Acids and Bases Flashcards
Define a Bronsted-Lowry acid
Proton donor
Define a Bronsted-Lowry base
Proton acceptor
What ions cause a solution to be acidic?
H+ (hydrogen 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)
H2O (l) ⇌ H+ (aq) + -OH (aq)
Derive Kw using the equation for the ionisation of water
Keq = [H+][OH-]/[H2O]
Kw=[H+][OH-]
What is the value of Kw at298K?
1.0 x 10-14
What physical factors affect the value of Kw? How do they affect it?
Temperature only - if temperature increases,
equilibrium shifts to the right so Kw increases and the pH of pure water decreases
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 = -log10[H+]
What is the relationship
between pH and
concentration of 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 x 10-14 at 298K) to calculate [OH-]
What is different when
finding [H+] from the
concentration of diprotic and triprotic acids?
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 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 mol per dm3, strong refers to amount of dissociation
What is a weak acid and a 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 (ethanoic), any organic acid
Give some examples of weak bases
NH3
What is 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 know titration to a base/acid of unknown titration 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 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 moles of one reactant
use balanced equation to work out moles 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 a reaction? (3)
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, or
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? 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 resist changes in pH when 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 a reaction 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 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 products are buffers found in?
Shampoos, detergents → important to keep pH right to avoid damage to skin, hair, fabrics
