Lecture notes: CHEM103 – November 13, 2008

LECTURE OUTLINE:

Nomenclature

Recognizing types of reactions

1. Balancing chemical equations

a. Balancing molecular equations

(Conservation of mass!)

b. Balancing ionic reactions (total & net)

(Both mass and charge balanced!)

BALANCING CHEMICAL EQUATIONS

I) Start with a complete, unbalanced equation

II) Count atoms on reactant and product side

a) Atoms found in large, complex polyatomic molecules

(note: balance polyatomic ions as a unit, when possible)

b) Atoms found only one time on each side

c) Atoms found multiple times on each side

d) Atoms in elemental form (e.g. Na, Cl₂, H₂, Fe, O₂)

III) Increase coefficients to each atom until atoms are balanced and repeat II) as needed

IV) Change coefficients to the lowest whole-number ratios if needed

(by multiplying or dividing by the common denominator)

V) Re-check for a balanced equation!

VI) Add states of matter (i.e. aq, s, g, l) to all chemical species

Notes: coefficients work on all atoms attached to it

can’t change subscripts within a molecule when balancing!

can’t add products or reactants (at least for now…)

examples:

calcium reacts in air (O₂) to form calcium oxide solid.

calcium phosphate solid reacts with sulfuric acid

CO₂ + H₂O à C₃H₈ + O₂

liquid nitroglycerine (C₃H₅N₃O₉) explodes to produce a mixture of gases: carbon dioxide, water, nitrogen and oxygen.

BALANCING IONIC CHEMICAL EQUATIONS (TOTAL IONIC & NET IONIC)

Rules for balancing molecular chemical equations

I) Start with a complete, unbalanced equation

II) Count atoms on reactant and product side

a) Atoms found in large, complex polyatomic molecules

(note: balance polyatomic ions as a unit, when possible)

b) Atoms found only one time on each side

c) Atoms found multiple times on each side

d) Atoms in elemental form (e.g. Na, Cl₂, H₂, Fe, O₂)

III) Increase coefficients to each atom until atoms are balanced and repeat II) as needed

IV) Change coefficients to the lowest whole-number ratios if needed

(by multiplying or dividing by the common denominator)

V) Re-check for a balanced equation!

VI) Add states of matter (i.e. aq, s, g, l) to all chemical species

If you stop here: molecular equation

VII) dissociate ALL soluble (aq) chemical compounds

If you stop here: total ionic equation

VIII) Cancel “spectator ions”

If you finish here: net ionic equation

examples:

iron (III) chloride + ammonium phosphate

sodium nitrate + potassium carbonate

silver (I) sulfate + barium chloride

sulfuric acid + calcium hydroxide

acetic acid + sodium hydroxide

nitric acid + potassium carbonate

Definition: “spectator ion”

How do you determine which ions are spectator ions?

When do you leave the spectator ions out?

Why is this helpful?

So why would you ever include the spectator ions at all?