it's professor Dave, let's talk about acids and bases
we've probably all heard the terms acid and base but what do these words mean on the molecular level
let's discuss the rigid definition according to a few different models. most
often when we talk about acids and bases we are referring to the brønsted-lowry
definition. according to this model an acid is something that donates a proton
and a base is something that accepts a proton. an H + ion is a proton because
if a hydrogen atom loses an electron all that is left is the proton that
comprises the nucleus. proton transfer is a big part of chemistry. an acid and base
will react to form a conjugate acid-base pair. the thing that gives up a proton is
the acid the thing that accepts it is the base. and on the product side the
base has become the conjugate acid and the acid has become the conjugate base
because the thing that gained the proton could potentially lose it again acting
as an acid and the thing that lost the proton might gain it back
acting as a base. the bronsted-lowry definition only applies to protic
species, or molecules with hydrogen atoms that can be lost as protons. but there
are aprotic species that also display acidity and basicity. this is what the
Lewis definition entails. under the Lewis definition a base is something that
donates a pair of electrons and an acid is something that accepts electrons. a
Lewis base is indistinguishable from a brønsted-lowry base but the acids
differ in the sense that a brønsted lowry acid must have at least one
hydrogen atom. a Lewis acid just needs some partially positive atom. a species
that can act as both a brønsted-lowry acid and base is said to be amphoteric
water is amphoteric because it can behave as an acid losing a proton to
become hydroxide
or it can act as a base gaining a proton to become a hydronium ion. in a sample
of water a tiny fraction of molecules transfer a proton from one to another
which we call an acid-base reaction. this is an equilibrium and it's shown here. we
can write an equilibrium expression by showing the concentrations of the
products over the concentrations of the reactants. we will only include gases and
aqueous substances so here it will look like this. at room temperature this
equals 1 x 10^14 or one in one hundred trillion. that's the
fraction of water molecules that are ionized at any moment. as temperature
increases Kw increases. hydrogen-ion concentration which is essentially
proton concentration, if above one times ten to the negative seven gives an
acidic solution, if below its basic, if equal its neutral. when looking at
conjugate acid-base pairs we must understand that a strong acid is
something that will very readily lose a proton. if something is a strong acid it
means its conjugate base is very stable, that's why it's so willing to lose a
proton. a strong acid will have a weak
conjugate base since the conjugate base will be very stable, meaning it isn't
dying to pick up a proton. a weak acid, something that isn't very willing to
lose a proton, will have a strong conjugate base, something that will very
easily gain a proton, so if the strength of an acid is proportional to the
stability of the conjugate base we have to be able to predict the stability of
conjugate basis in order to tell how strong an acid will be. whichever atom is
losing a proton the larger it is the more stable the conjugate base will be
comparing the acids made by combining a hydrogen with a halogen we can see that
HI is the strongest this is because when the proton leaves we are left with
an iodide ion. this is much larger than a fluoride ion and therefore can diffuse
the negative charge
around a greater volume thereby stabilizing itself. fluoride is much
smaller with a very localized charge and is therefore less stable so hydrofluoric
acid is a much weaker acid. also the more electronegative and
atom is the better it is at accommodating a negative charge so an oxygen atom can
more readily lose a proton than a carbon atom which is why something like water
is far more acidic than say methane.
acids are also stronger when their conjugate bases are resonance stabilized
a carboxylic acid is about a trillion times stronger an acid than water even
though both of them leave an oxyanion after deprotonating. this is because the
conjugate base of the carboxylic acid can share the burden of the negative
charge between two oxygen atoms by resonance while hydroxide can't. these
resonance structures don't actually exist but rather the composite resonance
structure does which shows delocalized pi electron density distributed about
this portion of the molecule. more on this in organic chemistry. there can be
monoprotic acids which can only lose one proton, or polyprotic acids which
can lose several. polyprotic acids become less acidic with each
deprotonation. in an acid-base equilibrium the side with the weaker
acid-base pair will always be favored. this is because the stronger acid or
base will have a greater tendency to react and generate the weaker species. we
can tell acid strength by looking at an acid's pKa. the lower the pKa the stronger
the acid. strong acids deprotonate completely, transferring every acidic
proton to molecules of solvent or other things in solution. weak acids only
deprotonate partially, generating some of the conjugate acid but won't react
completely. the weaker they are the less completely they will react. once again
the hydronium ion concentration measures the degree to which an acid
transfers protons to water molecules and therefore the strength of the acid
but hydronium concentrations have inconvenient exponents which can be
cumbersome to list so we have developed a more convenient way to describe acidity
the pH of a solution is the negative log of the hydronium concentration
logarithms are the inverse operations of exponents so the reason why a negative
log is useful here is that it takes an annoying number like 10 to the negative
seven and it turns it into seven which is much tidier so a solution with a pH
of 7 has a hydronium concentration that is considered neutral. we can also
measure the basicity of a solution by calculating the pOH. this is because a
strong base will generate hydroxide ions in aqueous solution by stealing protons
from water molecules. just like pH, pOH is the negative log of the hydroxide
concentration. another way to calculate pH and pOH is by using the formula for
Kw. from this we see that pH + pOH must equal 14. between these relationships we
can relate pH, pOH, hydronium concentration and hydroxide
concentration and we should be able to get from one to any of the others using
this chart. let's check comprehension
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