balancing chemical equations. the balanced equation l atoms can’t be created or destroyed. l all...
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Balancing Chemical Equations
The Balanced Equation
Atoms can’t be created or destroyed. All the atoms we start with we must end
up with. A balanced equation has the same
number of each element on both sides of the equation.
Balanced equations show how mass and atoms are conserved.
The Balanced Equation
C + O2 ® CO
This equation is NOT balanced. There is one carbon atom on the left
and one on the right. There are two oxygen atoms on the left
and only one on the right.
C + O® COO
C + O2 ® CO We need one more oxygen atom in the
products. We can’t change the formula, because it
describes what it is.
C + O® COO
In order to have two oxygen atoms, another CO must be produced.
But where did the other carbon come from?
C +O
®C
OO
OC
We must have started with two carbon atoms.
C
+O
®C
OO
OC
C
The balanced chemical equation is
2 C + O2 ® 2 CO
C
+O
®C
OO
OC
C
Chemical Equations
Numbers and types of atoms must balance.
Rules for Balancing
1. Write the correct formulas for all the reactants and products.
2. Count the number of atoms of each type appearing on both sides.
3. Balance the elements one at a time by adding coefficients (the numbers in front).
4. Check to make sure it is balanced.
Never
Never change a subscript to balance an equation.
If you change the formula you are describing a different reaction.
Never
Never put a coefficient in the middle of a formula.
2 NaCl is okay; Na2Cl is not.
Example
H2 + H2OO2 ®
Make a table to keep track of where youare in the balancing process.
Example
H2 + H2OO2®
We need twice as much O in the product.
Reactants
ProductsH
O
2
2
2
1
Example
H2 + H2
OO2
®
We need 2 oxygen atoms on the product side, so a coefficient of 2 should be placed in front of water.
2
Example
H2 + H2
OO2
®
We must recalculate the number of hydrogen and oxygen atoms on the right. Coefficients are used as multipliers. R P
H
O
2
2
2
1
2
4
2
Example
H2 + H2
OO2
®
Now we need 4 hydrogen atoms on the reactant side.
R P
H
O
2
2
4
2
2
Example
H2 + H2
OO2
®
A coefficient of 2 should be placed in front of hydrogen gas.
R P
H
O
2
2
4
2
22
4
Example
H2 + H2
OO2
®
The equation is balanced; it has the same number of each kind of atom on both sides!
R P
H
O
4
2
4
2
22
Example
H2 + H2OO2®
This is the answer,
R PH
O
2
2
2
1
2
2
4
2
4
not this.
Balancing Hints
Balance elements in the following order: metals nonmetals hydrogen oxygen
Balancing Hints
If an atom appears more than once on a side, balance it last.
If you fix everything except one element, and it is even on one side and odd on the other, double the first number, then move on from there.
Problem
Balance the following equation.
CH4 + O2 ® CO2 + H2O
CH4 + 2 O2 ® CO2 + 2 H2O
Problem
2 AgNO3 + Cu ® Cu(NO3)2 + 2 Ag
Balance the following equation.
AgNO3 + Cu ® Cu(NO3)2 + Ag
Problem
Balance the following equation.
Mg + N2 ® Mg3N2
3 Mg + N2 ® Mg3N2
Problem
Balance the following equation.
P + O2 ® P4O10
4 P + 5 O2 ® P4O10
Problem
Balance the following equation.
Na + H2O ® H2 + NaOH
2 Na + 2 H2O ® H2 + 2 NaOH
Objectives
PSc.2.2.5–Classify types of reactions such as synthesis, decomposition, single replacement, or double replacement.