1. 4.3: BALANCING EQUATIONS

2. 4.3: LEARNING OUTCOMES You Should Be Able To…
1. Define and explain the law of conservation of mass
2. Represent chemical reactions and the conservation of atoms, using molecular models
3. Write and balance (using the lowest whole number coefficients) chemical equations from formulae, word equations, or descriptions of experiments

3. 4.3: VOCABULARY Subscript Coefficient Law of Conservation of Mass
Molecule
Atom
Skeleton Equation
Balanced Equation
Word Equation

4. 4.3: Balancing Equations Reactants Products
Chemical reactions result in chemical changes.
Chemical changes occur when new substances are created.
The original substance(s), called reactants, change into new substance(s) called products.
Reactants
Products
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See pages

5. 4.3: Balancing Equations Reactants Products
(c) McGraw Hill Ryerson 2007
See pages

6. 4.3: Balancing Equations
Chemical reactions can be written in different ways.
A word equation:
Nitrogen monoxide + oxygen  nitrogen dioxide
A symbolic equation:
2NO(g) + O2(g)  2NO2(g)
STATE OF MATTER
Letters indicate the state of each compound.
(aq) = aqueous/dissolved in water
(s) = solid
( ) = liquid
(g) = gas
COEFFICIENTS
Indicates how many of each molecule there is.
Ie: there are 2 molecules of NO.
(c) McGraw Hill Ryerson 2007

7. Law of Conservation of Mass
When a chemical reaction occurs, new compounds are created, BUT…
No new matter is created or destroyed; atoms are just rearranged as the atoms change partners to form new compounds.
If there are 3 atoms of oxygen in the reactants, there MUST be 3 atoms of oxygen in the products.
Number of each atom in reactants = number of each atom in products.
The law of conservation of mass:
Mass of reactants = mass of products
If you could collect and measure all of the exhaust from this car, you would find that mass of reactants (gas + O2) = mass of products (exhaust).
(c) McGraw Hill Ryerson 2007

8. Writing & Balancing Equations
The simplest form of chemical equation is a word equation.
Potassium metal + oxygen gas  potassium oxide
A skeleton equation shows the formulas of the elements/compounds.
A skeleton equation shows which atoms are involved, but not how many molecules are involved.
K + O2  K2O

9. Writing & Balancing Equations
A balanced chemical equation shows all atoms and the coefficients tells us how many molecules (and atoms) there are.
Balancing ensures that the number of each atom is the same on both sides of the reaction arrow.
4K O  2K2O K K O O K O K K K K O K

10. Counting Atoms to Balance Equations
Using the law of conservation of mass, we can count atoms to balance the number of atoms in chemical equations.
Word equation: methane + oxygen  water + carbon dioxide
Skeleton equation: CH4 + O2  H2O + CO2
To balance the compounds, take note of how many
atoms of each element occur on each side of the
reaction arrow.
See Page 207
(c) McGraw Hill Ryerson 2007

11. Counting Atoms to Balance Equations
Skeleton equation: CH4 + O2  H2O + CO2
Carbon = 1 Carbon = 1
Hydrogen = 4 Hydrogen = 2
Oxygen = 2 Oxygen = 3
Balanced equation: CH4 + 2O2  2H2O + CO2 Carbon = 1 Carbon = 1
Hydrogen = 4 Hydrogen = 4
Oxygen = 4 Oxygen = 4
The same number of atoms must be on each side.
(c) McGraw Hill Ryerson 2007
See Page 207

12. Balancing Equations H2 + O2  H2O hydrogen + oxygen  water Reactants
Products H 2 O 2 1

13. YOU CANNOT CHANGE THE SUBSCRIPTS
Balancing Equations X
hydrogen oxygen  hydrogen peroxide
H O  H2O2
YOU CANNOT CHANGE THE SUBSCRIPTS
Reactants
Products H 2 O

14. Balancing Equations 2 H2 + O2  H2O hydrogen + oxygen  water
Reactants
Products H 2 O 1

15. Balancing Equations H2 + O2  H2O 2 hydrogen + oxygen  water
Reactants
Products H 2 4 O

16. Balancing Equations 2 H2 + O2  H2O 2 hydrogen + oxygen  water
Reactants
Products H 4 O 2

17. Birkeland’s Big Bag ‘O Balancing Tricks
Balance chemical equations by following these steps:
Trial and error will work but can be very inefficient.
USE A TABLE (write atoms underneath reactants and products)
If they look the same on both sides of the equation, treat polyatomic ions (such as SO42–) as a group & balance them as such.
If ‘OH’ and H2O are in the equation, write water as HOH.
Balance one compound at a time & rewrite the # of atoms in the chart as things change.
Only add coefficients; NEVER change subscripts!!!
If H and O appear in more than one place, attempt to balance them LAST.
Balance everything that isn’t ‘H’ or ‘O’ 1st.
Balance the ‘H’s 2nd to last.
Balance the ‘O’s last.
Always double-check after you think you are finished.
CHECK YOUR ANSWERS!!!
See pages
(c) McGraw Hill Ryerson 2007

18. ON BALANCING EQUATIONS
WATCH THIS LESSON
ON BALANCING EQUATIONS
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19. Balance the following:
Fe + Br2  FeBr3
Sn(NO2)4 + K3PO4  KNO2 + Sn3 (PO4)4
C2H6 + O2  CO2 + H2O

20. ____Ba + ____H2O  ____Ba(OH)2 + ____H2
 ____CO2 + ____H2O  ____H2CO3   
____Fe2O3 + ____C  ____Fe + ____CO
____Fe + ____H2O  ____H2 + ____Fe2O3

21. Turn Word Equations into
Balanced Equations
If you don’t transform your word into a skeleton equation properly, you won’t be able to balance the equation correctly.
Change chemical names into chemical formulas. 4 types:
Simple ionic compounds
Multivalent ionic compounds
Ionic compounds with polyatomic ions
Covalent compound
Be careful of diatomic elements -- remember the special seven!!
H2, N2, O2, F2, Cl2, Br2, I2
See page 208
(c) McGraw Hill Ryerson 2007

22. Turn Word Equations into
Balanced Equations
Several common covalent molecules containing hydrogen have common names that you should know and MEMORIZE!!
methane = CH4
glucose = C6H12O6
ethane = C2H6
ammonia = NH3
See page 208
(c) McGraw Hill Ryerson 2007

23. Turn Word Equations into
Balanced Equations
Example #1:
Word Equation: Solutions of lead nitrate react with potassium iodide to produce solid lead iodide and a solution of potassium nitrate.
Skeleton Equation: Pb(NO3)2(aq) + KI(aq)  PbI2(s) + KNO3(aq)
Balanced Equation: Pb(NO3)2(aq) + 2KI(aq)  PbI2(s) + 2KNO3(aq)
Example #2:
Word Equation: Copper reacts with hydrogen nitrate to produce copper (II) nitrate plus hydrogen.
Skeleton Equation: Cu + H(NO3)  Cu(NO3) H2
Balanced Equation: Cu + 2H(NO3)  Cu(NO3) H2
See page 208
(c) McGraw Hill Ryerson 2007