Chapter 20 - Acids, Bases and pH Flashcards
(42 cards)
define acids and alkalis using the Arrhenius model
- acids dissociate and release H+ ions in aqueous solution
- alkalis dissociate and release OH- ions in aqueous solution
what is the Bronsted-Lowry definition of a base
a base is a proton acceptor
what is the Bronsted-Lowry definition of an acid
an acid is a proton donor
define a conjugate acid-base pair and give an example of one
“a conjugate acid-base pair contains two species that can be interconverted by the transfer of a proton”
HCl acid with Cl- base
give an example of a reaction with two acid base pairs, identify them
HCl + OH- —> H2O + Cl-
HCl and Cl- are an acid-base pair
H2O and OH- are an acid-base pair
define a monobasic acid
“monobasic acids are acids where one hydrogen atom can be replaced per molecule”
E.g. HCl
define a dibasic acid
“dibasic acids are acids where two hydrogen atoms can be replaced per molecule”
E.g. H2SO4
define a tribasic acid
“Tribasic acids are acids where three hydrogen atoms can be replaced per molecule”
E.g. H3PO4
define pH, give the equation
-log([H+])
the negative log (base 10) of the hydrogen ion concentration in a solution
what are the things to note about the pH scale e.g. high/low values etc.
high value = low H+ conc.
low value = high H+ conc.
its logarithmic so pH 3 is 10x higher H+ conc. than pH 2
what can we say about the H+ ion concentration of a strong acid
it is the same as the concentration of the acid because the acid fully dissociates
what happens to the pH of a solution if it is diluted by 10 times
- the pH increases by 1 because the H+ ion concentration decreases by 10 times
what is the difference between a strong acid and a weak acid
- a strong acid completely dissociates/ionises in aqueous solution
- a weak acid partially dissociates/ionises in aqueous solution
what is Ka
a version of Kc to represent the strength of a weak acid/how much it dissociates/where equilibrium lies
how can we calculate Ka for a weak acid HA
HA –Reversible–> A- + H+
Ka = [A-][H+] / [HA]
what is the only thing that changes Ka
- it acts in the same way as Kc, the only thing that changes it is temperature
what is pKa
like pH,
pKa = -log(Ka)
(base 10)
what can we say about the strength of a weak acid compared to its Ka and pKa value
- the stronger the acid, the higher the Ka value, the lower the pKa value
- the weaker the acid, the lower the Ka value, the higher the pKa value
what two factors does the H+ ion concentration depend on in a weak acid
- Ka
- [HA]
how can we layout an equilibrium for a weak acid in order to find its Ka
HA –reversible–> A- + H+
[HA]start, 0,0 = initial concs
[HA]eqm, [A-], [H+] = eqm concs
[HA} eqm = [HA]start - [H+]eqm
so
Ka = [H+][A-] / [HA]eqm
what is the simplified equation for Ka of a weak acid that allows us to calculate [H+] more easily
Ka = [H+]eqm^2 / [HA]start
what are the two assumptions we make that allow us to form the simplified equation for Ka of a weak acid
- we can assume that the dissociation of water is negligible and therefore there are no additional H+ ions so [H+] = [A-], allowing us to have x^2 on the top of the equation
- we can assume that the change in concentration of HA is so small that it is negligible, thus [HA]eqm = [HA]start
how can we find [H+] and therefore pH from a Ka and initial conc. of acid
Ka = [H+]eqm^2 / [HA]start as in the simplified equation rearrange to [H+] = sqrt(Ka x [HA]start) then pH as normal
where do the two assumptions we make break down
- we can no longer assume dissociation of water is negligible if, pH > 6, so in very weak or very dilute acids
- we can no longer assume [HA]start = [HA]eqm if Ka is larger ( Ka > 10^-2) or in very dilute solutions
