Download Balancing_Chemical_Equations_20th_march

Balancing Chemical Equations
21st March 2012
Learning Objectives
Demonstrate how to
calculate balanced
chemical equations
(Level 6)
Describe the importance of
balancing equations with reference
to numbers
(Level 5)
Recall the symbols for certain
elements (Level 4)
What did we do last lesson?
• Can anyone remember what we did last
lesson?
Starter
• Complete the worksheet
Answers
What is my symbol?
Carbon
Nitrogen
Hydrogen
Oxygen
Sulfur
Boron
Helium
Calcium
Lead
Zinc
C
N
H
O
S
B
He
Ca
Pb
Zn
Can you….?
• Recall the symbols for certain elements
(Level 4)
Yes
Almost
No
Answers
• How do we write the symbols for these
things?
• Water
H 2O
• Carbon Dioxide CO2
• Oxygen
O2
• What do you notice about all these things?
Small numbers
• The small numbers we sometimes see are
called subscripts.
• O2
• This subscript number means there are
two oxygen atoms
O
O
Big numbers
• The big numbers we sometimes see are
called coefficients
• 2O2
• This coefficient number means there are
two lots of the oxygen molecules
O
O
O
O
Balancing equations
• When we talk about chemical reactions we
always refer to the reactants and the
products.
• When we write these chemical reactions
we can sometimes use the formula instead
of the word.
• Oxygen = O
Balancing Equations
• BUT, we must make sure that what goes
into a chemical reaction also comes out
• WE MUST BALANCE THE EQUATION
• Lets have a go…
Role play
• I need…
• 4 Hydrogen's
• 1 Carbon
• 2 oxygen
• Methane + Oxygen
CH4
+ O2
Carbon dioxide + Water
CO2
+ H2O
Balancing equations
Methane
CH4
+
+
Oxygen
2O2
Carbon Dioxide + Water
CO2
2H2O
H
H
O O
H C H
H
H
O
O C O
O O
H
What do you notice about this balanced
equation?
H
O
Can you….?
Describe the importance of balancing
equations with reference to numbers
(Level 5)
Yes
Almost
No
Balancing equations
• You are now going to use the molly mod
kits to try and balance these equations:
1.
2.
H2
Mg
+
+
O2
O2
H2O
MgO
Balancing equations
• Time for you to have a go…
H2
+
Hydrogen +
H H
O2
H2O
Oxygen
Water
O O
???????
Remember we need to balance it!
What do you notice??
Balancing equations
• Time for you to have a go…
H2
+
Hydrogen +
H H
O2
H2O
Oxygen
Water
O O
H
H
O
Remember we need to balance it!
Why is this wrong??
Because we haven't used both of the oxygen molecules
Balancing equations
• Time for you to have a go…
H2
+
Hydrogen +
H H
O2
Oxygen
O O
2H2O
Water
H
HH
O
Remember we need to balance it, BOTH SIDES
Is this correct??
H
H
O
Balancing equations
• Time for you to have a go…
2H2
+
Hydrogen +
H H
O2
Oxygen
O O
2H2O
Water
H
HH
O
H H
Remember we need to balance it, BOTH SIDES
Is this correct??
YES, There are now 4 hydrogen atoms and two oxygen
atoms on the left AND the same amount on the right 
H
H
O
Then start balancing:
Just count up the atoms on each side
Mg + O2  MgO
1
Mg 1
2
1
O
The numbers aren’t balanced so then add
“BIG” numbers to make up for any shortages
Mg + O2 
And adjust totals
2
MgO
1
Mg
1
2
2
O
1
2
But the numbers still aren’t equal, so add
another “BIG” number
2 Mg + O2  2 MgO
2
1
Mg
2
2
O
2
And adjust totals again
NOW BOTH SIDES HAVE EQUAL
NUMBERS OF ATOMS
WE SAY THAT THE
EQUATION IS BALANCED!!
Can you….?
Create balanced chemical equations
(Level 6)
Yes
Almost
No
Plenary
• I want you to write..…
• One learning objective you have achieved today
• One thing you have learnt today
• And if it was too easy, about right, or too hard