CLASS CONCEPTS

1. Introduction to Chemistry

2. The Periodic Table

3. Quantum Numbers

4. Electron Configuration

5. Chemical Families

6. Oxidation Numbers

7. Chemical Formulas

8. Chemical Names

9. Formula Mass

10. Percentage Composition

11. Reaction Types

12. Balancing Equations

13. The Mole Concept

14. Solution Concentration

15. Stoichiometry

16. Kinetic Theory

17. The Gas Laws

18. Enthalpy & Heat

19. Reaction Rates

20. Acids & Bases

21. pH Scale

22. Salts

23. Net Ionic Equations

24. Redox Reactions

25. Organic Chemistry

26. Nuclear Chemistry

12. Balancing Chemical Equations

 
Atoms are neither created nor destroyed during any chemical reaction. Chemical changes merely rearrange the atoms.

The statement above is supported by:
Chemical reactions are represented in a concise way by chemical equations.

2 H2 + O2 → 2 H2O
  • The reacting substances, called reactants, are located on the left side of the arrow.
     
  • The substances formed, called products, are located on the right side of the arrow.
     
  • In a chemical equation, the + sign is read as "reacts with" and the arrow is read as "produces".
     
  • Numbers in front of the formulas are coefficients, indicating the relative number of molecules or ions of each kind involved in the reaction.
    Coefficients of 1 are never written - they are understood.
  • Numbers to the lower right of chemical symbols in a formula are subscripts, indicating the specific number of atoms of the element found in the substance.
    Subscripts of 1 are never written - they are understood.
balanced A chemical equation must have the same number of atoms of each element on both sides of the arrow. When this condition is met, the equation is said to be balanced.

To count atoms, multiply the formula's coefficient by each symbol's subscript.

For example: 2Al2(SO4)3

  • For Al - coefficient of 2, times subscript of 2 = 4 aluminum atoms
     
  • For S - coefficient of 2, times subscript inside parenthesis of 1, times subscript outside parenthesis of 3 = 6 sulfur atoms
     
  • For O - coefficient of 2, times subscript inside parenthesis of 4, times subscript outside parenthesis of 3 = 24 oxygen atoms
 
The order in which the following steps are performed is important. While shortcuts are possible, (and you will learn about one), following these steps in order is the best way to be sure you are correct.

Balance equations "by inspection" with these steps:

  1. Check for diatomic molecules.
     
  2. Balance the metals (not Hydrogen).
     
  3. Balance the nonmetals (not Oxygen).
     
  4. Balance oxygen.
     
  5. Balance hydrogen.
     
  6. The equation should now be balanced, but recount all atoms to be sure.
     
  7. Reduce coefficients (if needed). ALL coefficients must be reducable before you can reduce. An equation is not properly balanced if the coefficients are not written in their lowest whole-number ratio.
HINT: NEVER change subscripts to balance equations.

The physical state of each substance in a reaction may be shown in an equation by placing the following symbols to the right of the formula:

  • (g)   for gas
  • (l)   for liquid
  • (s)   for solid
  • (aq)   for aqueous (water) solution

86 Equations to practice balancing

 
Balancing Equations #1

 
Balancing Equations #2

 
Balancing Equations

 
Balancing Equations