1.12 Acids and Bases Flashcards
Define a Bronstead-Lowry acid
A proton donor
Define a Bronstead-Lowry base
Proton acceptor
What ion causes a solution to be acidic ?
•H+ (hydrogen ion)
•H3O+ (Oxonium ion).
What ion causes a solution to be alkaline ?
Hydroxide ion (OH-)
Write an equation for the ionisation of water
•2H2O (l) ⇌ H3O+ (aq) + OH- (aq)
•H2O (l) ⇌ H+ (aq) + OH- (aq).
Derive Kw using the formula for the ionisation of water
•Kc = [H+][OH-] / [H2O]
•Kc [H2O] = [H+][OH-]
•[H2O] is so large that it can be assumed that it is constant
•Therefore Kw = [H+][OH-].
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?
•Only temperature effects the value of Kw
•If temperature increases, the equilibrium moves to the right
•Kw increases therefore and the pH of pure water increases.
Why is pure water still neutral, even if 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 concentration of H+?
Lower pH = higher concentration of H+.
By what scale does the concentration of H+ increase if you jump from pH 2 to 1 ?
x10.
How do you find the [H+] from pH ?
[H+] = 10 ⁻ᵖᴴ
How do you find [OH-] from pH ? (298K)
•Find [H+]
•Use Kw / [H+] = [OH-].
What is different when finding the concentration of H+ in diprotic and triprotic acids ?
You need to multiply 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 into water : XH -> X- + H+.
Define the term strong base
One which fully dissociates into water: XOH -> X+ + OH-
What is the difference between a concentrated acid and strong acid ?
•Concentrated means many moles per dm3
•Strong refers to amount of dissociation.
What are weak acids and bases ?
Acids / bases which only partially dissociate into their ions in a solution.
Give some examples of strong acids
HCl, H2SO4, HNO3, H3PO4
Give some examples of strong bases
NaOH, CaCO3, Na2CO3
Give an example of a weak acid
CH3COOH, any organic acid.
Give an example of a weak base
NH3
What is Ka ?
•Equilibria value for weak acid dissociation
•Eg. HA ⇌ H+ + A-
•Ka = [H+][A-] / [HA].
How would you calculate the pH of a weak acid ?
•Use the formula for Ka to find value for [H+]
•Use -log([H+]).
What is a titration ?
•The addition of an acid/base of a known solution to an acid/base of an unknown solution to determine its concentration
•An indicator or pH meter is used to see when neutralisation occurs.
Describe the pH dissociation curves for all combinations of weak acid/bases and strong acid/bases
•Strong acid-strong base starts very high, has a large sudden drop and finishes very low
•Strong acid-weak base starts slightly above 7, has a drop then finishes low
•Weak acid-weak base starts slightly above 7 and finishes slightly below 7, not much of a clear drop.
Define the term equivalence point
The point at which the exact volume of the base has been added to to exactly 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 the pH, except in the weak-weak neutralisation
How would you calculate the concentration of a reactant in a reaction if you know it’s volume and the other reactants volume/conc ?
•Calculate mols of one reactant
•Use balanced equation to work out the mols of the other
•Use conc=mol/vol to get answer
What is the end point in a titration ?
•The volume of acid or alkali when the just changed colour: showing neutrality
•If the correct indicator is used, equivalence point should equal end point.
What are the properties of a good indicator of a reaction ?
•A sharp colour change to show when end point has been reached
•End point should be equivalence point.
What indicator would you use for a strong acid-strong base titration ?
•Phenolphthalein
•Colourless in acid and pink in base
What indicator would you use for a strong acid-weak base titration ?
•Methyl orange
•Red in acid, yellow in bases.
What indicator would you use for a strong base-weak acid titration ?
•Phenolphthalein
•Colourless in acid, pink in base.
What indicator would you use for a weak acid-weak base titration ?
•Neither methyl orange or phenolphthalein are suitable
•This is because there will be no sharp change in colour at the end point.
What colour is methyl orange in acid/alkali ? At what pH does it change ?
•Red in acid, yellow in alkali
•Changes in around pH 4-5.
What colour is phenolphthalein in acid/alkali ? At what pH does it change ?
•Colourless in acid, pink in weak alkali, red in strong alkali
•Change around pH 9-10.
What is the half neutralisation point ?
When volume = half volume that has been added at the equivalence point
Define a buffer solution
A solution that can resist changes in pH when a small amount of acid/alkali is added
What do acid buffer solutions contain in general terms ?
A weak acid and a soluble salt of that acid that fully dissolves
Write an equation for an acidic buffer with added acid
A- + H+ -> HA (salt which opposes addition of H+)
Write an equation for an acidic buffer with added alkali
HA + OH -> H2O + A-
How else can you achieve an acid buffer solution other than just mixing it’s constituents ?
By neutralising half of a weak acid with an alkali - forming a mixture of the unneutralised acid and soluble salt.
What do basic buffer solutions contain in simple terms ?
A weak base and a soluble salt of the 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 an acid or base is added ?
•Calculate the number of moles of H+, A- and HA before the new acid or base is added
• Use equations to work out new moles of A- and HA -> find [H+] -> find pH
Which buffer system maintains blood at pH 7.4 ? What happens when an acid or base is added ?
•H+ + HCO3- ⇌ CO2 + H2O
•Adding OH- -> reacts with H+ to form H2O, then shifts equilibrium to left to restore H+
•Adding H+ -> equilibrium shifts to the right, removing excess H+.
What products are buffers found in ?
Skin and beauty products.
