CHEMISTRY
November 7th, 2011
Warm Up
What is wrong with the following equation??
Can you fix it?
 HINT: think Law of Conservation of Mass

H2 + O2  H2O
CHEMISTRY LAUGHS
Why does hamburger have lower energy than
steak?
Agenda
Unit 7: Balancing Equation
 Practice

Balancing Equation
Word Equations
 Describes a chemical reaction that occurs
using the names of the chemicals involved.
 Reactants versus Products
 An arrow is written between the reactants
and products to indicate that chemical
change is occurring
– For example: 2H2 + O2  2H2O
Balancing Equation

Atoms involved in a chemical reaction must be
balanced.
– Same number of atoms of each element on both
sides of a chemical equation.

Unbalanced equation
– Balance the equation by adding coefficients in front
of the compounds
 Coefficients: number of molecules of a particular
compound
 Subscripts: number of atoms of each element in the
compound
Balancing Equations
A.
B.
From the word equation, write the correct formula
for all reactants and products.
If given an unbalanced equations first, balance the
equation by using coefficients:
1. NEVER change the subscripts in the formula to balance the
equation
2. Elements need to be balanced from either side of the
equation
3. Start with the first element and work right of the equation
4. Balance the number of oxygen and hydrogen atoms after
all the other elements have been balanced.
Go back and double check that all elements are
balanced
D. BOX your coefficients when finished
E. USE PENCIL only! (you will be erasing A LOT!)
C.
Balancing Equation
Inspection Method
 Algebraic Method

Practice and Reasoning

Unbalanced Equation:
– SnO2 + H2 → Sn + H2O

Look at the equation and see which
elements are not balanced. In this case,
there are two oxygen atoms on the lefthand side of the equation and only one on
the right-hand side. Correct this by putting
a coefficient of 2 in front of water.
– SnO2 + H2 → Sn + 2 H2O
– SnO2 + H2 → Sn + 2 H2O

This puts the hydrogen atoms out of balance.
Now there are two hydrogen atoms on the left
and four hydrogen atoms on the right. To get
four hydrogen atoms on the right, add a
coefficient of 2 for the hydrogen gas.
Remember, coefficients are multipliers, so if we
write 2 H2O it denotes 2x2=4 hydrogen atoms
and 2x1=2 oxygen atoms.
– SnO2 + 2 H2 → Sn + 2 H2O

The equation is now balanced. Be sure to
double-check your math! Each side of the
equation has 1 atom of Sn, 2 atoms of O, and 4
atoms of H.
Practice Problems

__Fe + __Cl2  __FeCl3

__Cr + __O2  __Cr2O3

__Na + __H2O  __NaOH + __H2

__Ca(CN)2 + __AlF3  __CaF2 + __Al(CN)3
Word Problem

Potassium phosphate + Calcium hydroxide 
Calcium phosphate + Potassium hydroxide

__K3(PO4) + __Ca(OH)2  __Ca3(PO4)2 + __KOH

2K3(PO4) + 3Ca(OH)2  __Ca3(PO4)2
Reactants
K=6
P=2
Ca = 3
O = 14
H=6
+ 6KOH
Products
K=6
P=2
Ca = 3
O = 14
H= 6
Homework

Unit 7 Balancing Equation Worksheet 1