Acid base equilibria Flashcards
what is PH defined as in formula
-log[H+]
where [H+] is the concentration of hydrogen ions in mol dm–3
How can you calculate concentration of hydrogen ions
[H+] = 10-pH
What is a strong acid
an acid that dissociates almost completely in aqueous solutions
what is a weak acid
an acid that partially dissociates in aqueous solutions so fewer hydrogen ions are formed
What is the Ka expression
acid dissociation constant and has the units mol dm-3
why is the concentration of hydrogen ions assumed
due to the ionisation of water is negligible
what does a higher Ka value indicate
the more dissociated the acid and the stronger it is
Why is total [H+] the same as the [HA]
The number of hydrogen ions formed from the ionisation of water is very small relative to the [H+] due to ionisation of the strong acid and can therefore be neglected
what does the ionisation of sulphuric acid suggest about the concentration of hydrogen in dibasic acids
H2SO4 → HSO4- + H+
HSO4- ⇌ SO42- + H+
the first step is thought to be fully ionised, the second step is suppressed by the abundance of hydrogen ions from the first step creating an equilibrium
The result is that the hydrogen ion concentration is less than double the acid concentration
When can the pH of weak acids be calculated
when the following are known:
- The concentration of the acid
- The Ka value of the acid
what can we assume about Ka expression of equilibrium concentration of [H+] and [A-]
the same since one molecule of HA dissociates into one of each ion
What is Kw
[H+ (aq) ][OH- (aq) ]
for all aqueous solutions is 1x10-14 mol2dm-6
what is the relationship between Kw and pKw
pKw = -logKw
what can you tell from a pH curve
pH of the acid
pH at equivalence point
volume of base at equivalence point
Draw the pH curve for:
strong acid + strong base
weak acid + strong base
strong acid + weak base
weak acid + weak base
what is an alkali
a soluble base
what does end point refer to in comparison to equivalence point
change in colour of indicator whereas equivalence point is volume of base required to neutralise the acid
what is a buffer solution
a solution which resists changes in pH when small amounts of acid or alkalis are added
what happens when hydroxide ions are added to buffer solution
hydrogen ions react with them to form water and decrease in hydrogen ions would mean the pH will increase but equilibrium move to right to replace removed hydrogen ions and keep pH constant
how is a acidic buffer solution created
from a weak acid and one of its salts
what does a buffer consist of
weak acid and its conjugate base or a weak base and its conjugate acid
what exactly is half equivalence point
stage of the titration at which exactly half the amount of weak acid has been neutralised
The equivalence point in a weak acid - strong base titration is….
above 7
whats a key thing to talk about in titration curve questions
ph range lies completely in vertical region and indicator will change colour in vertical region
devise an experiment to find the Ka of a solution of ethanoic acid with access to a pH meter and a solution of NaOH
titrate ethanoic acid with sodium hydroxide
measure pH at regular intervals and plot a pH graph against volume
use the graph to find the pH half equivalence point
use phenolphthalein to find end point and add the same volume of acid to the mixture at end point
measure pH of resultant mixture with a pH meter
Ka = 10-pH
how to obtain Ka from titration curve
read pH at half equivalence point as pH is pKa
what happens if you use an unsuitable indicator for a titration
colour change will be complete before equivalence point is reached
when can you use equilibrium amount in moles rather than equilibrium concentrations for calculating Kc
Same number of moles/molecules on both sides of the equation
(1)
(so) volume / V cancels in K expression
why does pink colour of phenolphthalein fade after end point of titration is reached
the equilibrium shifts to the left
ОГ
the mixture absorbs carbon dioxide from the atmosphere
(1)
so the mixture is (becoming more)
acidic / the acid reforms
describe a buffer region for potassium hydroxide and propanoic acid
buffering region
as a result of both acid and its conjugate base
as excess Ch3Ch2COOh has reacted with KOH
Why is a pH greater than 7 formed using potassium hydroxide and propanoic acid
propanoate ions are present (conjugate base) at equivalence point
proponoate ions react with water to form KOH in excess
OH > H+
weak acid strong base titration
How to find PH in such circumstances
HA in excess
A- in excess
HA = A-
HA in excess = Use [HA] and [A] along
A- in excess = with Ka to find [H*], then pH.
HA = A-
Use Kw to find [H*], then pH.
In this case, pKa is equal to pH, therefore find pa.
