1. Balancing Equations and The Law of Conservation of Mass
Unit 5: Balancing Equations
John Dalton said “In ordinary chemical reactions, atoms may be combined, separated, or rearranged, but they may never be created or destroyed.”
During a chemical reaction, atoms present in the reactant compounds are rearranged to form new & different product compounds.
A chemical equation must always show equal numbers of each element on the reactants and products side of a chemical equation.

2. Law of Conservation of Mass
In ordinary chemical reactions, the total mass of reacting substances is equal to the total mass of products
All atoms on the reactant side must appear on the product side, and in equal numbers
No new elements may appear
No elements may disappear

3. Reaction Equation Basics
H Cl2 2 HCl
REACTANTS
PRODUCTS
Reactants are on the left side of the arrow.
Products are on the right side of the arrow.
The arrow is read as “yields” or “produces.” It separates the 2 sides of the reaction equation.

4. Reaction Equation Basics
H Cl2 2 HCl
REACTANTS
PRODUCTS
Reactants are on the left side of the arrow.
Products are on the right side of the arrow.
Subscripts (indicate how many of that atom or ion there are)

5. Reaction Equation Basics
H Cl2 2 HCl
REACTANTS
PRODUCTS
Reactants are on the left side of the arrow.
Products are on the right side of the arrow.
Subscripts (indicate how many of that atom or ion there are)
Coefficients (tell how many of that molecule or compound)

6. Count the # of atoms
CH4
5 H2O
2 C6H6
3 Cr2O7
Ca (OH)2
3 Ca (OH)2

7. CH4 1 C; 4 H 5 H2O 2 C6H6 3 Cr2O7 Ca (OH)2 3 Ca (OH)2
Count the # of atoms
CH4 1 C; 4 H
5 H2O
2 C6H6
3 Cr2O7
Ca (OH)2
3 Ca (OH)2

8. CH4 1 C; 4 H 5 H2O 10 H; 5 O 2 C6H6 3 Cr2O7 Ca (OH)2 3 Ca (OH)2
Count the # of atoms
CH4 1 C; 4 H
5 H2O 10 H; 5 O
2 C6H6
3 Cr2O7
Ca (OH)2
3 Ca (OH)2

9. CH4 1 C; 4 H 5 H2O 10 H; 5 O 2 C6H6 12 C; 12 H 3 Cr2O7 Ca (OH)2
Count the # of atoms
CH4 1 C; 4 H
5 H2O 10 H; 5 O
2 C6H6 12 C; 12 H
3 Cr2O7
Ca (OH)2
3 Ca (OH)2

10. CH4 1 C; 4 H 5 H2O 10 H; 5 O 2 C6H6 12 C; 12 H 3 Cr2O7 6 Cr; 21 O
Count the # of atoms
CH4 1 C; 4 H
5 H2O 10 H; 5 O
2 C6H6 12 C; 12 H
3 Cr2O7 6 Cr; 21 O
Ca (OH)2
3 Ca (OH)2

11. CH4 1 C; 4 H 5 H2O 10 H; 5 O 2 C6H6 12 C; 12 H 3 Cr2O7 6 Cr; 21 O
Count the # of atoms
CH4 1 C; 4 H
5 H2O 10 H; 5 O
2 C6H6 12 C; 12 H
3 Cr2O7 6 Cr; 21 O
Ca (OH)2 1 Ca; 2 O; 2 H
3 Ca (OH)2

12. CH4 1 C; 4 H 5 H2O 10 H; 5 O 2 C6H6 12 C; 12 H 3 Cr2O7 6 Cr; 21 O
Count the # of atoms
CH4 1 C; 4 H
5 H2O 10 H; 5 O
2 C6H6 12 C; 12 H
3 Cr2O7 6 Cr; 21 O
Ca (OH)2 1 Ca; 2 O; 2 H
3 Ca (OH)2 3 Ca; 6 O; 6 H

13. Balanced Equations show that Matter Is Conserved
H Cl2 2 HCl
Total atoms = Total atoms
2 H, 2 Cl 2H, 2 Cl
Total Mass = Total Mass
2(1.01) (35.45) 2(36.46)
72.92 g = g

14. Balance Equations with Coefficients
Unit 5: Balancing Equations
Balance Equations with Coefficients
Always use the coefficients in front of formulas to balance each type of atom.
DO NOT EVER alter subscripts!!
4NH O NO + 6H2O
4 N = 4 N
12 H = 12 H
10 O = 10 O

15. Balancing Equations Practice:
Ex. 1
_MgCl2 + __K3N → __Mg3N __KCl
Ex.2: Hint:If it shows up twice, balance it last!!
__C6H12O6 + __O2 → __CO __H2O
Ex 3.
___Na2O + ___Al2S3 → ___Al2O3 + ___Na2S
Ex 4. : Hint: Look for Least Common Multiples
___BF3 + ___Ca3P2 → ____BP + ____CaF2

16. Balancing Equations Practice:
If there is not a coefficient shown, assume it to be 1
Ex. 1
___MgCl2 + ___K3N → ____Mg3N ____KCl
Ex.2: Hint: If it shows up twice, balance it last!!
___C6H12O6 + ___O2 → ____CO ____H2O
Ex 3.
___Na2O + ___Al2S3 → ____Al2O3 + ____Na2S
Ex 4. : Hint: Look for Least Common Multiples
___BF3 + ___Ca3P2 → ____BP + ____CaF2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

17. Balancing Equations Practice:
Ex. 1
___MgCl2 + ___K3N → ____Mg3N ____KCl
Ex.2: Hint: If it shows up twice, balance it last!!
___C6H12O6 + ___O2 → ____CO ____H2O
Ex 3.
___Na2O + ___Al2S3 → ____Al2O3 + ____Na2S
Ex 4. : Hint: Look for Least Common Multiples (LCM)
___BF3 + ___Ca3P2 → ____BP + ____CaF2 3 2 1 6 1 6 6 6 3 1 3 1
LCM of 6
for F 2 2 1 3

18. Word Equations, Skeleton Equations, & Balanced Equations
Sentence: Hydrogen gas is combined with oxygen gas in an exothermic reaction producing water vapor and releasing 252 kJ of energy
Word Equation:
Hydrogen + oxygen water + energy
Skeleton Equation (formulas, but not balanced):
H O2 H2O + energy
Balanced Chemical Equation:
2H2(g) + O2(g) 2H2O(g) + 252kJ
exo means energy is released!

19. Unit 5: Balancing Equations
Useful Symbols
Unit 5: Balancing Equations
(g) = gas (l) = liquid (s) = solid (aq) = aqueous
(dissolved in water)
 = gas evolution  = precipitate
→ = yields/produces ↔ = reversible reaction
Energy (usually in kJ) can also be listed as a reactant or as product.
Energy on left of arrow = heat energy absorbed = endothermic reaction
Energy on right of arrow = heat energy released = exothermic reaction

20. Endothermic reaction
C2H6(g) kJ C2H4(g) + H2(g)
Gas made (bubbles!)
Physical States
Zn(s) + HNO3(aq) → Zn(NO3)2(aq) + H2(g)  + Energy
Exothermic Reaction