acids and bases Flashcards
what is a brønsted-lowry acid?
a proton donor
what is a brønsted-lowry base?
a proton acceptor
what does ‘strong’ mean in terms of acids and bases?
completely dissociates in solution
what are some examples of strong acids?
HCl, H2SO4 + HNO3
what does monoprotic mean?
1mol of acid produces 1mol of H+
what does diprotic mean?
1mol of acid produces 2mol of H+
what are strong bases?
they produce OH- if the base dissolves in water
what are some examples of strong bases?
KOH, NaOH, CuOH
what is meant by ‘weak’ in terms of acids and bases?
only slightly dissociate in solution
constituent elements: weak acids are mainly…
“carbon containing” (organic)
e.g., ethanoic acid
what is an example of a weak base?
ammonia
where does the position of equilibrium lie for weak acids and bases?
far to the left
what scale is used to measure pH?
the pH scale (logarithmic)
a difference of 1 on the pH scale is a 10x increase/decrease of [H+]
what formula do we use to calculate pH?
pH = –log10[H+]
what formula do we use to calculate [H+] using pH?
[H+] = 10(-pH)
what is the pH of neutral solutions?
pH 7 at room temp.
what is an alkali?
a soluble base
alkaline solutions = when bases dissociate in water
what is Ka?
the acid dissociation constant - this is only applicable for WEAK ACIDS, due to them only partially dissociating
what is the relationship between Ka and acid dissociation?
the higher the value for Ka, the more the acid will dissociate when placed in solution; strong acids have exceptionally high Ka values
what equilibrium do we need to consider when looking at Ka?
HA ⇌ H+ + A-
what is the formula for Ka?
Ka = [H+][A-]/[HA]
what are the units used for Ka?
mol dm-3
how would the formula look for a PURE SOLUTION of a weak acid?
Ka = [H+]^2/[HA]
what is the formula for calculating pKa?
pKa = -log(Ka)
what is the formula for calculating Ka from pKa?
Ka = 10^(-pKa)
what assumption do we make about HA?
that the [HA] at equilibrium is the same as the [HA] initially - we assume this as we say that the acid is so weak, barely any of it has dissociated. Again, this is only applicable for pure solutions of the weak acid.
what does Kw denote?
the ionic product of water
what equilibrium are we considering when looking at Kw?
H2O ⇌ H+ + OH-
what is the formula for calculating Kw?
Kw = [H+][OH-]
what are the units for Kw?
mol2 dm-6
what is the value of Kw AT ROOM TEMP?
1.00 x10^-14 ; if the value is not given within the question, we assume this value
why does pure water remain pure, despite a pH different to 7?
for water to be pure, [H+] has to be equal to [OH-]
how would the formula look different for Kw of PURE water?
Kw = [H+]^2
how many types of different titration curves are there?
four:
weak acid-weak base
weak acid-strong base
strong acid-weak base
strong acid-strong base
what is the definition of equivalence point?
the point at which the moles of acid and the moles of base present would neutralise each other
what is the definition of end-point?
the point at which the indicator changes colour
what pH does the inflection occur for strong acid-strong base?
between pH2 and pH11
what pH does the inflection occur for strong acid-weak base?
between pH2 and pH8
what pH does the inflection occur for weak acid-strong base?
between pH6 and pH11
what pH does the inflection occur for weak acid-weak base?
there is no major point of inflection
what is the choice of indicator dependent on?
the pH in which the indicator changes colour must work with the pH at which the equivalence point is - it must be within the inflection pH of the curve
what are the two main indicators for titrations?
phenolphthalein and methyl orange
what is the colour change for phenolphthalein?
colourless in acid –> pink in base
what is the pH range of the colour change for phenolphthalein?
pH8.3 - pH10.
appropriate for titrations including a strong base
what is the colour change of methyl orange?
red in acid –> yellow in base
what is the pH range of the colour change for methyl orange?
pH3.1 - pH4.4
what does dilution involve?
changes in the conc. of H+/OH- in the solution - this is therefore accompanied by a change in the pH of the solution
what is the formula for calculating the new conc. of a solution following dilution?
new conc. of solution = moles of solute/new total volume.
what is the definition of a buffer?
buffers are solutions which can resist small changes in pH caused by dilution/addition of small amt. of acid/base.
what are the two main types of buffer?
acidic buffers and basic buffers
buffer action: what happens when a small amt of dilute acid is added?
- addition of extra acid involves addition of extra H+
- A- (acid) reacts with the H+ and removes them from the solution - this keeps the pH almost constant
buffer action: what happens when a small amt of dilute base is added?
- addition of extra base involves addition of extra OH-
- OH- reacts with the H+ (small amt), removing some H+ from solution - however, position of equilibrium shifts right to restore the H+, which keeps the pH almost constant
what does an acidic buffer consist of?
an excess of weak acid and its salt
e.g. CH3COONA and CH3COOH can be mixed to form a buffer.
alternatively, the CH3COONA may be formed indirectly from the reaction of NaOH with excess CH3COOH
what does a basic buffer consist of?
an excess of weak base and its salt
what formula do we consider when finding the pH of a buffer solution?
Ka = [H+] x ([salt]/[acid])
