Physical 12: Acids and Bases Flashcards
What is a Bronsted-Lowry acid?
A substance that donates protons
What is a Bronsted-Lowry base?
A substance that accepts protons
How does water act as a base?
When it accepts protons
e.g.
HCl + H2O –> H3O+ +Cl-
How does water act as an acid?
When it donates protons
e.g.
H2O + NH3 –> NH4+ + OH-
What is the ionic product of water?
Kw = [H+][OH-]
How do you calculate pH using [H+]?
How do you calculate [H+] from pH?
How does concentration of [H+] change as pH increases?
As pH increases, the concentration of H+ ions decreases
How does Kw change with temperature?
As temperature increases, Kw increases
Because the equilibrium reaction of water is endothermic in the forward direction
So the reaction shifts in the endothermic direction (forward)
What is a strong acid?
One that completely dissociates into its ions in water
What is a strong base?
One that completely dissociates into its ions in water
What is a weak acid?
One that only partially dissociates into its ions in water
What is a weak base?
One that only partially dissociates into its ions in water
What is the acid dissociation constant?
Do strong acids have a smaller or larger acid dissociation constant?
The stronger the acid, the larger the value of Ka
Because the equilibrium will lie further to the right if the acid is more dissociated
What is pKa?
Often used to express Ka for a weak acid
Gives a measure of how strong a weak acid is
Small pKa = stronger acid
How do you calculate pKa?
Give three examples of strong acids.
Hydrochloric acid HCl
Nitric acid HNO3
Sulfuric acid H2SO4
Chloric acid HClO3
Give three examples of weak acids.
Ethanoic acid
Citric acid
Benzoic acid
Give three examples of strong bases.
Sodium hydroxide NaOH
Potassium hydroxide KOH
Calcium hydroxide Ca(OH)2
Barium hydroxide Ba(OH)2
Give two examples of weak bases.
Ammonia NH3
Ammonium hydroxide (NH4OH)
What is an acid-base titration used for?
To determine the amount of acid/alkali needed to neutralise a given amount of acid/alkali
How do you calibrate a pH meter?
Place the probe in a buffer solution of known pH
Draw the titration curve of a strong acid and strong base.
Draw the titration curve of a strong acid and weak base.
Draw the titration curve of a weak acid and strong base.
Draw the titration curve of a weak acid and weak base.
What is the equivalence point?
The point on a titration curve at which sufficient acid/alkali has been added to neutralise the solution
The number of moles of H+ is the same as the number of moles of OH-
What is the end point?
The point in a titration where the sufficient amount of acid/alkali has been added to make the indicator change colour
What are the three properties of a suitable indicator?
1) The colour change must be sharp rather than gradual at the end point
2) The end point must be the same as the equivalence point
3) The indicator should give a distinct colour change
What is the half-equivalence point?
The point half way between zero and the equivalence point
Half of HA has been converted into A-
So [HA] = [A-]
Prove that pKa = pH at the half equivalence point.
What is a buffer solution?
A solution that resists small changes to pH when small volumes of acid or alkali are added
They are based on an equilibrium reaction which will move in the right direction to either remove additional H+ or OH-
How do you make an acidic buffer?
Weak acid + soluble salt of that acid
How do acidic buffers work?
The dissociation of a weak acid is an equilibrium reaction
So if alkali is added, the OH- will react with HA to produce more H+ and A-, making the solution more acidic again
How do you make a basic buffer?
Weak base + soluble salt of that base
How do basic buffers work?
If acid is added, it reacts with the aqueous base
