1. Balancing Equations
Unit 5.2

2. Law of conservation of mass states :Matter cannot be created or destroyed, it can only change forms

3. Balancing Equations What’s wrong with this equation?
H2(g) + O2(g)  H2O (l)
2 oxygen atoms on reactant side but only 1 on product side
Does not obey law of conservation of mass- appears one O is destroyed
H2(g) + O2(g)  H2O2 (l) Did we fix it?
Not a good solution- changed what the equation said- no longer forms water, it forms hydrogen peroxide

4. Balancing Equations
Balanced equation: equation which has the same number of atoms of each element on both sides of the equation.
involves changing ONLY the coefficients in an equation
Coefficients are the big numbers in front of formulas that tells you how many molecules there are
You can’t change subscripts because this would change the identity of the substances in the equation!
Subscripts are the little numbers that tell how many of each atom there is

5. How to balance equations:
Make an element inventory (how many of each element there is on the reactant side and the product side)
NEVER CHANGE A SUBSCRIPT!!!
Choose coefficients so the number of atoms of each element is the same on both sides of the arrow
Trial and error
ALWAYS RECHECK YOUR ELEMENT INVENTORY!!!!!!!!

6. Examples
H2 + O2  H2O
Na + Cl2  NaCl
Rb + S8  Rb2S