1. Chemical Reactions
Chapter 11

2. Writing Chemical Reactions
Chemical Equation:
an expression that represents a chemical reaction.
A balanced chemical equation tells:
What elements and/or compounds are involved
The relative amounts of each substance involved
Chemical equations MUST:
Include the correct formula for the species involved
Satisfy the Law of Conservation of atoms
Tell the relative number of moles (quantity) of each species

3. Symbols you might see: Physical States: (g) – gas (l) – liquid
(s) – solid or precipitate
(aq) – substances dissolved in water
 - reversible arrow
reaction can go forward or backwards
Δ – heat involved in the reaction
MnO2 – catalyst
Substance that speeds up the reaction but is not used up in the reaction

4. How to write a balanced chemical equation:
1. Decide which species are reactants & which are products
2. Write each species as a correct chemical formula.
3. Write the formulas for the reactants on the left side of the paper.
4. Place an arrow () after the reactants to indicate what is produced.
5. Write formulas for the products on the right side of the arrow.
6. Balance the chemical equation by adjusting the coefficients in front of the species to satisfy the Law of Conservation of Atoms.
Same number of atoms of each element on both sides of the arrow
Reactants
Products
Yields

5. Law of Conservation of Atoms
During any chemical reaction, the mass of the products is always equal to the mass of the reactants.
Matter cannot be created or destroyed.
In any chemical reaction, mass is conserved.
We use coefficients to balance chemical equations so that it obeys the
Law of Conservation of Atoms!

6. Rules for Balancing an Equation:
1. Write out the numbers of each element for each side.
2. Add coefficients to balance elements ONE AT A TIME.
Keep a running tally of the numbers of each element.
Check each number after each coefficient added.
Balance complicated compounds FIRST.
Balance elements that appear alone or as diatomic molecules LAST.
Save Oxygen & Hydrogen for LAST.
3. Reduce coefficients to simplest ratio, if necessary.

7. Example:
Ammonium chloride reacts with calcium hydroxide to produce calcium chloride, ammonia, and water. 2
NH4Cl +
Ca(OH)2
CaCl2 + 2
NH3 + 2
H2O
N :
H :
Cl :
Ca :
O : 1 2
N :
H :
Cl :
Ca :
O : 1 2 6 10 5 8 10 1 2 2 1 1 2 1 2

8. Practice:
Write the chemical equation & balance:
Sodium hydrogen carbonate decomposes into sodium carbonate, carbon dioxide, and water.
NaHCO3  Na2CO CO H2O 2
Na 1
H 1
C 1
O 3
---- 2
---- 6
Na 2
H 2
C 2
O 6