Writing and Balancing Chemical Equations

A Review

by Mrs. Volpe

What is an equation?• An equation is a sentence, with either words or formulas, that

describes what happens in a given chemical reaction.

• Parts of an equation:– Subscripts and Coefficients

• Subscript tells how many of one particular element in the molecule.

• Coefficient tells how many of the molecule.

– Reactants and Products• Reactants are starting materials• Products are ending materials

– Symbols: yields (s) solid (g) gas fire (l) liquid (aq) aqueous

What does a balanced equation look like?

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

Reactants Products

Law of Conservation of Matter:Matter can be neither created norDestroyed, only changed in form.

“If you start with it, you must end with it.”

subscript

coefficient

How to Balance An Equation

• Make a list of the elements present on both sides of the reaction.

• Count the number of atoms on both sides of the reaction.

• Look for the ones that do not match, and change COEFFICIENTS ONLY to multiply the number of atoms present.

• Start with metal atoms• End with Hydrogen• Keep polyatomic ions grouped together, unless they split up

in the products.

Balancing in Action

Al + Fe2O3 Al2O3 + Fe

Al 1 Al 2

Fe 2 Fe 1

O 3 O 3

2

2

2

2

Dealing with word equations

• When going from words to formulas:– Check subscripts in formulas

– Remember naming rules

– Remember to balance!

Ex: Hydrogen chloride and silver (II) oxide are combined to form water and silver (II) chloride.

2 HCl + AgO H2O + AgCl2

•When going from formulas to words:

2 Na + CaCl2 2 NaCl + Ca

Sodium + calcium chloride sodium chloride + calcium

Sodium is combined with calcium chloride to form sodium chloride and calcium.