3.1.12: Acids and bases Flashcards
(25 cards)
What is a bronsted-lowry acid?
proton donor
What is a bronsted-lowry base?
proton acceptor
What is the difference between a monoprotic and diprotic acid?
- monoprotic- acids releasing one H+
- diprotic- acids releasing 2 H+
What is the equation for pH and [H+]?
pH= -log [H+]
[H+]= 10^-pH
What is the equation for the dissociation of water?
H2O ⇌ H+ + OH-
▲H= endothermic
What is Kw?
ionic product of water
Kw= [H+][OH-]
(no H2O as conc. of water in water is 1)
Why is the pH of water at room temperature 7?
- pH of water= 7
- [H+] and [OH-]= 10^-7
- Kw @ 25°C= 10^-14
Why is water neutral?
has the same concentration of hydrogen and hydroxide ions
[H+] = [OH-]
How does temperature effect pH and the neutrality of water?
- as temperature increases, equilibrium shifts to endothermic direction to oppose the change in temperature
- therefore [H+] and [OH-] increase, Kw increase and pH decrease
- water still neutral as [H+] = [OH-]
What is Ka?
acid dissociation constant
Ka= ([H+][A-]) ÷ [HA]
or
Ka= [H+]^2 ÷ [HA] in pure weak acids ONLY
What is the equation for pKa and Ka?
pKa= -log Ka
Ka = 10^-pKa
What is the equation for the dissociation of acid?
HA ⇌ H+ + A-
▲H= endothermic
How are Ka and pKa used to determine acid strength?
- the larger Ka. the smaller the pKa value, and the stronger the acid
- the lower the Ka value, the higher the pKa value, and the weaker the acid
What is the definition of a strong and weak acid?
- strong acid- all molecules fully ionise/ dissociate in water
- weak acid- only partially ionises/dissociates in water
What’s a pH indicator?
- weak acids where HA & A- are different colours
- HA ⇌ H+ + A-
- at low pH, HA main species present | at high pH, A- main species present
When does a pH indicator change colour?
- when mol of acid=mol of base
- indicator must change colour with range of rapid pH change at end point
What are pH curves used for?
to show how pH changes as alkali added to acid (or vice versa)
What is the equivalence point?
- when moles of acid= moles of alkali present
- pH not always 7 at eq. point
- (vertical point)
- where rapid pH change in most acid-alkali reactions
What is the end point of pH curves?
- where indicator changes colour
- = equivalence point
How do you know what indicator to use for acid/base strength combos?
- equivalence point/ pH colour change within range of indicator
- weak acid/ weak base has no vertical region, can’t be studied with indicator, use pH probe
What is a buffer?
a solution that resists changes in pH when small amounts of acid or alkali are added
(pH still changes, not by much)
How do you make an acidic buffer?
- weak acid and one of its salts (e.g. HA & A-: ethanoic acid and Na/K ethanoate (as Na & K always dissolve in water))
- mixing excess weak acid with strong alkali (as results in mix of HA & A- (partial neutralisation))
- [acid] and [salt] much higher than [H+]
What is the equation for dissociation of acid in buffers?
HA ⇌ H+ + A-
- equilibrium favours acid as weak acid so high concentration of it (only small amount ionises/ dissociates)
HA fully dissociates
What happens when you add H+ to buffer?
- H+ removed by reaction w/ CH3COO-
- equilibrium shifts to favour acid by Le Chatelier to oppose change
- remove H+ due to salt in high concentration,
- increase H+, decrease A-, increases HA
