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?
H+ (hydrogen ion)
H3O+ (oxonium ion) as protons react with H2O to form it
What ions causes a solution to be alkaline?
OH-
hydroxide ion
Write an equation for the ionisation of water
2 H2O (l) H3O+ (aq) + OH- (aq)
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
Identify the acid pairs for the reaction below
CH3COOH + H2O CH3COO- + H3O+
CH3COOH = ACID 1 CH3COO- = BASE 1
H30+ = ACID 2 H2O = BASE 2
Define strong acid
Acids dissociates 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 the amount of dissociation
Define weak acids
Acids that only partially dissociate
Give some examples of weak acids
Methanoic acid, any organic acid
What is the constant that is used to measure the extent of acid dissociation called?
Acid dissociation constant
What is the symbol of acid dissociation constant?
Ka
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. the greater the extent of disccociation
Write the equation used to convert Ka into pKa
pka = -log (Ka)
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 equation used to convert pH into conc of H+
[H+] = 10^-ph
Why is a pH scale useful compared to the conc of H+?
pH scale allows a wide ranger of H+ concentrations to be expressed as a simple positive value
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]
Write the equation used to calculate [H+] of weak acids
[H+] = sqrt ka x [HA]
What is the assumption made when calculating pH of weak acids?
It is assumed that the concentration of acid at equillibrium is equal to the concentration of acid after disscoaition. 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 298k?
1..0 x 10^-14
What physical factors affect the value of kw? How do they affect it?
Temperature only
if temp increases, the equilibrium moves to the right so kw increases and the pH of pure water decreases
Indices 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 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 concentrations 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 acid/ alkali is added?
H+ + HCO3 CO2 + H2O
Adding OH-
reacts with H+ to form H2O
then shifts equilibrium left to restore H+ lost
Adding H+
equilibrium shifts to the right to remove 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 good properties of a good indicator for a reaction?
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
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 for a weak acid - weak base titration?
neither methyl orange of phenolphthalein is suitable as neither gives a sharp change at the end point
What colour is methyl orange in acid or alkali?
Red in acid
Yellow in alkali
What colour is phenolphthalein in acid or alkali?
Colourless in acid
red in alkali
What colour is bromothymol blue in acid or alkali?
yellow in acid
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
