1. Writing and Balancing Equations
Chemical Equations
Writing and Balancing Equations

2. A chemical reaction is the process by which one or more substances are
Chemical Reactions
A chemical reaction
is the process by
which one or more
substances are
changed into
different
substances.

3. Reactants are the beginning substances in a chemical reaction (left of arrow).
Products are the substances produced in a chemical reaction (right of the arrow).

4. Chemical equations – represent chemical reactions using formulas and symbols for the substances, their states, and other conditions.
(NH4)2Cr2O7 (s) → N2 (g) + Cr2O3 (s) + 4 H2O
(reactant) (products)

5. It can only change forms.
Law of Conservation of Mass – states that matter can neither be created or destroyed in a chemical reaction.
It can only change forms.
Antoine-Laurent de Lavoisier

6. Therefore, the same number of each kind of atom must be found on the left and on the right sides of a chemical equation.
2H2 + O2
      
2H2O
This is accomplished by adding coefficients to the front of compounds and elements.
Do not change subscripts.

7. 5 grams 40 grams __ grams Example: assuming all reactants are
used completely
2H O2 → 2H2O
5 grams grams __ grams

8. Step 1 – Determine the reactants and products.
Writing Balanced Equations
Step 1 – Determine the reactants and products.
Important – The following elements exist as diatomic molecules in their free (uncombined) state:
O2, H2, N2, Cl2, Br2, F2, and I2

9. Step 2 – Write the chemical equation with the correct symbols for all reactants and products.
Step 3 – Balance the equation by adding coefficients to the front of formulas
Hint: Balance elements that appear more than once on a side last.

10. Step 4 – Add symbols, when appropriate.
COMMON SYMBOLS
yields (separates reactants from products)
 heat required (this symbol is placed over the arrow)
(l) liquid
(aq) aqueous (dissolved in water)
(g) gas ( an  is sometimes used for a gas product)

11. A formula over the yield arrow indicates a catalyst.
(s) solid (and  is sometimes used for a precipitate)
(cr) crystalline solid
**Note:
A formula over the yield arrow indicates a catalyst.

12. Example 1

13. Example 2
CH3OH (l) + O2 (g) → CO2 (g) + H2O (g)
Example 3
Liquid water is formed from the combining of hydrogen and oxygen gases. (Write the Chemical Equation)