Acids and Bases Flashcards
Define bronsted-Lowry acid
Give an example
Proton donor
HA + H2O → H3O+ + A-
Define bronsted-Lowry base
Give an example
Proton acceptor
B + H2O → BH+ + OH-
Define strong acid
Completely dissociates
Define weak acid
Slighty dissociates
Define strong base
Completely ionises
Define weak base
Slightly ionises
What happens during acid base equilibria?
Protons are transferred from acid to base
Does water fully dissociate or slightly?
Slightly dissociates
Where is kw derived from?
Kc x [H2O]
Ionic product of water
Unit
Kw = [H+] [OH-]
mol2 dm-6
Does the value of Kw change as the temp changes?
Yes, it varies w/ temp
What is the value of Kw at 298k?
1.00 x 10-14 mol2 dm-6
Why is a solution w/ pH 6 still considered neutral?
[H+] = [OH-]
Equation for pH
pH = -log [H+]
pH to conc equation
[H+] = 10-pH
Give the expression of Ka
Unit
Ka = [H+] [A-] / [HA]
Ka = [H+]2 / [HA]
mol dm-3
When is Ka used?
Weak acid
How do you work out Ka from pKa?
Ka = 10-pKa
How do you work out pKa from Ka?
pKa = -log (Ka)
Would you expect strong acids to have higher or lower pKa value than weak acids?
Lower
What is the vertical line in a pH curve?
Equivalence point - neutral
How do you work out which indicator to use?
The one that changes colour over narrow pH range that lies on vertical part of pH curve
Methyl orange
Colour change
pH range
- Red to yellow
- 3.1 - 4.4
Phenolphthalein
Colour change
pH range
- Colourless to pink
- 8.3 - 10
Which indicator do you use for
Strong acid/strong base titration
- Either indicator
- Rapid pH change over range of both indicator
Which indicator do you use for
Strong acid/weak base titration
Methyl orange
Which indicator do you use for
Weak acid/strong base titration
Phenolphthalein
Which indicator do you use for
Weak acid/weak base titration
- Neither bc no sharp pH change
- Use pH meter
Define buffer
Sol that resists changes in pH when small amounts of acid/base are added or when diluted
What is an acidic buffer?
- pH less than 7
- Mixing weak acid w/ salt
- Weak acid slightly dissociates, salt fully dissociates
What happens when you add acid to an acidic buffer?
- H+ conc inc
- Equil shifts to left
- Oppose inc in H+
- pH doesn’t change
What happens when you add base to an acidic buffer?
- OH- conc inc
- OH- reacts w/ H+ to form water, removing H+ from solution
- Equil shifts to right bc acid dissociates to form H+
- pH doesn’t change
What is a basic buffer?
- pH greater than 7
- Mixing weak base w/ salt
- eg. ammonia + ammonium chloride
- Salt fully dissociates
- Weak base reacts w/ water
- NH3 + H2O ⇔ NH4+ + OH-
What happens when you add base to a basic buffer?
- OH- conc inc
- Sol = more basic
- OH- reacts w/ NH4+ → NH3 + H2O
- Equil shifts to left to remove OH-
What happens when you add acid to a basic buffer?
- H+ conc inc
- Sol = more acidic
- H+ reacts w/ OH- → H2O
- Equil shifts to right to replace in OH-
- H+ reacts w/ NH3 → NH4+
What are buffers used for?
- Shampoo (pH 5.5) - hair becomes rougher if exposed to alkaline
- Biological washing powder - keep pH right level for enzymes to work efficiently
