Theory Flashcards
What is a strong bronsted acid
a proton doner that dissociates completely in water to give H3O+ ions and donates protons more easily compared to the H3O+ ions (single arrow)
the conjugate base of a strong acid has very low tendency to accept a proton
What is a weak bronsted acid
is dissociation single / double arrow ?
a proton doner that dissociates partially in water to give H3O+ and donates a proton less readily than H3O+. (reversible arrow) as the conjugate base of a weak acid has high tendency to accept a proton (also, may need to add H2O to weak and strong acid when dissociating them)
What are strong and weak bronsted base
is the dissociation single / double arrow ?
a strong bronsted base is a proton acceptor that dissociates completely in water to form OH- ions while a weak bronsted base is a proton acceptor that dissociates partially in water to form OH- ions (double headed arrow)
For weak bases, may need to add H2O when dissociating, strong ones no need
pH of solution of salt derived from weak acid and strong base
the conjugate base of the weak acid is a stronger base than water, and hence it will be able to undergo hydrolysis to form OH- ions and since [OH-] > [H+], PH>7 and solution is alkaline (Reversible arrow)
equation of dissociation of Al(NO3)3 in water
single / double arrow ?
Al(NO3)3 (s) + 6H2O (l) —> [Al(H2O)6]3+ (aq) + 3NO3- (aq)
complex ion formed with water as Al3+ cations are coordinated to water molecules through dative bonding
note: not a hydrolysis reaction as hydrolysis is when IONS of a salt reacts with water to produce H3O+ or OH- ions.
Explain the hydrolysis equation of complex ion of aluminium nitrate
[Al(H2O)6]3+ (aq) + H2O(l) ⇌ [Al(H2O)5(OH)]2+ (aq) + H3O+ (aq) (DOUBLE ARROW)
Al3+ ion is small and highly charged, hence having high charge density, hence withdrawing sufficient electron density from the O-H bonds of the coordinated H2O molecules, weakening the OH bonds. When the OH bonds break, a proton is released. Hence, the complex ion acts as a weak bronsted acid, and since [H3O+] > [OH-], pH < 7 at 25 degree celcius
What is a buffer solution
A buffer solution is one that is able to resist pH changed upon addition of small amounts of acids or base and consists of 2 species which makes up a conjugate acid base pair of a weak acid (means other one is strong conj base) or weak base (means other one is strong conj acid)
How does the buffer consisting of large reservoirs of CH3CO2H and its conjugate base CH3CO2- resist pH changes on addition on small amounts of acid / base ?
- When small amounts of H+ is added, the H+ ions react with the large reservoir of CH3CO2-
CH3CO2- + H+ –> CH3CO2H - This causes the conc of CH3CO2H to increase slightly and conc of CH3CO2- to decrease slightly,
- but since the original amounts of CH3CO2H and CH3CO2- are large compared to the amount of H+ ions added,
- the ratio [CH3CO2-] / [CH3CO2H] remains almost constant
- and since Ka = [CH3CO2-][H+] / [CH3CO2H] and Ka is constant at constant temp, [H+] and pH remains almost constant
- When small amounts of OH- ions added, it reacts with the large reservoir of unionised CH3CO3H molecules
- This causes [CH3CO2H] to decrease slightly and [CH3CO2-] to increase slightly
- However since the original amounts of [CH3CO2H] and [CH3CO2-] are large compared to the amount of [OH-] ions added
- the ratio [CH3CO2-] / [CH3CO2H] remains almost constant
- and since Ka = [CH3CO2-][H+] / [CH3CO2H] and Ka is constant at constant temp, [H+] and pH remains almost constant
Buffer equations are written with SINGLE DIRECTION arrows
What is an end point and equivalence point ?
End point is the point in a titration at which the indicator changes colour when 1 drop of excess titrant is added
Equivalence point is the point in a titration at which the exact number of moles of acid and base react completely
What makes an indicator suitable ?
- The indicator must have a distinct colour change
2. The working pH range of the indicator coincides with the rapid pH change at the equivalence point for the titration
Methyl orange
pH range 3.1-4.4, red in acidic region (below 3.1), yellow in basic region (above 4.4), In between the colour is orange
screened methyl orange
3.0-4.6 violet/green (make grey)
bromothymol blue
6.0-7.6 yellow/blue (make green)
phenolphthalein (not used in lab) because cannot diff between acidic and weakly basic solutions since colourless at both times
8.0-9.6 colourless/pink
thymol blue (replace phe in lab)
8.0-9.6 yellow/blue (green)
