Topic 12 Acid-Base Equilibria Flashcards
Define an acid
a substance that can donate a proton i.e. a proton donor
Define an alkali
a substance that can accept a proton i.e. a proton acceptor
What is a requirement of an alkali/proton acceptor?
needs to have a lone pair of electrons to accept a proton and form a dative covalent bond
What is a conjugate acid-base pair?
consists of either a base and its conjugate acid or an acid and its conjugate base
What is a conjugate acid?
when a base accepts a proton
What is a conjugate base?
when an acid donates a proton
Define an amphoteric substance
a substance that can act as a base and an acid e.g. water
What ion causes a solution to be acidic?
H+ ion, hydrogen ion or H3O+ ion, oxonium ion (protons react with H2O to form it)
What ion causes a solution to be alkaline?
OH- ion, hydroxide ion
Write an equation for the ionisation of water
2 H2O(l) ---> H3O+(aq) + -OH(aq) OR H2O(l) ---> H+ (aq)+ -OH(aq)
Define a monoprotic acid
an acid that releases one H+ ion per molecule
Define a diprotic acid
an acid that releases two H+ ions per molecule
How do you work out pKa?
pKa= -lgKa
How do you work out pH using H+ ions?
pH= -lg[H+] or pH=lg 1/H+
What is Kw/ how do you work it out?
Kw is the ionic product of water Kw=[H+(aq)][OH-(aq)]
What is the equation to work out pH using acid and salt concentrations?
pH=pKa-lg[acid]/[salt]
What is a buffer solution?
a solution that minimises the change in pH when a small amount of either acid or base is added
What do acid-base reactions involve the transfer of?
protons
What are the 2 equations for the ionisation of water?
1) H2O(l) –> H+(aq) + OH-(aq)
2) 2H2O(l) –> H3O-(aq) + OH-
What is Kw value for water?
Kw= [H+][OH-]
What is the value of Kw at 298K?
1.00 x 10-14 (to the power of -14)
What physical factor effects the value of Kw and how?
temperature, an increased temperature moves the equilibrium to the right so Kw increases and the pH of pure water decreases
How do you calculate pKw?
pKw= -logKw Kw= 10-pKw (to the power of -pKw)
If 2 solutions have a pH difference of 1, what is the difference in [H+]?
a factor of 10
What is different when finding [H+] from the concentration of diprotic and triprotic acids?
need to multiply the concentration of the acid by the number of protons to find [H+]
Define a strong acid
one which fully dissociates in water
Define the term strong base
one which fully dissociates in water
What is the difference between strong and concentrated?
concentrated means many mols per dm3. strong refers to the amount of dissociation
What is a weak acid and weak base?
one which partially dissociates
Give some examples of strong acids
- HCL
- H2SO4
- H3PO4
Give some examples of strong bases
- NaOH
- CaCO3
- NaCO3
Give some examples of weak acids
- CH3COOH
- any organic acids
Give some examples of weak bases
-NH3
What is Ka
acid dissociation constant:
Ka=[H+][A-]/[HA]
Define equivalence point
the point at which the exact volume of base is added to the acid to neutralise it, or acid added to base
What generally happens to the pH of the solution around the equivalence point?
there is a large and rapid change in pH, except in a weak base and weak acid titration
What is the end point?
the volume of acid or alkali added when the indicator just changes colour, with the right indicator equivalence point= end point
State the properties of a good indicator
- sharp colour change- no more than a drop of acid or alkali needed to change colour
- end point must be the same as the equivalence point
- distinct colour change
What indicator would be used for a strong-acid strong-base titration?
phenolphthalein or methyl orange- phenolphthalein is the clearest colour change
What indicator would be used for a strong-acid and weak-base titration?
methyl orange
What indicator would be used for a strong base and weak-acid titration?
phenolphthalein
What indicator would be used for a weak base and weak acid titration?
neither would be suitable as there is no steep change
What is the half-neutralisation point?
when volume= half the volume that has been added at the equivalence point
Define a buffer solution
a solution that resists changes to pH when small amounts of acid or alkali are added
What do acidic buffer solutions contain?
- weak acid
- soluble salt that an acid fully dissociates
Write an equation for an acidic buffer with added alkali
HA + OH- –> H2O + A-
What do basic (base) buffer solution contain?
- weak base
- soluble salt of that weak base
How can you calculate the pH of buffer solutions?
use the Ka of the weak acid, sub in [A-] and [HA], calculate [H+] –> pH
How can you calculate the new pH of a buffer solution when acid or base is added?
calculate the number of moles of H+ and A- and HA before acid or base is added (initial moles). use equations to work out the new moles of A- and HA, work out [H+] –> pH
Why is there a difference in enthalpy changes of neutralisation values for strong and weak acids?
- enthalpy changes of neutralisation are always exothermic, value is similar for weak and strong acids because same reaction is occurring: H+ + OH- –> H2O
- weak acids have a less exothermic value because energy is absorbed to ionise the acid and break the bond to the hydrogen in the un-dissociated acid
What does a lower pKa value tell you?
that the acid is strong
Give the equation for the ionisation of HCl
HCl(aq) –> H+(aq) + Cl- (aq)
What does a large Ka value tell you?
that the acid is strong (small pKa value also tells you the acid is strong)
What is a neutral solution?
where the concentration of hydrogen ions and hydroxide ions are equal
How do you determine pKw?
pKw= -lgKw
What does diluting a strong acid by a factor of 10 do?
increase the pH by 1
What does diluting a weak acid by a factor of 10 do?
increase the pH by 0.5
