Topic 12 Acid Base Equilibria Flashcards
What are brønsted Lowry acids and bases
Proton donators and acceptors
What is transferred in acid base reactions
Protons
What is pH
-log[H+]
What do acids actually form in water
H3O+ / hydroxonium ions
What’s the difference between weak and strong acids
Strong acids fully dissociate in water but weak acids dissociate poorly
What behaves as a base when acid is added to water
The water
What do acids and bases react to form
Salts
Is the enthalpy change of neutralisation always exo or endothermic
Exothermic
What are the 2 enthalpys involved in neutralisation of a weak acid
- Enthalpy of dissociation
- Enthalpy for the reaction of OH- and H+
What enthalpy is involved in neutralisation of a strong acid
Enthalpy when OH- and H+ react
What is Ka
Acid dissociation constant
When working with Ka for a weak acid what 2 assumptions do you have to make
- [HA] at equilibrium = initial conc
- [H+]=[A-] at equilibrium
Does water exist in equilibrium or not
Yes
Water dissociation equation non simplified
2H2O <—> H3O+ + OH-
Water dissociation equation simplified
H2O <—> H+ + OH-
Kc expression for water
Kc = [H+][OH-]/[H2O]
Formula of Kw
Kw =[OH-] x [H+]
What is the value of Kw
1.00 x 10^-14
Does Kw change with temp and why
Yes - dissociation of water increases with temperature so there is more OH- and H+
What is special about pure water
[OH-] = [H+]
What is the simplified definition of Kw when working with pure water
Kw = [H+]^2
Define pKw and pKa
pKw = -logKw
pKa = -logKa
What do you use to measure pH
pH meter
Qualities of strong base into strong acid titration curve
Equivalence point at 7, starts at pH 1 ends at pH 13
Qualities of weak base into strong acid titration curve
Equivalence point < 7 , start at pH 1 end at pH 9
Qualities of strong base into weak acid titration curve
Equivalence point > 7 , starts at pH 5 ends at pH 13
What is the half neutralisation point and what can it be used to calculate
Half way between start and equivalence points, pH at this point can be used to calculate pKa as [HA] = [A-]. Cancels out in the Ka equation so Ka = [H+] which can be found from the pH (pKa = pH)
What makes an indicator “suitable”
Change colour completely within the vertical section of a titration curve
Methyl orange colour change
Red-Orange acid - base
Phenolphthalein colour change
Clear - pink
Acid - base
Why are no indicators suitable for weak acid/base titrations
There is no sharp pH change
What is a buffer
A solution that resists changes in pH when a small amount of acid or base is added
What makes up an acidic buffer
A weak acid and a salt of its conjugate base
What happens when you add an acid to an acidic buffer (2 steps)
- H+ ions react with A- (high conc from salt)
- More HA is produced as equilibrium has shifted left
What happens when you add a base to an acidic buffer (2 steps)
- OH- ions react with H+ ions in solution
- Equilibrium shifts right so HA dissociates to H+ and A- to restore the concentration of H+ ions
What other way can a buffer be formed and where does this happen.
Give 2 steps to explain this process
Excess weak acid + strong base - in a titration.
1. All of the base reacts HA + OH-—-> A- + H2O
2.A- and excess HA left in solution which forms a buffer solution
What are basic buffers made of
Weak base and its salt
What weak acid and ion form the buffer in blood. Give two equilibrium equations for this buffer
Carbonic acid and hydrogencarbonate ions
1. H2CO3 <—> H+ + HCO3-
2. H2CO3 <—-> H2O + CO2
What do you have to remember when calculating pH of buffers
[H+] ≠ [A-]
What is the Henderson Hasselbalch equation and what is it used for
pH = pKa + log [A-]/[HA]
Used to work out how to make a buffer of a specific pH
