Types of Chemical Reactions
By Jason Doub
Most reactions in chemistry take place with water as the solvent.
Solutions where water is the solvent are called aqueous solutions. Water is known
to be a polar molecule because the unequal distribution of charge caused by the
electronegative oxygen. This polarity gives the water the ability to dissolve ions and
other polar substance.
A useful method for characterizing an aqueous solution is its electrical conductivity.
If a solution conducts electricity well, it is considered a strong electrolyte.
If it only conducts slightly, it is considered a weak electrolyte; if it
doesn’t conduct, it’s a nonelectrolyte.
Strong electrolytes are substances that are completely ionized in water. Such examples
are strong acids, strong bases, and soluble salts. Acids ionize into H+ and A-;
bases ionize into OH- and X+.
A weak electrolyte is a substance that only slightly ionizes when added to water. These
substances include weak acids, weak bases, and slight soluble salts. Weak acids are those
that only dissociate slightly into H+ and A-. Similarly, weak bases
are those that dissociate only slightly into OH- and X+.
Nonelectrolytes are substances that dissolve in water, but that don’t break up
into ions. These substances are mostly polar molecules; the reason they don’t conduct
electricity is that no ions are formed
One type of reaction is called a precipitation reaction.
This occurs when two solutions are mixed resulting, and a solid or precipitate forms. The
precipitate contains ions that when combined are insoluble with water. However, these
individual ions do dissolve in water. So, the result of mixing these ions is an insoluble
solid. One example of a precipitation reaction is when a solution containing Ca2+
and a solution containing SO42-; the result of mixing these the
formation of solid CaSO4.
Another type of reaction is an acid-base reaction. An
acid-base reaction is one when the net reaction is the combination of a proton and
hydroxide to form water. The acid base reaction is also called a neutralization reaction.
When performing calculations fo an acid-base reaction follow these simple steps:
- List all the species before any reaction occurs and decide what reaction will occur.
- Write the balance net-ionic equation
- Calculate the moles of reactants
- Determine the limiting reactant where appropriate
- Calculate the moles of the required reactant or product.
- Convert to grams or volume, as required.
The final type of aqueous reaction is an oxidation-reduction
reaction. An oxidation-reaction is characterized by the transfer of one or more
electrons. These reactions are often used for energy production; in fact, these reactions
are often used in the human body to provide energy.
The concept of oxidation states provides a means for keeping track of the
movement of electrons in a redox reaction. The oxidation states can be assigned according
to these rules:
- The oxidation state of an atom in an element is 0.
- The oxidation state for a single atom ion is the charge of the ion.
- Oxygen is assigned the oxidation state of –2 in covalent compounds, except in
peroxides where oxygen is assigned a –1 state.
- In covalent compound hydrogen is assigned a +1 state.
- In a covalent compound, fluorine is always –1 state.
- The sum of the oxidation states must equal to overall charge of the molecule.
When balancing oxidation it is convenient to divide the reaction into two half-reactions;
one reaction involves the oxidation, the other the reduction. Then, follow these steps if
it’s in acid:
- write the half reactions
- for each half reaction balance the element except H and O, balance the O with water,
balance the H with H+, and balance the charge with electrons.
- If necessary, multiple the reaction by an integer to equalize the number of electrons.
- Add the half reaction
If the reaction occurs in base repeat the above steps, but after balancing the hydrogen
with H+, add hydroxides to cancel out the H+’s. Then, continue
the above steps as before.