S C I E N C E 1 0

4.3 – BALANCING EQUATIONS

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

Subscript

Coefficient

Law of Conservation of Mass

Molecule

Atom

Skeleton Equation

Balanced Equation

Word Equation

• 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.

See pages 202 - 203

Reactants Products

(c) McGraw Hill Ryerson 2007 See pages 202 - 203

Reactants Products

• 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

( l ) = liquid

(g) = gas

COEFFICIENTS

- Indicates how many of each

molecule there is.

-Ie: there are 2 molecules of

NO.

• 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

(c) McGraw Hill Ryerson 2007

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).

• 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

• 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 + O2 2K2O

K

K

K

K

K

K

K

K

O O O

O

• 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

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.

See Page 207

BALANCING EQUATIONS

O2

Reactants Products

H 2 2

hydrogen + oxygen water

H2O

H2 +

O 2 1

BALANCING EQUATIONS

H2 + O2 H2O2

Reactants Products

H 2 2

O 2 2

hydrogen + oxygen hydrogen peroxide

YOU CANNOT CHANGE THE SUBSCRIPTS

BALANCING EQUATIONS

H2 + O2 H2O

Reactants Products

H 2 2

O 2 1

hydrogen + oxygen water

2

BALANCING EQUATIONS

H2 + O2 H2O

Reactants Products

H 2 4

O 2 2

hydrogen + oxygen water

2

BALANCING EQUATIONS

2 H2 + O2 H2O

Reactants Products

H 4 4

O 2 2

hydrogen + oxygen water

2 (g) (g) (l)

• 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 SO4

2–) 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 209 - 211

Practice!

• Balance the following:

• Fe + Br2 FeBr3

• Sn(NO2)4 + K3PO4 KNO2 + Sn3 (PO4)4

• C2H6 + O2 CO2 + H2O

____Ba + ____H2O ____Ba(OH)2 + ____H2

____CO2 + ____H2O ____H2CO3

____Fe2O3 + ____C ____Fe + ____CO

____Fe + ____H2O ____H2 + ____Fe2O3

• 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

See page 208

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

Example #1:

Word Equation: Solutions of lead nitrate reacts 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)2 + H2

Balanced Equation: Cu + 2H(NO3) Cu(NO3)2 + H2