1.12 Acids and Bases
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What is a bronsted lowry acid and base
A bronsted-lowry acid is a proton donor eg NH4+ A bronsted-lowry base is a proton acceptor eg OH-
Define strong acid
An acid that completely dissociates to ions when in solution
Define weak acid
An acid that only slightly dissociates when in solution
Equation for pH
pH= -log10[H+]
Equation for conc of H+
[H+]=10^-pH
Equation for Kw and the constant value of it at 25C
Kw = [H+][OH-] 1x10^-14
What is Kw
The equilibrium constant when water slightly dissociates to ions
Equations involving Ka
Ka = 10^-pKa pKa= -log10Ka Ka=[H+][A-] / [HA]
What to use when HA is in excess
[HA] [A-] and Ka to find H+ then pH
What to use when A- is in excess
Use Kw to find[H+] then pH
What to use when HA=A-
Find pKa as pKa=pH
When does neutralisation occur for 1. Strong acid strong base 2. Strong acid weak base 3. Weak acid strong base 4. Weak acid weak base
• pH 7 • < pH7 (more acidic) • > pH7 (more basic) • normally pH 7 but hard to determine
When is methylorange used and what colour does it turn when
More acidic neutralisation point Red in acids and turns yellow at neutralisation point
When is phenolphthalein used and what colour does it turn when
More basic neutralisation point Pink in alkalis and colourless at neutralisation point
What is a buffer solution
A solution which is able to resist changes in pH when small volumes of acid or base are added
What do buffer solutions contain
Either a weak acid or a weak base and the salt of the weak acid/base
How to work out pH for acid and base
Find moles of each species Calc conc when at equilibrium using total volume Use Ka to find [H+] and pH
How to work out pH for acid and salt
Find moles of salt Use Ka to find pH
Why are buffers used
To keep systems regulated Important as enzymes or reactions in living organisms often require a specific pH which can be maintained using a buffer solution
Equation for ionisation of water
H2O(l) <–> H+(aq) + OH-(aq)
Give examples of strong acids
HCl H2SO4 H3PO4
Examples of strong bases
NaOH CaCO3 Na2CO3
Examples of weak acids
CH3COOH
Examples of weak bases
NH3