car, you would find that mass of reactants (gasoline + O2) = mass of products (exhaust)!]

3. Chemical Equations
Chemical Reaction when one or more chemical changes occur at the same time
Word Equation
Maltose + Oxygen Water + Carbon Dioxide
Reactants Products

4. Equations Chemical reactions can be written in different ways.
here, there is twice as much NO and NO2 than there is O2
A word equation: Nitrogen monoxide + oxygen → nitrogen dioxide
State of matter
Letters indicate the state of each compound
A symbolic equation: 2NO(g) + O2(g) → 2NO2(g)
(aq) =aqueous/dissolved in water
Coefficients
(s) = solid
indicate the ratio of compounds in the reaction
(l) = liquid
(g) = gas

5. Balancing Writing and Balancing Chemical Equations
word equation – uses the names of the chemicals
Example:
K O → K2O
Potassium metal + oxygen gas → potassium oxide

6. Writing and Balancing Chemical Equations
A balanced chemical equation shows the correct number of each atom
Writing and Balancing Chemical Equations
A skeleton equation shows only the formulas of the elements/compounds
Balancing ensures that the number of each atom is the same on both sides of the reaction arrow
Shows atoms, but is not balanced
Always use the smallest whole number ratio
K(s) + O2 (g) → K2O(s)
4K(s) + O2 (g) → 2K2O(s)

7. Hints for writing chemical equations
Use the chemical symbol to represent single elements.
Be careful of diatomic and polyatomic elements such as O2, P4, and S8
The “special seven” are all diatomic elements (hockey stick & puck)
H2, N2, O2, F2, Cl2, Br2, I2

8. rules for balancing Do Not Use Trial and Error!
Begin with the most complex compound in the equation
Balance one part of the compound first (anion or cation)
Completely balance one compound at a time
Only add coefficients to the formula: do not change any of the chemical formulas
Balance polyatomic ions as one substance (do not balance their parts, unless you have to)
Balance hydrogen and oxygen last oxygen last of all

9. Balancing equations examples
Balance the following:
__Sn(NO2)4 + __K3PO4 → __KNO2 + __Sn3(PO4)4
___ C2H6 + ___ O2 → ___ CO2 + ___ H2O