Acids, Bases and Buffers Flashcards
define a Brønsted–Lowry acid
a species that can donate a proton
define a Brønsted–Lowry base
a species that can accept a proton
describe and use the term conjugate acid– base pairs
A conjugate pair is a pair of acid and base that differ by H+. e.g. CH3COOH and CH3COO-
H2SO4 and HSO4-
H3O+ and H2O
What is the difference between strong and weak acids?
Strong: fully dissociates in solution HA –> H+ + A-What
Weak: partially dissociates in solution to form equilibrium HA H+ + A-
What is the significance of the acid dissociation constant, Ka?
Shows the extent of acid dissociation, and therefore the relative strength of weak acids: the higher the value of Ka the stronger the acid
How can Ka values be more easily compared?
Use the pKa scale, -log10(Ka)
The lower the pKa the stronger the acid
Define pH
pH=–log[H+];
[H+]=10^–pH
How to find the pH of a strong acid?
Fully dissociated, so [H+] for a monobasic acid is equal to the concentration of that acid.
[HA] = [H+]
pH = -log[H+]
find pH of a weak monobasic acid?
use Ka
[H+] = square root of Ka x [HA]
Ka = [H+]^2/[HA] (or [H+]x[A-])
pH = -log[H+]
What assumptions are made in the calculation of the pH of a weak acid using Ka?
assumes negligible acid dissociation so that [HA] = concentration of acid
Assumes that water contributes very little H+ so that [A-]=[H+]
What is Kw? when is it used?
the ionic product of water, 1x10^-14
Used to convert [OH-] into [H+] and thus find the pH of a strong base.
Kw = [H+][OH-]
[H+] = 1x10^-14/[OH-]
What is a buffer solution?
a system that minimises pH changes on addition of small amounts of acid or base
how can a buffer solution be set up?
by reacting excess weak acid with a salt of the weak acid. An equilibrium is set up
e.g.
CH3COOH CH3COO- + H+
Position of equilibrium shifts to cancel out pH changes when H+/OH- is added
How is the pH of a buffer solution calculated?
[H+] = Ka x [SALT]/[ACID]
OR
pH = pKa + log([SALT]/[ACID])
explain the role of carbonic acid– hydrogencarbonate in the body
acts as a buffer in the control of blood pH;
When asked to sketch a pH titration curve, what are the marking points that must be covered?
- initial points at 1 for a strong acid, 3 for a weak acid and final points of the curve at 13 for a strong base or 11 for a weak base.
- line is vertical at the equivalence point/ at the end point volume
- vertical point of the grap hcovers the values: 3-11(SA/SB), 7-11(WA/SB) or 3-7(SA/WB)
- smooth S shaped curve
which indicators can be used for which titrations?
strong acid/strong base: methyl orange or phenol phthalein
weak acid/strong base: phenol phthalein
strong acid/weak base: methyl orange
weak acid/weak base: neither - no rapid pH change
wht criteria must an indicator have to be used in a titration?
In order to change colour at the equivalence point the pH range of the indicator must coincide with the sharp rise in pH at the equivalence point
what is enthalpy of neutralisation?
the enthalpy change when 1 mol of water is formed during the neutralisation of an acid by an alkali
H+ + OH- –> H2O
It is always exothermic
how is enthalpy change of neutralisation calculated?
q(J) = mc∆T ∆Hneut = q/mol water formed
What is a thermometric titration?
temperature change during a titration is used to deduce the end-point.
The temp. will increase during neutralisation and decrease once the equivalence point has been reached.
Point at which temp. mixture is highest = end-point.
