AND HOW TO BALANCE THEM

Chemical Equations

What is a Chemical Equation?

A chemical equation is a symbolic representation of a chemical reaction

Indications of a Chemical Reaction

Evolution of heat and light

Production of a gas

Formation of a precipitate

Color change

All chemical reactions are written in the following format:

Reactants Products

Law of Conservation of Mass

Atoms are neither created nor destroyed in ordinary chemical reactions.

equations must be balanceduse coefficients

Coefficient – small whole number that appears in front of a formula in a chemical equation (1 is assumed)

More on equations….

Each formula in an equation shows the physical state of the compound it represents

2HCl(aq)+Ca(OH)2(aq) CaCl2(s)+2H2O(l)

(s) – solid (l) – liquid (g) – gas (aq) – aqueous (dissolved in water)

More on equations…

A catalyst speeds up a reaction without being changed written over the arrow in chem. eq.

reactants catalyst products

Look on wiki for other equation symbols you should know . . .... Or look at chart on pg. 246 of your book for other equation

symbols

Diatomic Molecules

Some elements that are gases in their elemental form exist as diatomic molecules, or two atoms bonded together Halogens Hydrogen Oxygen Nitrogen

Br2, H2, O2, N2, Cl2, I2, F2 (brother honclif, lucky 7)

Writing equations

1. reactants on left of arrow2. products on right of arrow3. plus sign between each compound4. write correct states of matter (s,l,g,aq)

Balancing

Each side of the arrow must have the same # of each element

Check to make sure compounds are written correctly You CANNOT balance an equation until all compounds

are written correctly

Balance equation using coefficients

Balancing

1. Get yourself an unbalanced equation. Given or figured out (must have correct formulas!)

2. Draw boxes around all the chemical formulas.

Never, ever, change anything inside the boxes. Ever.

3. Make an element inventory. Keep it current throughout the whole problem.

4. Write numbers in front of each of the boxes until the inventory for each element is the same both before and after the reaction.

Whenever you change a number, update the inventory

Balancing

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

1 N 2 N3 H 2 H

2NH3(g) N2(g) + H2(g)

2 N 2 N6 H 2 H

2NH3(g) N2(g) + 3H2(g)

2 N 2 N6 H 6 H

This equation is now balanced and the coefficients are in the simplest form.

Balance the following equations as a class

Al + O2 Al2O3

CO2 + H2 CH4 + H2O

C2H6 + O2 CO2 + H2O

Fe2O3 + H2SO4 Fe2(SO4)3 + H2O

Balance the following equations as a class

2Al + 3O2 2Al2O3

CO2 + 4H2 CH4 + 2H2O

2C2H6 + 7O2 4CO2 + 6H2O

Fe2O3 + 3H2SO4 Fe2(SO4)3 + 3H2O

Please balance at your desk

Mg (s) + O2 (g) MgO (s)

K2O + H2O KOH

C3H8 + O2 CO2 + H2O

Al + CuO Al2O3 + Cu

Fe2(SO4)3 + KSCN K3Fe(SCN)6 + K2SO4

Please balance at your desk

2Mg (s) + O2 (g) 2MgO (s)

K2O + H2O 2KOH

C3H8 + 5O2 3CO2 + 4H2O

2Al + 3CuO Al2O3 + 3Cu

Fe2(SO4)3 + 12KSCN 2K3Fe(SCN)6 + 3K2SO4

Balancing Tips!

You may only put numbers in front of molecules, never change the subscripts!!

Al + O2 Al2O3

Al2 + O3 Al2O3

2Al + 3O2 2Al2O3

Balancing Tips!

You can reduce OR multiply

C2H6 + O2 CO2 + H2O

C2H6 + 7/2O2 2CO2 + 3H2O

2C2H6 + 7O2 4CO2 + 6H2O

Balancing Tips!

Start with large formulas or elements that appear only once on each side

Fe2(SO4)3 + KSCN K3Fe(SCN)6 + K2SO4

Make sure this one balances first

When balancing equations, keep in mind:

Balance polyatomic ions as one item

Fe2(SO4)3 + KSCN K3Fe(SCN)6 + K2SO4

Fe2X3 + 12KY 2K3FeY6 + 3K2X

When balancing equations, keep in mind:

Balance elements and diatomic elements lastOxygen and hydrogen last (usually)

Al + 3CuO Al2O3 + Cu

2Al + 3CuO Al2O3 + 3Cu

Balance these first

Balance these second

Writing Equations from Words

Hydrogen gas and oxygen gas combine to form liquid water.

H2 , O2 , H2O

H2(g) + O2(g) H2O(l)

2 H 2 H2 O 1 O

H2(g) + O2(g) 2H2O(l)

2 H 4 H2 O 2 O

2H2(g) + O2(g) 2H2O(l)

4 H 4 H2 O 2 O

The equation is balanced and in it’s simplest form

Try these…

When calcium carbonate is heated, calcium oxide and carbon dioxide are produced.

Solid sodium hydrogen carbonate reacts with hydrochloric acid to produce aqueous sodium chloride, water, and carbon dioxide gas.

Aluminum metal reacts with oxygen in the air to form aluminum oxide.

Try these…

When calcium carbonate is heated, calcium oxide and carbon dioxide are produced.

CaCO3 CaO + CO2Solid sodium hydrogen carbonate reacts with

hydrochloric acid to produce aqueous sodium chloride, water, and carbon dioxide gas.

NaHCO3 + HCl NaCl + H2O + CO2

Aluminum metal reacts with oxygen in the air to form aluminum oxide.

Al + O2 Al2O3

Writing Words from Equations

K2O + H2O 2KOHPotassium oxide

combined with water makes potassium hydroxide.

2C2H6 + 7O2 4CO2 + 6H2O

Ethane combusts in the presence of oxygen to form carbon dioxide and water

2Al + 3CuO Al2O3 + 3Cu

Aluminum and copper (II) oxide make aluminum oxide and copper

2NH3(g) N2(g) + 3H2(g)

Nitrogen gas and hydrogen gas are formed when ammonia decomposes.

Try These . . .

Fe2(SO4)3 + 12KSCN 2K3Fe(SCN)6 + 3K2SO4

CO2 + 4H2 CH4 + 2H2O

HCl + Ca(OH)2 CaCl2 + H2O

2KClO3(s) --> 2KCl(s) + 3O2(g)