Acids, Bases and Buffers Flashcards
Define a Brønsted-Lowry acid?
Proton donor
Define a Brønsted-Lowry base?
Proton acceptor
Define Lewis acid?
Electron pair acceptor
Define Lewis base?
Electron pair donor
What ion causes a solution to become acidic? (2 answers)
H+
H3O+
Write an equation for the ionisation of water.
H2O(l) —> H+ (aq) + OH- (aq)
Give an example of a monobasic acid.
HCl
Give an example of a dibasic acid.
H2SO4
Give an example of a tribasic acid.
H3PO4
Define strong acid.
An acid that dissociates completely
Give some examples of strong acids
- HCl
- H2SO4
- HNO3
Define weak acids.
Acids that only partially dissociate
Give some examples of weak acids.
- methanoic acid
- any organic acid
Write the acid dissociation constant expression.
For acid HA, HA —> H(+) + A-
Ka = [H+][A-]/[HA]
What does a larger Ka value mean?
Larger the Ka — greater the extent of dissociation
Write the equation used to convert Ka into pKa
pKa = -log10(Ka)
What is the relationship between pKa and strength of the acid?
smaller the pKa the stronger the acid
Write the equation used to convert the concentration of H+ into pH.
pH = -log[H+]
Why is a pH scale useful compared to the concentration of H+?
pH scale allows a wide range of H+ concentration to be expressed as simple positive values.
[H+] of a strong acid is equal to what?
[H+] = [HA]
What is the assumption made when calculating pH of weak acids?
It is assumed that the concentration of acid at equilibrium is equal to the concentration of acid after dissociation. This is because only very little of the acid dissociates.
Write the expression for the ionic product of water, Kw.
Kw = [H+][OH-]
What are the units for Kw?
mol^2 dm^-6
What physical factors affect the value of Kw? How?
Temperature only — if temp is increased, the equilibrium moves to the right so Kw increases and the pH of pure water decreases.
Define the term strong base.
Base that dissociates 100% in water.
Give some examples of strong bases.
- NaOH
- KOH
- Ca(OH)2
Give an example of a weak base.
Ammonia
Write the equation uses to calculate [H+] of strong bases.
[H+] = Kw/ [OH-]
Define a buffer solution
A mixture that minimises pH change on addition of small amounts of an acid or a base
What are the 2 ways in which buffers can be made?
- weak acid and its conjugate base
- weak acid and a strong alkali
In which direction does the equilibrium shift when an acid is added to a buffer solution? Why?
Equilibrium shifts to the left because [H+] increases and the conjugate base reacts with the H+ to remove most of the H+
In which direction does the equilibrium shift when an alkali is added to a buffer solution? Why?
Equilibrium shifts to the right, because [OH-] increases and the small concentration of H+ reacts with OH-. To restore the H+ ions HA dissociates shifting the equilibrium.
Write the equation used to calculate [H+] of buffer solution.
[H+] = Ka x [HA]/[A-]
Which buffer system maintains blood pH at 7.4? What happens when an acid/alkali is added?
H+ + HCO3- —> CO2 + H2O
Add OH- —> reacts with H+ to form H2O, then shifts equilibrium left to restore H+ lost
Add H+ —> equilibrium shifts to the right, removing excess H+
What is a titration?
The addition of an acid/base of known concentration to a base/acid to determine the concentration. An indicator is used to show that neutralisation has occurred, as is a pH meter
Define the term equivalence point.
The point at which the exact volume of base has been added to just neutralise the acid, or vice versa.
What is the end point?
The point at which pH changes rapidly
What are the properties of a good indicator for a reaction? (3)
- sharp colour change
- end point must be the same as the equivalence point
- distinct colour change so it is obvious when the end point has been reached
What indicator would you use for a strong acid-strong base titration?
Phenolphthalein
What colour is methyl orange in acid? In alkali?
Red in acid; yellow in alkali.
What colour is phenolphthalein in acid? In alkali?
Colourless in acid; red in alkali
What colour is bromothymol blue in acid? In alkali?
Yellow in acid and blue in alkali
Describe how to use a pH meter
- remove the pH probe from storage solution and rinse with distilled water
- dry the probe and place it into the solution with unknown pH
- let the probe stay in the solution until it gives a settled reading
