Acids, Bases and Buffers Flashcards
Define a Bronsted-Lowry acid
Proton donor
Define a Bronsted-Lowry base
Proton acceptor
What ion causes a solution to be acidic? (Name and formula)
H+ ion or more accurately H3O+ (oxonium ion) as protons react with H2O to form it
What ion causes a solution to be alkaline
-OH (hydroxide ion)
Write an equation for the ionisation of water (2)
2H2O (l) ⇌ H3O+ (aq) + -OH (aq)
OR
H2O (l) ⇌ H+ + -OH (aq)
Derive Kw using the equation for ionisation of water
K= [H+][OH-] / [H2O]
[H2O] K = [H+][OH-]
[H2O] is so large compared to [H+] and [OH-] that [H2O]K can be considered a constant. [H2O] K = Kw
Therefore Kw = [H+][OH-]
What is the value of Kw at 298K
1.0 x10^-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
Why is pure water still neutral even if the pH does not equal 7?
[H+] = [OH-]
Give an expression for pH in terms of H+
pH = -log[H+]
What is the relationship between pH and [H+]
Lower pH = higher concentration of H+
If two solutions have a pH difference of 1, what is the difference in [H+]
A factor of 10
How do you find [H+] from pH?
[H+] = 10^-pH
How do you find [OH-] from pH? (at 298K)
Find [H+], use Kw (equal to 1 x10^-14 at 298K) to calculate [OH-]
What is different when finding [H+] from the concentration of diprotic and triprotic acids
Need to multiple the concentration of the acid by the number of protons to find [H+]
How do you calculate the pH of a strong alkaline solution?
Use Kw to calculate [H+] from [OH-]
Use pH = -log[H+]
Define the term strong acid
One which fully dissociates in water (HX —> H+ + X-)
Define the term strong base
One which fully dissociates in water (XOH —> X+ + OH-)
What is the difference between concentrated and strong
Concentrated means many moles per dm3, strong refers to amount of dissociation
What is a weak acid and weak base?
Weak acids and bases do not fully dissociate in water. They only partially dissociate into their ions
Give some examples of strong acids
HCl
H2SO4
H3PO4
Give some examples of strong bases
NaOH
CaCO3
Na2CO3
Give some examples of weak acids
CH3COOH or any organic acid
Give some examples of weak bases
NH3
What is Ka (expression)
For acid HA, HA ⇌ H+ + A-
Ka = [H+][A-] / [HA]
How would you work out the pH of a weak acid
Use the equation for Ka, subbing in values for [A-] and [HA]
Use pH = -log[H+] equation to find pH
What is a titration
The addition of an acid / base of known titration to a base / acid of unknown titration 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 generally happens to the pH of the solution around the equivalence point
There is a large and rapid change in pH, except in the weak-weak titration
How would you calculate the concentration of a reactant if you know the volume and conc of the other reactant and the volume of that reactant added
Calculate mols of one reactant
Use balanced equation to work out mols of the other
Use conc = mol/vol to calculate concentration
What is the end point
The volume of acid or alkali added when the indicator just changes colour. If the right indicator is chosen, equivalence point = end point
What are the properties of a good indicator for the reaction (3)
Sharp colour change - no more than a drop of acid or alkali needed for colour change
End point must be the same as the equivalence point or the titration gives the wrong answer
Distinct colour change so it is obvious when the end point been reached
What indicator would 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 for 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? At what pH does it change
Red in acid
Yellow in alkali
Changes at about pH = 4-5. Approx same as pKa value
What colour is phenolphthalein in acid? In alkali? At what pH does it change?
Colourless in acid
Red in alkali
Changes at about pH = 9-10. Approx same as pKa value
What is the half-neutralisation point?
When volume = half the volume that has been added at the equivalence point
Define a buffer solution
A solution that resists changes in pH when a small amount of acid/alkali are added
What do acidic buffer solutions contain in general terms
A weak acid and a soluble salt of that acid that fully dissociates
Write a reaction for an acidic buffer with added acid
A- + H+ —> HA, opposes addition of H+
Write an equation for an acidic buffer with added alkali
HA + OH- —> H2O +A-
How else can you achieve an acidic buffer solution other than just mixing the constituents
Neutralise half of a weak acid (meaning the acid must be in excess) with an alkali - this forms a weak acid / soluble salt mixture
What do basic buffer solutions contain in general terms
Weak base and soluble salt of that weak base
How can you calculate the pH of buffer solutions
Use the Ka of the weak acid, sub in [A-] and [HA], calculate [H+] –> pH
How can you calculate the new pH of a buffer solution when acid or base is added
Calculate number of moles of H+ and A- and HA before acid or base is added. Use equations to work out new moles of A- and HA —> find [H+] —> pH
Which buffer solution 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 products are buffers found in?
Shampoos, detergents —> important to keep pH right to avoid damage to skin, hair, fabrics
