Topic 12 - Acid-based Equilibria Flashcards
According to Bronsted-Lowry, what is an acid?
a proton donor
According to Bronsted-Lowry, what is a base?
a proton acceptor
What is a conjugate base?
the product of an acid donating a proton which can return to its original species by acting as a base
What is a conjugate acid?
the product of a base accepting a proton which can return to its original species by acting as an acid
What is pH?
the potential of H+
- poor indicator of acid strength
- on a scale of -1-14
What is a strong acid?
an acid that fully dissociates
eg. HCl → H+ + Cl-
What is a weak acid?
an acid that partially dissociates
eg. CH3COOH ⇌ CH3COO- + H+
pH =
-log[H+]
[H+] =
10 ^-pH
How do you calculate pH of a strong acid?
use pH = -log[H+]
How do you calculate pH of a weak acid?
use Ka rearranged to calc [H+]
then use pH = -log[H+]
How do you calculate pH of a strong base?
either use pOH = -log[OH-]
pH = 14-pOH
or use Kw rearranged to find [H+]
and then pH = -log[H+]
How do you calculate pH of a weak base?
use Kb rearranged to find [OH-]
then use use pOH = -log[OH-]
pH = 14-pOH
What is Ka?
the acid dissociation constant
- larger Ka = greater acid dissociation
- better indicator of acid strength than pH
Ka =
[H+][A-]
______
[HA]
How can Ka be calculated experimentally?
- make a standard solution of a known conc
- calc pH (using pH meter)
- use pH to find [H+] (pH = -log[H+])
- use Ka expression to calc Ka
What is Kw?
the equilibrium constant for the dissociation of water
-used to calc pH of strong bases
Kw =
[H+][OH-]
What is the value of Kw?
1x10^-14
How does pH of water vary with temperature?
- dissociation of water is endothermic
- so when temp increases, eqm shifts to right
- so [H+] increases
- so pH decreases
What are acid-base titration curves?
graphs of volume against pH which show how pH changes as an acid is added to a base (or vice versa) during a titration
What is the equivalence point?
the half way point on a titration curve where there are equal concentrations of acid and base
-during the vertical section of the titration curve
Why is the equivalence point not always 7 on a titration curve?
- when a strong acid is titrated with a weak base, a slightly acidic product is formed, which lowers the equivalence point
- when a weak acid is titrated with a strong base, a slightly alkali product is formed, which raises the equivalence point
How can you tell what indicator to use by using a titration curve?
the indicator should have a colour change (normally a range) during the vertical section of the titration curve (aka the end point)
pH at half equivalence point =
pKa at half equivalence point
Why is pH equal to pKa at half equivalence point?
-at the equivalence point, the vol of acid that has reacted is the same as the vol of base that has reacted
-so at the half equivalence point, half of the acid remains but half of the salt has been produced
-so at the half equivalence point, the concs of acid and salt are equal so can be cancelled out in the Ka expression
∴ we are left with Ka = [H+]
so pKa must = pH
How do you find the Ka of a weak acid using a titration curve?
-find the equivalence point (half way on vertical section of graph)
-find the half equivalence point (half the vol of the equivalence point, and then use to find pH)
pKa = pH
so Ka = 10^-pKa
What is a buffer?
a solution which minimises changes in pH when a small amount of acid or base is added
What is in an acidic buffer?
- weak acid
- its conjugate base (aka a salt)
the salt fully dissociates and the weak acid only partially dissociates (which is suppressed by the ion from the salt) ∴ the buffer has a large reservoir of the undissociated weak acid and a large reservoir of the conjugate base (as well as some H+)
How does a buffer work when an acid is added?
-the [H+] increases in the buffer solution
-this causes eqm to shift to the left (to lower [H+])
-BUT the concs of the weak acid and conjugate base don’t change significantly
∴ pH does not change significantly
How does a buffer work when a base is added?
-the OH- ions react with the H+ ions (producing water)
-this causes [H+] to decrease in the buffer solution
-this causes eqm to shift to the right (to increase [H+])
-BUT the concs of the weak acid and conjugate base don’t change significantly
∴ pH does not change significantly
How do you calculate the pH of a buffer?
- rearrange the Ka of the acid to find [H+]
- use [H+] to calc pH (using pH = -log[H+])
What is the Henderson-Hasselbalch equation?
pH = -logKa -log [acid]/[salt]
What is in an alkali buffer?
- weak base
- its salt
How can the Henderson-Hasselbalch equation be used to calculate an acid:base ratio needed to be mixed to obtain a buffer with a particular pH?
-plug numbers you do have into equation
-do 10^x to remove log
should get left with a value equal to [A]/[B]
the [A] = value and [B} = 1
-these can then be placed as a ratio
How is buffer action seen on a titration curve?
the curve levels off after a quick, small pH change before it eventually increases again
How is the pH of the blood controlled?
- by carbonic acid (which dissociates into H+ and CO3-)
- when [H+] rises in blood, HCO3- ions react with excess H+ (shifts eqm to left) to reduce [H+] (prevents blood pH dropping)
- when [H+] decreases in blood, H2CO3 dissociate (shifts eqm to right) to increase [H+] (prevents blood pH rising)
