Acids, Bases and Buffers Flashcards
What is an acid?
proton donors
What is a base?
proton acceptors
The definitions we use for acids and bases are called
Bronsted Lowry acids and bases
oxonium ion?
H3O+ - H2O forms a dative covalent bond to H+
when acids dissociate?
(x)H+ + negative ion always forms
When a monoprotic acid dissociates?
1 (mol) of H+ forms
When a diprotic acid dissociates?
2 H+ form
When a triprotic acid dissociates?
3 H+ form
Idm if acid is di/triprotic, when it dissociates how many moles of the salt forms?
1 mole
could say ? instead of protic
basic
Acid base pairs?
- involves 2 acid base pairs
* 2 particles that change into each other by losing/ gaining a H+
ABP: what does the acid form?
Conjugate base by donating a H+
the CB has a negative charge
ABP: what does the base form?
Conjugate acid.
the CA has a positive charge
water is amphoteric meaning?
can act as acids and bases
strong acid definition?
Fully dissociate into H+ e.g. HCl
weak acid definition?
only partially dissociate into H+
conc acid definition?
high no. of moles per dm3
dilute acid definition?
fewer moles per dm3
for the reaction to be a neutraliation?
water must be formed, so metal + acid is not a neutralisation
⭐ionic equations
only include ions that change e.g. state
pH equation?
-log [H+]
[H+] =
10 to the power of -pH
calc pH of strong acid?
• in strong acid, all H+ dissociate so [H+] dissociated = [H+] in acid
give pH to?
2dp
calc pH of di/triprotic acid?
x the [H+] by 2/3 b4 putting in pH equation
What is Kw used for?
- the ionic product of water, 10^-14mol2dm-6 at 298K
* used to find the pH of alkaline solutions
Kw equation?
Kw/ [OH-] = [H+]
when do u use Kw?
when the [OH-] is given, get [H+] and then calc pH
why does an increase in T change the pH of water?
• endo forwards reaction favoured, eq shifts to the right
• [H+] inc
so pH ⬇
remember that lower pH =
more acidic
to calc pH of weak acids?
DIFFERENT METHOD - can’t just use [acid] as [H+]
neutral solution =
equal conc of H+ and OH- - does not have to be at pH 7
when 2x OH in the alkali?
times the [OH-] by 2
dissociation of a weak acid is?
reversible
Ka =
- the acid dissociation constant
- larger the Ka value, the stronger the acid - eq lies towards products
- Units = moldm-3
Ka equation?
Ka = [H+][A-] / [HA]
simplified to Ka = [H+]2/ [HA]
Ka (in relation to pKa)?
Ka =10-^pKa
10 is not just 10 it’s…
shift log
pKa =
pKa = -logKa
think of Ka like [H+]
the stronger the acid, the lower the?
pKa value
3 steps to calculating the pH of a weak acid?
- get Ka if pKa given
- get [H+]
- get pH
weak acid: depending on the no. of marks,
show the rearranging of the Ka equation
how can u tell if an acid is weak?
must be given Ka or pKa
relationship between pH and [H+]
as pH ⬇, [H+] ⬆
relationship between pKa and Ka?
as pKa ⬇, [Ka] ⬆
same relationship as pH and [H+]
What is a bufffer?
- solution that can resist a change in pH when small amounts of acid or base are added
- can be acidic or basic
What do buffer contain?
• a weak acid and a large amount of a salt of a strong base
e.g. ethanoic acid and sodium ethanoate - G1 metals used cos soluble
What’s the point of the salt of a strong base?
forms the same negative ion as the acid when the acid dissociates. creates a reserve
What happens to a buffer when H+ are added?
- equilibrium shifts to the left
- uses up H+
- we’d run out of the negative ion
- but bc of the salt, there’s a reserve of negative ions and pH doesn’t change
if OH- are added to the buffer solution?
- would react w H+, reducing H+ so eq would shift to the right inc amount of H+
- not gonna run out of acid bc it’s weak so only a small amount has ionised - lots left
- this is why weak acid is used
- pH doesn’t change
salt not involved here
why can we used the moles instead of conc to calc Ka of buffer solution?
bc all solutions are in the same total V so conc ratio = moles ratio
for buffers don’t ⭐
simplify [H+][A-} to [H+]2
buffers work out?
moles of both then do Ka
If acid added to buffer?
amount of acid inc and salt decreases by that amount of moles
If you want an buffer of a specific pH the choose a an acid of a similar ?
pKa value
3 factors that influence pH?
Ka, [acid], [base]
What is the most important factor that determines the pH of a buffer?
Ka/ pKa
what can alter the pH of a buffer solution?
ratio of acid to salt
how is the pH of a buffer affected by dilution?
not diluted, Salt and acid are in the same container so the ratio of them doesn’t change - stays constant and so pH doesn’t change
Weak acid weak base indicator?
unclear where the equivalence point is so diff to select an indicator
indicator pHs?
over 7: P
under 7: methyl orange
equivalence point?
[H+] = [OH-]
What can be done from an acid base/ graph?
read pH and calc [H+]
strong acid and strong base equivalence point at?
ph 7
For an indicator to work in a titration?
the range of the indicator’s colour change must be within the range of the pH change at the equivalence point
