L3: Constructing and Balancing Equations

Objectives

• Construct and use word equations

• Construct, balance and use symbol equations, with state symbols, including ionic equations

Names, symbols and state symbols

Names are used in word equations. For example:

magnesium + oxygen → magnesium oxide

Symbols are used in symbol equations. For example:

Mg_(s) + O_2(g) → MgO_2(s)

In symbol equations, we also indicate the state of each reactant and product using the following _subscripts:

• Solid (s) Fe_(s) CaCO_3(s)

• Liquid (l) Hg_(l) H₂O_(l)

• Gas (g) CO_2(g) H₂O_(g)

Aqueous solution (solution in water):

• Aqueous (aq) NaCl_(aq) CuSO_4(aq)

Rules for Balancing Equations

Write the symbol equation

Add coefficients (large numbers) to the left of the symbols until the number of atoms of each element in the reactants is the same as in the products.

Start by balancing the more complicated chemicals first! Leave the simplest substances (e.g. Oxygen) for the end

If there are polyatomic ions, treat them as if they were their own element

NEVER, EVER balance by adding subscripts (small numbers) to your symbols. If you do this, YOU ARE CHANGING THE CHEMICALS THEMSELVES!