1. Balancing Chemical Equations

2. Balancing Chemical Equations
5.3
Balancing Chemical Equations
Chemical equation
– an equation that uses
chemical formulas and symbols to show
the reactants and products in a
chemical reaction
– the smallest relative
number of moles of the substances involved in a
reaction
mole ratio

3. Parts of an equation Reactants – yield sign - products
The left side of an equation- chemicals that react with each other – the reactants
The arrow separating the beginning and end result – the yield sign
The right side of an equation – the NEW substances that are formed as a result of a chemical reaction - products

4. Balancing equations – balancing atoms
Remember the Law of Conservation of mass: …..
Mass cannot be created or destroyed…
# of atoms of each element on reactants side must equal # of atoms of each element on the products side!

5. Is this equation balanced?
Count the atoms……
HCl + NaOH NaCl+ H2O
left side has 2-H, 1-Cl, 1-Na, and 1-O
right side has 1-Na, 1-Cl,
2-H, and 1-O
Yes!!!!!!

6. Is this equation balanced?
NaCl + H2O  NaOH + Cl2
left side has 1-Na atom, 1-Cl, 2-H, and 1-O
right side has 1-Na, 1-O,
1-H, and 2-Cl
NO!!!!!!
How can we make it balance????

7. Use coefficients…. List all elements and # of atoms
Add more molecules of a substance by adding coefficients…… to balance
Ca + H2O  Ca(OH)2 + H2
Ca 1 atom Ca 1 atom
H 2 atoms H 4 atoms
O 1 atom O 2 atoms

8. Continued…… Ca + H2O  Ca(OH)2 + H2 Ca 1 Ca 1 H 2 H 4 O 1 O 2
H = 2 x 2 = H = 4
O = 2 x 1 = O = 2

9. How to balance equations
Identify the individual elements in reactants and products
Count atoms on each side
“Try” different coefficients to balance the numbers of atoms.
Balance the equation one element at a time.*
*In general – balance the metals (and any other non-metals)
first and save hydrogen and oxygen for last

10. 1 Ca + 2 H2O  1 Ca(OH) H2
Just like in in formula writing the “1’s” are understood. This equation should be written in a simpler way:
Ca + 2 H2O Ca(OH)2 + H2

11. Practice Balancing Chemical Equations
Copper (II) sulfate, CuSO4, and aluminum react to form aluminum sulfate, Al2(SO4) and copper. Write the balanced equation for this single-displacement reaction.
In a double-displacement reaction, sodium sulfide, Na2S, reacts with silver nitrate, AgNO3 to form sodium nitrate, NaNO3 and silver sulfide, Ag2S. Balance this equation.
CuSO Al  Al2(SO4) + Cu
Na2S + 2AgNO3  2NaNO3 + Ag2S

12. Balancing Equations
Hydrogen peroxide, H2O2, is sometimes used as a bleach or as a disinfectant. Hydrogen peroxide decomposes to give water and molecular oxygen. Write a balanced equation for the decomposition reaction.
Hydrogen sulfide, H2S, is a gas that smells like rotten eggs. Write and balance an equation for the oxidation by molecular oxygen of hydrogen sulfide to make sulfuric acid, H2SO4.
2H2O2 -> 2H2O + O2
H2S + 2O2 -> H2SO4

13. Determining Mole Ratios
Balanced equations indicate particles and moles
Balanced equations show the conservation of mass
The Law of Definite Proportions:
“A compound always contains the same elements in the same proportions, regardless of how the compound is made or how much of the compound is formed.”

14. Mole Ratios
Look at the reaction of magnesium with oxygen to form magnesium oxide:
magnesium + oxygen magnesium oxide
2Mg + O2  2MgO
The single molecule of oxygen in the equation might be shown as 1O2. However, a coefficient of 1 is never written.
The equation can also be read as describing mole quantities – 2 mol of magnesium can react with 1 mol of oxygen to produce 2 mol of magnesium oxide.
Mole ratios can only be derived from balanced equations.

15. Information from the balanced equation 2Mg + O2  2MgO
Amount (Mol)
Molecules Mg = (6.02 x 1023) x 2
O = (6.02 x 1023) x 1
MgO = (6.02 x 1023) x 2
Mass (g) Mg = g/mol x 2 mol
O = g/mol x 1 mol
MgO = 40.3 g/mol x 2 mol
Total Mass (g)
Mole Ratio is 2:1:2

16. Assignment Balancing Equations - worksheet Word Problems - worksheet
1 - write word equation
2 – write skeletal equation
3 – balance
EX: When a match is lit, sulfur (S8) reacts with oxygen to release energy and form sulfur dioxide, SO2.
sulfur + oxygen  sulfur dioxide
S O2  SO2
S O2  SO2