Acids and pH Flashcards
What is an acid-base pair?
Pair of two species that transform into each other by gain or loss of a proton
Draw titration curves for:
- weak acid strong base
- strong acid strong base
- strong acid weak base
- weak acid weak base
-
What is neutralisation?
Chemical reaction in which an acid and a base react together to produce a salt and water
Units of Ka
Always mol dm^-3
What does a large Ka show?
A strong acid because lots of dissociation
How does a buffer work?
An addition of acid the conjugate base reacts with H+ ions and equilibrium shifts to left removing H+ ions added
When alkali is added - conc. OH- ions increases - H+ ions react with the OH- ions, HA dissociates, equilibrium shifts to right to restore reacted H+ ions
HA H+ + A-
What is a buffer?
A mixture that minimises pH change on addition of small amounts of acid or base. It is a mixture of a weak acid and its conjugate base. Equilibrium mixture formed contains a high conc. of undissociated weak acid and A-. Conc. H+ is very low.
What is the equivalence point?
The point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution. It is the centre of the vertical section of a tritration curve.
What is half neutralisation point?
Volume is half of volume at equivalence point, here pKa = pH
What is an acid-base indicator and how is it represented?
A weak acid, represented as Hln
Hln H+ +ln-
Different colours in acid and conjugate base form
What is equation when indicator is at its end point?
[Hln] = [ln-]
How do you choose an indicator?
Chose one so that the pH at the end point is as close as possible to the titration’s equivalence point
What assumption is made when doing pH calculations for a strong acid?
[H+] = [HA]
Equation for [H+] of weak acid
square root of Ka x [HA]
Equation for pH of a buffer system
-log (Ka x ([HA]/[A-]))
Equation for ionic product of water
Kw = [OH-][H+] = 1.00 x 10^-14 mol2 dm-6 at 25 c
[OH-] for a strong base
[OH-] = conc. base
How does carbonic acid-hydrogen carbonate buffer system in blood work?
H2CO3 H+ + HCO3 -
(Then the carbonic acid is converted into aqueous carbon dioxide through the action of an enzyme, then converted to gas in the lungs and exhaled)
What is standard enthalpy change of neutralisation?
The enthalpy change which accompanies the neutralisation of an aqueous acid by an aqueous base to form 1 mole of H2O under standard conditions
What is a conjugate acid?
Species formed when a proton is added to a base
What is a conjugate base
A species formed when a proton is added to an acid
Ethanoic acid is a strong acid in liquid ammonia but is weak in water, why?
The solvents are acting as bases and ammonia is a better base than water
Suggest why some acid are weak in water while others are strong
The weaker or more polar the bond between the hydrogen and the rest of the molecule, the stronger the acid is likely to be because it can dissociate better
Under what circumstances, during the calculation of pH for strong acids and alkalis, is the assumption that [H+] and [OH-] due to ionisation of water is negligible valid and invalid?
Valid:
If the [H+] or [OH-] from the acid or alkali is very large compared with that of the water and INVALID if it is comparable to or smaller than
