3.1.12 Acids and bases Flashcards
the point that represents the solution is acting as a buffer
the straight part of the graph - no change in pH
what is a Bronsted - Lowry acid
proton donor
strong acid definition
completely ionising to give H+ ions in water
what do you use when calculating the HA
use the equivalence point
HA ->
H+ + A-
Ka =
conc H+ x conc A-
divided by
conc ACID HA
pH
-Log10 H+conc
pka
-Log 10 ka
kw
CONC H+ x conc OH-
what assumption can you make in weak acid calculations but not in buffer calculations
H+ conc = A- conc at eqm
p =
- P = - log
pure water
- Pure water = conc of H+ = conc of OH-
- Dilution questions
- Strong acid and water - conc H+ final = conc H+ initial x ( initial volume/ final volume)
just use OH- for bases
half neutralization point
- Half neuralization point
- Conc HA = conc A-
- Meaning pH = pKa
- Buffer calculations
- Initial to final
1. Start with mols of initial HA and A- ( and H+ / OH- from substance added)
2. Draw out the weak acid dissociation equation and the equation of the substance added
3. Workout mols of final - If an acid is added – then add h+ mols to the acid and take them away form the A-
- If a base is added – Oh- mols take away from acid and add them to A-
4. Rearrange Kw and wat not to workout what needed
why is pure water not acidic
as conc OH- = conc H+ by definition
what to do with a di basic and diprotic acid
- Di basic / di protic – have to multiple the conc of H+ or OH- by 2
- Example of a di protic acid = sulfuric acid – example of a di basic = group 2
working out the end poInt Ha
- Color change of the indicator = the end point
Alkali added - n(end point ) = (concentration x volume at the eq point)
- n end point HA = n point (oh-)
workout the pH of a neutralization Acid + Base
- workout the pH of a neutralization Acid + Base
1. workout which is in excess between the Oh- and the H+
2. use the leftover to calculate the pH
relation between H+ and pH
- as the concentration of H+ increases pH decreases
explain why H2O is not shown in the Kw equation
- explain why H2O is not shown in the Kw equation – the concentration of water is (almost) constant
- suggest why the Ph probe is washed with distilled water before new reading
– different solutions must not contaminate each other
- why volume added of base to acid decreases as the end point get closer –
to avoid missing the end point
why indicator is suitable
all have color change within the vertical/ steep part of the titration curve
if a sample is diluted
- if a sample is diluted – have to workout moles and then scale to the correct size (use the new volume) to workout correct concentration
to workout the mass of salt
- to workout the mass of salt – sort it out as an ice table ( initial – final )
the highest pH at the equivalence point
- the highest pH at the equivalence point = weak acid and strong base
- a buffer solution has a constant pH even when diluted show mathematically
ratio between conc HA/ conc A- = as the H+ conc is irrelevant at constant pH
WHAT IA TRUE ABOUT H+ conc at a concstant pH
it is irelevent
half neutralisation point important
I was also told that the half-equivalence point is when the concentration of a weak acid equals concentration of conjugate base: [HA]=[A−].
conjugate bace = proton acceptor
difference between the half neutralization point and the equivalence point
The equivalence point is where the amount of moles of acid and base are equal, resulting a solution of only salt and water.
If you are titrating an acid against a base, the half equivalence point will be the point at which half the acid has been neutralized by the base
End point
The volume of alkaline or acid added when the indicator has changed colour
Properties of a suitable indicator
The colour change must be sharp - no more than one drop needed to give a complete colour change
The endpoint of titration must be the same as the equivalence point - change colour in the vertical section of the graph
The indicator should give a distinct colour change
strong acid important point
mol of h+ and A- are the same
buffer solution made when base is added to weak acid
OH- mols = A- mols as 1:1
how to make a buffer
weak base and its salt (chloride) must be mixed together, for example, ammonia and ammonium chloride. To prepare acidic buffer solution, weak acid and its salt must be mixed.
example of a weak alakali
nh3
what does a beurette allow
variable volumes to be added more easily
