NOTES: 11.1 – Writing and Balancing Chemical Equations

Effects of chemical reactions:

• Chemical reactions rearrange atoms in the reactants to form new products.

• The identities and properties of the products are completely different from that of the reactants.

What is a Chemical Reaction? • Chemical Reaction – one or more substances

change into new substances• Process involves reactants and products• Reactant – a starting substance • Product – a substance formed • Example:

Nitrogen and hydrogen gas can react to form ammonia under certain conditions.

Reactants Yield Products

N2 (g) + H2 (g) NH3 (g)

How Can You Tell Whether or Not a

Chemical Reaction Has Taken Place?

• Chemical Change – alters a given material by changing its chemical composition

• Production of gases and color changes are signs of chemical reactions

• Examples: burn, rust, decompose, corrode, explode

Chemical Reactions:

• we can describe a chemical reaction with words:

“Iron metal reacts with oxygen gas to produce iron(III) oxide, or rust.”

• we can then write a word equation:

Iron + oxygen iron(III) oxide

Chemical Equations:• Chemical equations are used to represent or describe chemical reactions.

• Chemical equations use chemical symbols and formulas for the reactants and products

• For example when hydrogen, H2,burns, it reacts with oxygen, O2, in the air to form water. We write the chemical equation for this reaction as follows:

2H2 + O2 2H2O

Chemical Equations:

An equation shows…

Chemical formulas of reactants;

Chemical formulas of products;

Molecule / Mole ratios of all compounds in the reaction.

Chemical Equations:

We read the (+) sign as “reacts with” and the arrow ( ) as “produces” or “yields”.

2H2 + O2 2H2O

Reactants Products

To show physical states of each substance:

• (s) = solid • (l) = liquid• (g) = gas• (aq) = aqueous

**aqueous means dissolved in water

• Consider again the reaction of iron with oxygen to form iron(III) oxide, or rust.

Fe(s) + O2 (g) Fe2O3 (s)

(unbalanced)

**this is a skeleton equation in that is NOT “balanced” and does not show the relative amounts of reactants and products

Coefficients & Subscripts

COEFFICIENTS: numbers in front of compound that represents the number of molecules/moles of that compound

SUBSCRIPTS: small numbers within a formula that help define the compound.

2H2SO4

Coefficient Subscript

H2O: One molecule of water

2H2O: Two molecules of water

H2O2: One molecule of Hydrogen Peroxide

• During a chem. rxn. atoms are rearranged (NOT created or destroyed!)• Chemical equations must be BALANCED to show the relative amounts of all substances.• Balanced means: each side of the equation has the same # of atoms of each element.

CH4 + O2 H2O + CO2

Unbalanced

CH4 + 2O2 2H2O + CO2 Balanced

In order to balance…

• Write correct formulas for all reactants and products

• Reactants Products• Count the number of atoms of each

element in reactants & products.• Balance one at a time using

coefficients.• Check for balance• Are the coefficients in the lowest

possible ratio?

Balancing EquationsBalancing Equations

NOTE: When balancing equations, you may change coefficients as much as you need to, but you may

never change subscripts because you can’t change what substances are involved.

Examples:

CuCl2 (aq) + Al (s) Cu (s) +AlCl3 (aq)

3CuCl2 (aq) + 2Al (s) 3Cu (s) + 2AlCl3 (aq)

(3:2:3:2)

Examples:Propane, C3H8, burns in oxygen, O2, to form carbon dioxide and water.

C3H8 + O2 CO2 + H2O

Balance C – then H – then O

C3H8 + 5O2 3CO2 + 4H2O

(1:5:3:4)

Examples:Pentane, C5H12, burns in oxygen, O2, to form carbon dioxide and water.

C5H12 + O2 CO2 + H2O

Balance C – then H – then O

C5H12 + 8O2 5CO2 + 6H2O

(1:8:5:6)

Examples:Silver nitrate reacts with copper to produce silver and copper (II) nitrate.

AgNO3 + Cu Ag + Cu(NO3)2

2AgNO3 + Cu 2Ag + Cu(NO3)2

(2:1:2:1)

Examples:Phosphorus reacts with oxygen gas to produce diphosphorus pentoxide.

P + O2 P2O5

4P + 5O2 2P2O5

(4:5:2)

Examples:

C7H14 + O2 CO2 + H2O

Balance C – then H – then O

C7H14 + 10½O2 7CO2 + 7H2O

2C7H14 + 21O2 14CO2 + 14H2O

(2:21:14:14)