Download Chapter 2 An Introduction Chemistry

4/1/2015
Balancing Chemical Equations
Chapter 2
An Introduction Chemistry
Lecture 4: Balancing Chemical Reactions
Balancing Chemical Equations
• In a balanced equation, the number of atoms
on the left will equal the number of atoms
on the right
• An equation can be balanced by placing
coefficients in front of one or more of the
chemical formulas
• When the reaction is completely balanced,
coefficients must be whole numbers
• Fractions may be useful in the process of
balancing
Na(s) + H2O(l) --->> H2(g) + NaOH(aq
aq))
• Matter is neither created nor destroyed in a
chemical reaction
• Every atom on the reactant side of the equation
must be accounted for on the product side of the
equation
• In this equation, there are 2 hydrogens on the
left, but 3 hydrogens on the right
•THIS EQUATION IS NOT BALANCED!!
Balancing Chemical Equations
Na(s) + H2O(l) --->> H2(g) + NaOH(aq
aq))
Na = 1
H=2
O=1
Na = 1
H=3
O=1
• Let’s try to balance the H atoms by putting a 2 in
front of the water
Balancing Chemical Equations
Balancing Chemical Equations
Na(s) + 2H2O(l) --->> H2(g) + NaOH(aq
aq))
Na(s) + 2H2O(l) --->> H2(g) + 2NaOH(aq
aq))
Na = 1
H=4
O=2
Na = 1
H=3
O=1
• Now the H atoms and the O atoms are out of
balance!
• Let’s try balancing the O atoms by placing a 2 in
front of NaOH
Na = 1
H=4
O=2
Na = 2
H=4
O=2
• Now the H atoms and the O atoms balance, but
the Na atoms don’t!
• Let’s try to balance the Na atoms by placing a 2 in
front of Na
1
4/1/2015
Balancing Chemical Equations
2Na(s) + 2H2O(l) --->> H2(g) + 2NaOH(aq
aq))
Na = 2
H=4
O=2
Na = 2
H=4
O=2
• All atoms are equal on both sides
• This equation is now balanced!!
Balancing Chemical Equations
•There are 2 things you must NEVER EVER do when
balancing a chemical equation
• Never change a correct chemical formula
H2 + O2 --->> H2O
H2 + O2 --->> H2O2
• Never add (or remove) some real or imaginary
chemical species to either side of a reaction
H2 + O2 --->> H2O
H2 + O2 --->> H2O + O
Balancing Chemical Equations
Balancing Chemical Equations
• This just takes practice (and a good eraser!)
• You just need to play around with the coefficients
so that the atoms on both sides are equal
• There is no particular atom you need to start
with (although there are some tips I will give you)
• It may take a few attempts to get it balanced –
don’t give up!!
• Tips for balancing equations…
• Start with an element that appears in only one
compound on each side of the equation
• If an element appears by itself on either side
of the equation, save it for last
• If an element appears in more than one
compound on either side of the equation, save it
for last
• Use fractions in the process of balancing if you
find it helpful
• Make sure that your coefficients cannot be
reduced
Balancing Chemical Equations
Balancing Chemical Equations
C6H12O6 + O2 --->> CO2 + H2O
C6H12O6 + O2 --->> 6CO2 + H2O
C=6
H = 12
O=8
C=1
H=2
O=3
• This equation is not balanced
• Let’s start by putting a 6 in front of CO2
C=6
H = 12
O=8
C=6
H=2
O = 13
• This fixes our carbon problem
• Now let’s balance hydrogen by putting a 6 in front
of H2O
2
4/1/2015
Balancing Chemical Equations
Balancing Chemical Equations
C6H12O6 + O2 --->> 6CO2 + 6H2O
C6H12O6 + 6O2 --->> 6CO2 + 6H2O
C=6
H = 12
O=8
C=6
H = 12
O = 18
• This fixes our hydrogen problem
• Can you see how to balance oxygen now?
Activity
• Balance the equations found on page 22-13 in your
study guide
Fe + Cl2 --->> FeCl3
C=6
H = 12
O = 18
C=6
H = 12
O = 18
• Now everything is balanced!
END OF MATERIAL FOR EXAM #1
• Good review materials:
• Study packet
• Textbook chapters
Zn + HCl -> ZnCl2 + H2
Cu + AgNO3 --->> Cu(NO3)2 + Ag
C3H8 + O2 --->> CO2 + H2O
3