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Alevel Chemistry Edexcel > Topic 12 - Acid-based Equilibria > Flashcards

Topic 12 - Acid-based Equilibria Flashcards

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1
Q

According to Bronsted-Lowry, what is an acid?

A

a proton donor

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2
Q

According to Bronsted-Lowry, what is a base?

A

a proton acceptor

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3
Q

What is a conjugate base?

A

the product of an acid donating a proton which can return to its original species by acting as a base

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4
Q

What is a conjugate acid?

A

the product of a base accepting a proton which can return to its original species by acting as an acid

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5
Q

What is pH?

A

the potential of H+

  • poor indicator of acid strength
  • on a scale of -1-14
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6
Q

What is a strong acid?

A

an acid that fully dissociates

eg. HCl → H+ + Cl-

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7
Q

What is a weak acid?

A

an acid that partially dissociates

eg. CH3COOH ⇌ CH3COO- + H+

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8
Q

pH =

A

-log[H+]

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9
Q

[H+] =

A

10 ^-pH

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10
Q

How do you calculate pH of a strong acid?

A

use pH = -log[H+]

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11
Q

How do you calculate pH of a weak acid?

A

use Ka rearranged to calc [H+]

then use pH = -log[H+]

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12
Q

How do you calculate pH of a strong base?

A

either use pOH = -log[OH-]
pH = 14-pOH

or use Kw rearranged to find [H+]
and then pH = -log[H+]

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13
Q

How do you calculate pH of a weak base?

A

use Kb rearranged to find [OH-]
then use use pOH = -log[OH-]
pH = 14-pOH

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14
Q

What is Ka?

A

the acid dissociation constant

  • larger Ka = greater acid dissociation
  • better indicator of acid strength than pH
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15
Q

Ka =

A

[H+][A-]
______
[HA]

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16
Q

How can Ka be calculated experimentally?

A
  • 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
17
Q

What is Kw?

A

the equilibrium constant for the dissociation of water

-used to calc pH of strong bases

18
Q

Kw =

A

[H+][OH-]

19
Q

What is the value of Kw?

A

1x10^-14

20
Q

How does pH of water vary with temperature?

A
  • dissociation of water is endothermic
  • so when temp increases, eqm shifts to right
  • so [H+] increases
  • so pH decreases
21
Q

What are acid-base titration curves?

A

graphs of volume against pH which show how pH changes as an acid is added to a base (or vice versa) during a titration

22
Q

What is the equivalence point?

A

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

23
Q

Why is the equivalence point not always 7 on a titration curve?

A
  • 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
24
Q

How can you tell what indicator to use by using a titration curve?

A

the indicator should have a colour change (normally a range) during the vertical section of the titration curve (aka the end point)

25
Q

pH at half equivalence point =

A

pKa at half equivalence point

26
Q

Why is pH equal to pKa at half equivalence point?

A

-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

27
Q

How do you find the Ka of a weak acid using a titration curve?

A

-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

28
Q

What is a buffer?

A

a solution which minimises changes in pH when a small amount of acid or base is added

29
Q

What is in an acidic buffer?

A
  • 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+)

30
Q

How does a buffer work when an acid is added?

A

-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

31
Q

How does a buffer work when a base is added?

A

-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

32
Q

How do you calculate the pH of a buffer?

A
  • rearrange the Ka of the acid to find [H+]

- use [H+] to calc pH (using pH = -log[H+])

33
Q

What is the Henderson-Hasselbalch equation?

A

pH = -logKa -log [acid]/[salt]

34
Q

What is in an alkali buffer?

A
  • weak base

- its salt

35
Q

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?

A

-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

36
Q

How is buffer action seen on a titration curve?

A

the curve levels off after a quick, small pH change before it eventually increases again

37
Q

How is the pH of the blood controlled?

A
  • 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)

Alevel Chemistry Edexcel (19 decks)

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