So we see that while KNO3 is soluble, PbI2is
not, and must be the yellow solid.
A step further will allow us to simplify this equation dramatically.
The aqueous (aq) subscript indicates that a species is soluble in
water, and exists mainly as ions. Therefore, we write:
Then, we cancel out the species that appear on both sides of the
equation. Essentially, these do not change in the reaction (that's
why we call them spectator ions), and we have:
You'll run into many reactions of this type today, and you can
use this procedure on each one.
Acid-Base Reactions, for our purposes,
there are two ways of thinking about acids and bases. In the Arrhenius
definition, acids produce protons (H+) in water, and
bases produce hydroxide ions (OH-). It's a straightforward
and useful definition for many cases, but it doesn't include all
acids and bases. The Brønsted-Lowry concept describes acid-base
reactions as the transfer of a proton from one compound to another.
A molecule that donates a proton is called an acid, and
a molecule that accepts a proton is called a base.
We see that HBr is the acid because it gives up a proton, and that
H2O is the base because it accepts the proton. In equation
form, we have:
Often, you will see this reaction written in a simpler form. The
idea is the same, however.
This equation reflects the fact that HBr can be described with
the Arrhenius definition of an acid, as well as the Brønsted-Lowry
HBr is a fairly simple case, and you will come across reactions
that are more difficult to write. The key is to remember that for
it to be an acid-base reaction, one compound has to gain a proton,
and the other has to lose one. But what if you just can't tell which
species will donate and which will accept? What if you have a reaction
between ammonia (NH3) and water? Both contain hydrogen. Experimentally,
we use indicators. Indicators are substances that respond differently
to acidic and basic solutions-generally in terms of color. For example,
phenolphthalein is a common indicator that turns pink in basic solutions
and is clear in acidic solutions. In this lab, you will use Universal
Indicator. Just like phenolphthalein, its color depends on the acidity
of the solution. The trick is that you'll have to figure out how!
Complexation Reactions are, well, a little complicated, but that's
not the source of their name. A complexation reaction can be described
as a reaction that forms a "complex". For instance, in
adding a cobalt salt, such as CoCl2 (s), to water, we form [Co(H2O)6]2+ (aq).
The part of this substance
written in brackets is called a
complex ion; it involves more than one species but still carries
an overall charge (making it an ion) while in solution. The entire
group has a charge, much like a polyatomic ion (ex: [Cu(CN)2]-,
[Mn(OH2)6]2-). The list below helps describe complexation reactions
in more detail:
- Complexes generally form around a transition metal cation (ex:
Ni2+, Au3+, Cu+).
- Whether or not a complex forms depends on concentration, among
other factors. A reaction between two compounds may proceed normally
until it is "flooded" by one of the reagents, at which
point a complex ion is more likely to form.
- The formation of a complex is generally indicated by a color
change. In fact, most complex ions are very brightly colored.
- Don't worry! You won't be expected to figure out the formulas
for any complex ions you create.
Redox Reactions (oxidation reduction reactions)
involve the transfer of electrons. That's what oxidation and reduction
Loss of electrons
Gain of electrons
A useful way to remember this is with the mnemonic: Leo
Let's look at an example:
It may not look unusual at first, but this isn't like the other
reactions we've seen. No atoms are moving from one compound to another,
and nothing is dissociating! We need a way to explain how solid
aluminum and zinc ions transformed into aluminum ions and solid
zinc. To do this, chemists break the reaction into two half reactions,
one for each element.
Equation 11 is an oxidation reaction; each mole of solid aluminum
loses three moles of electrons to become a mole of aluminum cations.
Equation 12 is a reduction reaction; as each mole of zinc cations
gains two moles of electrons to become a mole of solid zinc. It's
important to note that for a reaction to occur there must be both
oxidation and reduction. One species can only lose electrons if
another species is in solution to take them.
It isn't always obvious from observation alone that a redox reaction
is taking place, but keep the option in mind. If you observe a reaction,
and you can't think of a combination of anions and cations that
could form new products, consider redox before you give up!