Hey it’s professor Dave, let’s talk about electrolytes.
Anyone who is a fan of sports beverages has heard of the word electrolyte before, but
what does this word really mean?
This term refers to a substance that when dissolved in water will dissociate into positively
charged and negatively charged ions, which have the ability to conduct electricity in
solution.
A substance that will not dissociate into ions when dissolved in water will not be able
to conduct electricity and is therefore called a nonelectrolyte.
The greater the extent to which a substance dissociates, the stronger an electrolyte it
is, so as we look at different types of electrolytes, we will be focusing on the efficiency of the
dissociation.
If a substance completely dissociates in solution, it is a strong electrolyte, and will conduct
electricity very well.
If a substance only partially ionizes in solution, meaning some particles dissociate and some
do not, this will be called a weak electrolyte, which will still conduct electricity, but
not as well as a strong electrolyte.
And as we said, a substance that does not dissociate at all will be a nonelectrolyte.
We can categorize substances as being one of these three things by measuring the electrical
conductance of their corresponding aqueous solutions.
In order to conduct electricity, a substance must contain freely mobile, charged species,
such as the case with metallic wires, which allow for the free movement of electrons.
In a solution, these freely mobile charged species are the ions that result when a substance
dissociates.
The more ions there are in solution, the greater the conductivity, and the stronger the electrolyte,
and we can measure the current flow, or simply observe the brightness of a light bulb included
in the circuit to gather this kind of data.
Let’s categorize electrolytes a bit further.
Something like potassium chloride will dissociate entirely, so it is a strong electrolyte.
It is also an example of an ionic electrolyte.
These are ionic compounds that will dissociate in aqueous solution to give individual cations
and anions.
The dissolution occurs because the cations and anions will be able to make ion-dipole
interactions with the water molecules that comprise the solvent, and this energy storage,
combined with the greater disorder of the solution over a crystallized ionic solid,
is what results in spontaneous dissociation.
The ions will therefore be dispersed throughout the solution in order to maximize solvation,
that is, to maximize the ion-dipole interactions that are occurring, by allowing for as many
water molecules to interact with each individual ion as possible.
This will be the case with most ionic solids, which is why ionic solids, whether comprised
of monoatomic ions or polyatomic ions, tend to be strong electrolytes.
Acetic acid, being a weak acid, will only partially dissociate, so it is a weak electrolyte.
This is also an example of a covalent electrolyte because the molecule contains strictly covalent
bonds, and unlike ionic solids that allow their interionic attractions to dissipate
in favor of solvation, these covalent species undergo an actual chemical reaction with the
solvent molecules in solution to form charged products.
These will typically be strong acids and bases that either donate a proton to a water molecule
or remove a proton from a water molecule.
For example, hydrogen chloride is a covalent compound.
There are no formally charged particles in this substance, only neutral atoms.
But when dissolved in water, it will become hydrochloric acid, which is a strong acid,
and will completely transfer all of its protons to water molecules, resulting in a certain
amount of hydronium ions and chloride ions.
This means there will be enough ions in solution for it to be considered a strong electrolyte,
so it will conduct electricity.
Of course, the identity of the solvent plays a big part in this, as in order for hydrochloric
acid to donate a proton, something must accept that proton, so if hydrochloric acid were
to be placed in an aprotic solvent like ether or hexanes, this would not result in an electrolytic
solution, as these solvents can’t accept any protons the way a water molecule can.
But for aqueous solutions, let’s keep in mind that strong acids and bases will be strong
electrolytes because they ionize completely in solution, whereas weak acids and bases,
like acetic acid, will only partially ionize, and will therefore be weak electrolytes, as
they will generate relatively small concentrations of ions in solution.
Lastly, something like ethanol won’t dissociate almost at all, so it won’t generate any
current, making it a nonelectrolyte.
That’s because it doesn’t have appreciable acidity or basicity, so the reactions it will
make with water molecules in solution will be negligible.
The same can be said for sugar, and many other covalent compounds, or water insoluble ionic
compounds.
We want to make sure we can identify substances as being strong electrolytes, weak electrolytes,
or nonelectrolytes, so let’s check comprehension.