ACIDS BASES AND PH Flashcards
(21 cards)
Definition of a Bronsted Lowry acid
acids are proton donors and they release H+ in water to form H3O+ ions
Definition of a Bronsted Lowry base
a proton acceptor that forms OH- ions in water
What is a conjugate acid and base
a conjugate acid is a species that gains a proton
a conjugate base is a species that has lost a proton
Kw expression and units
Kw = [H+][OH-]
mol2dm-6
Kw constant value and how can it be affected
1x10-14 mol2dm-6
unless the temperature changes
Kw expression for pure water
Kw = [H+]2
pH equation
-log[H+]
How to use pH to figure out concentration of H+
10^-pH
How does H+ concentration relate to acid concentration in monoprotic acids
H+ concentration = concentration of acid
How does H+ concentration relate to acid concentration in diprotic acids
H+ concentration = 2 x acid concentration
2 assumptions of dissociation of weak acids
- the concentration of acid in equilibrium is roughly equal to the concentration of acid before dissociation
- all H+ ions have come from the acid in solution, so [H+] = [A-]
Ka expression for a weak acid
Ka = [H+]2 / [HA]
What can we assume at the equivalence point on a titration curve
that [H+] = [OH-]
Colour change point of methyl orange
pH 3 - 4.5
Colour change point of phenolphthalein
pH 8.2 - 10
What is a buffer
a chemical that resists a change in pH when small amounts of acid or base are added
What is the role of acid buffers and how are they composed
resist pH change to keep the solution below pH7
they are made from a weak acid and ITS salt
The two equations of an ethanoic acid buffer
CH3COOH <–> CH3COO- + H+
(weak acid dissociates weakly so equilibrium lies left)
CH3COO-Na+ <–> CH3COO- + Na+
(salt dissociates strongly so equilibrium lies to the right)
What happens when acid is added to an acidic buffer
H+ ions will react with the negative ions in solution, there is a high concentration of these from the salt
more weak acid is produced so equilibrium shifts to the left
What happens when a base is added to an acidic buffer
The OH- ions will react with the H+ ions from the dissociated weak acid
as this removes H+ ions from solution, more H+ ions will be dissociated from the weak acid to counteract the change in concentration
What values are needed to calculate pH of a buffer
The Ka value
The concentration of the weak acid and its salt
