Balancing Redox Reactions by 1/2 Reactions

Redox Reactions

Redox reactions are reactions in which particles change charge by either losing or gaining electrons.  Whether a particle loses are gains is determined by its electronegativity, or attraction for shared electrons.  We've actually used this before in single replacement reactions.  A more active metal or nonmetal can replace a less active metal because it can either take or force another element to take electrons.

Reduction occurs when a particle gains electrons.  In other words by gaining negative charges, its charge is reduced.  Oxidation means a particle has lost electron, therefore its charge will become more positive.  There are several pneumonics you can use to remember this.

Thus far we have been balancing reaction only by mass.  We could do this because all the substance were written in neutral form.  Now we are using ions so we also have to balance a reaction by charge.

If the electrons are produced, they are being lost to the particle- they are no longer attached.  If the electrons are a reactant, they are being stuck on the particle- they've been gained.

Net Ionic Reactions

In a molecular or complete reaction, all substances are shown in neutral (molecular) form.

In an ionic reaction, the substances that all always dissociate (strong) are broken into their ions.

Because lead (II) nitrate, sodium chloride and sodium nitrate all all soluble salt (strong electrolytes), they will all completely dissociate in water.  They will not exist as molecules, but as ions.  Remember you can't lose any mass in a reaction!  The Law of Conservation of Matter still applies!

 Notice that sodium and nitrate are EXACTLY the same on the reactant and product sides?  They are spectator ions.  Spectator ions do not change in a reaction.  When you cancel the spectator ions, you're left with the net ionic reaction.

This shows us what is really changing in a reaction- the driving force for the reaction to proceed.

Aqueous Solutions- Dissociation

Water is a very polar molecule, meaning it has a partial positive charge on the hydrogen end and a partial negative charge on the oxygen end.  This is caused by the unequal sharing of electrons by the hydrogen and oxygen atoms.

Because water is polar, it will dissolve most ionic compounds.  Since ionic compounds are composed of a positive ion (cation) and a negative ion (anion), the opposite charged end of a water molecule will be attracted and break a large crystal into smaller pieces.  This is called hydration. If the molecules are completely broken into their ions by water, it is called dissociation.

For instance, table salt (NaCl) will completely dissociate in water. Every single molecule will be broken apart into ions and kept apart by the water molecules.

NaCl (aq) --> Na+ (aq) + Cl- (aq)

In chemistry, STRONG means that every molecule will dissociate when dissolved in water.  WEAK means that it partially dissociates, or that only some of the particles will dissociate while others will remain in neutral/molecular form.

Strong acids, strong bases and strong electrolytes will always dissociate when dissolved in water.  Weak acids, weak bases and weak electrolytes may or may not dissociate.

Oxidations Numbers

Oxidation numbers are the "effective charge" a particle has in a molecule or ion.  While all atoms WANT to have a full outer shell, and they TRY to lose or gain electrons, they sometimes aren't able to.  We all know that you don't always get what you want.  Sometimes 2 non-metals are forces to share electrons.  Both WANT to gain electrons, but the more electronegative element will get the electrons most of the time.  In other words, they don't share equally.  Oxidation numbers tell us what the charge really is in a particle situation.  Manganese can form a +2, +4, +5 and even +7 charge depending on what other atoms are around to take its electrons.  While sulfur wants to gain 2 electrons and form a -2 charge, it is very common for oxygen to grab its electrons and sulfur is left with a +6 charge.  It now has a full outer shell because its lost ALL its valence electrons.

Follow these rules to determine the oxidation number of an ion-

You can calculate the charge of an ion by using the entire compound (must =0) or a polyatomic ion (must = the charge given).  Here are 2 ways to calculate the charge of sulfur in sulfuric acid.

No matter which method you use, the oxidation number of sulfur in sulfuric acid is +6.