5.1.3: Acids, bases and buffers Flashcards
Define a Bronsted-Lowry acid
Proton donor
Define a Bronsted-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) Name and formula
H+ (hydrogen ion) or, more accurately, H3O+ (oxonium ion), as protons react with H2O to form it
What ion causes a solution to become alkaline?
-OH (hydroxide ion)
Write an equation for the ionisation of water (2 marks)
2H2O (l) ⇌ H3O+ (aq) + -OH (aq) OR H2O (l) ⇌ H+ (aq) + -OH (aq)
Give example of a monobasic acid
HCl
Give example of a dibasic acid
H2SO4
Give example of a tribasic acid
H3PO4
Identify the acid base pairs for the reaction below
CH3COOH + H2O ⇌ CH3COO- + H3O+
Acid 1: CH3COOH
Base 2: H2O
Base 1: CH3COO-
Acid 2: H3O+
Define strong acid
Acids dissociate completely
Give some examples of strong acids
● Hydrochloric acid
● Sulfuric acid
● Nitric acid
What is the difference between concentrated and strong?
Concentrated means many mol per dm3,
strong refers to amount of dissociation
Define weak acids
Acids that only partially dissociate
Give some examples of weak acid
Methanoic acid, any organic acid
What is constant that is used to measure the extent of acid dissociation called?
Acid dissociation constant
What is the symbol of acid dissociation constant?
Ka
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 = -log10Ka
Write the equation used to convert pKa
into Ka
Ka= 10-pKa
What is the relationship between pKa and strength of the acid?
Smaller the pKa stronger the acid
Write the equation used to convert concentration of H+ into pH
pH = -log[H+]
Write the equation used to convert pH into concentration of H+
[H+] = 10-pH
Why is a pH scale useful compared to concentration of H+?
pH scale allows a wide range of H+
concentration to be expressed as simple
positive values
What is the relationship between pH and [H+]?
High pH value means a small [H+]
If two solutions have a pH difference of 1, what is the difference in [H+]?
A factor of 10
[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 ionic product of water, Kw
Kw = [H+][OH-]
What is the units for Kw?
mol2dm-6
What is the value of Kw at 298 K?
1.0 x 10-14
What physical factors affect the value of Kw? How do they affect it?
Temperature only - if temperature is increased,
the equilibrium moves to the right so Kw
increases and the pH of pure water decreases
Indices of of [H+] and [OH-] always adds up to what value?
-14
Define the term strong base
Base that dissociates 100% in water
Give examples of some strong bases
NaOH
KOH
Ca(OH)2
Give example of a weak base
Ammonia
Write the equation used 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
Which buffer system maintains blood pH at 7.4? What happens when 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 neutralization 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 marks)
Sharp colour change (not gradual) - no more than one drop of acid/alkali needed for colour change
End point must be the same as the equivalence point otherwise titration gives wrong answer.
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 or methyl orange, but phenolphthalein is usually used as clearer colour change.
What indicator would you use for a strong acid-weak base titration?
Methyl orange
What indicator would you use for a strong base-weak acid titration?
Phenolphthalein
What indicator would you use from a weak acid-weak base titration?
Neither methyl orange or phenolphthalein is suitable, as neither give a sharp change at the end point.
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 metre
● 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
