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Chemistry organic speedrun v2 + other topics > 12 - Acid-base equilibria > Flashcards

12 - Acid-base equilibria Flashcards

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1
Q

Bronsted-lowry acid

A

A proton donor

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2
Q

Bronsted-lowry base

A

A proton acceptor

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3
Q

What do acid-base reactions involve?

A

Transfer of protons.

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4
Q

What is pH?

A

pH = -log10[H+]

Define pH using this equation.

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5
Q

How do you calculate [H+] when given pH?

A

[H+} = 10^-pH

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6
Q

What is a conjugate acid-base pair?

A

It consists of either a base and its conjugate acid, or an acid and its conjugate base.

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7
Q

How is a conjugate acid formed?

A

When a base accepts a proton, the species formed is the conjugate acid of the base.

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8
Q

How is a conjugate base formed?

A

When an acid donates a proton, the species formed is the conjugate base of the acid.

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9
Q

What is an atmospheric substance?

A

A substance that can act both as an acid and as a base.

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10
Q

What is a strong acid?

A

Strong acids dissociate almost completely in aqueous solution.

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11
Q

What is a weak acid?

A

Weak bases dissociate only partially (less than 10%) in aqueous solution.

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12
Q

How do you calculate pH of a STRONG acid?

A
  • Strong acids almost completely dissociate in aqueous solution.
  • monoprotic acids: same number of moles of H+ as original acid.
  • Diprotic acid: 2x number of moles of H+ as original acid.
  • Then use pH = -log10[H+] to find pH.
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13
Q

What does a low pH mean?

A

More acidic, higher [H+].

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14
Q

What does a high pH mean?

A

More basic, lower [H+].

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15
Q

What is Ka (acid dissociation constant)?

A

Ka expresses how easily an acid releases a proton (strength as an acid).

  • Large Ka = large extent of dissociation.
  • Small Ka = small extent of dissociation.
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16
Q

Expression for Ka?

A

Ka = ([H+][A-])/([HA])

units are moldm^-3.

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17
Q

How do you calculate the pH of a WEAK acid?

A

Use the equation:
Ka = ([H+][A-])/([HA]), to find [H+].
Then use this to find pH.

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18
Q

What are the ASSUMPTIONS when calculating the pH of a weak acid?

A
  1. [acid]initial = [acid]equilibrium.

2. [H+] = [A-]

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19
Q

Define the ionic product of water, Kw?

A

Kw = [H+][OH-]

(aqueous for both)

units are mol^2dm^-6

20
Q

What is the value of Kw at 298K?

A

1.00x10^-14 mol^2dm^-6

21
Q

How do you calculate pH of a STRONG base from its concentration?

A

Strong bases almost completely fully dissociate in aqueous solution, so [OH-] = [strong base].
Then use Kw = [H+][OH-], with Kw = 1.00x10^-14 (at 298K) to calculate OH-. Then with found [H+], find pH.

22
Q

Calculate pH between strong acids and strong bases?

A
  • Calculate moles of unreacted H+ (if acid in excess), or unreacted OH- (if base in excess).
  • Calculate excess concentration of the unreacted H+ or OH-.
  • Calculate pH using pH = -log10[H+] if we have [H+], or use Kw = [H+][OH-] first if we have [OH-].
23
Q

Calculate pH of a solution of weak acid and strong base (excess base)?

A
  • Calculate moles of each
  • Check which is in excess.
  • If base is in excess, substract moles of H+ from moles of OH- to know moles of excess OH-.
  • Calculate excess [OH-].
    Use Kw to find pH of a strong base as there is no acid left.

^^ For every mole of strong base added, 1 mole of weak acid dissociates.

Not the same case as a strong acid, where it completely dissolves straight away in aqueous solution.

24
Q

Calculate pH of a solution of weak acid and strong base (excess acid)?

A
  • Calculate moles of each
  • Check which is in excess (this case acid).
  • Calculate moles of excess weak acid and moles of A- formed in reaction (EXCESS HA AND NOT H+).
  • Calculate concentrations of HA and A-.
  • Use Ka = ([H+][A-])/([HA]) to find [H+].
  • Find pH.
25
Q

Define pKa?

A

pKa = -log10Ka.

Ka = 10^-pKa

26
Q

Define pKw?

A

pKw = -log10Kw

At 298K, Kw = 1.00x10^-14 mol^2dm^-6, pKw = 14.00

27
Q

Strong and weak acids pH meaning?

A

Higher the pH, weaker the acid.

28
Q

Strong and weak bases pH meaning?

A

Higher the pH, stronger the base.

29
Q

pH of salt solutions?

A

Salt of a strong acid and strong base, pH = 7 (neutral).

Salt of a weak acid and a strong base, pH > 7 (alkaline).

Salt of a strong acid and a strong base, pH < 7 (acidic).

Salt of a weak acid and a weak base (a for acid, b for base):
if Ka = Kb, pH = 7.
if Ka > Kb, pH < 7.
if Ka < Kb, pH > 7.

30
Q

How does pH change as you dilute aqueous solutions of acids?

A

Strong acid: pH increases by 1 when diluted by a factor of 10.

Weak acid: pH increase by 0.5 when diluted by a factor of 10.

31
Q

What is a buffer solution?

A

A buffer solution is a solution that minimises the change in pH when a small amount of either acid or base is added.

32
Q

How to make a buffer solution?

A
  • mix weak acid with its conjugate base.

mix weak base with its conjugate acid.

33
Q

Important things to remember when finding pH of a buffer solution?

A
  • initial HA concentration = equilibrium HA concentration, because we assume that the dissociation of the acid is negligible.
  • The salt (conjugate base of acid) dissociates fully in aqueous solution, and since we assume that the dissociation of the acid is negligible, then we can say the A- is entirely made up from the salt. So concentration of salt = A- concentration.
34
Q

Henderson-Hasselbalch equation (to calculate pH of a buffer solution)?

A

pH = pKa + log10([base]/[acid]).

35
Q

How to make a buffer solution with a required pH?

A

Overall you work out:
[acid]/[salt].

You basically find a ratio of the concentrations needed of the acid and salt, using the same volumes.

36
Q

How do you know which indicator to use when looking at a titration curve?

A

The pH range over which the indicator changes colour has to be within the vertical region (end point) of the titration curve.

37
Q

On titration curve, strong acid pH?

A

1 ish

38
Q

On titration curve, weak acid pH?

A

3 ish

39
Q

On titration curve, strong base pH?

A

13 ish

40
Q

On titration curve, weak base pH?

A

11 ish

41
Q

Explain the straight horizontal part of a titration curve?

A

buffer action.

42
Q

What is the equivalence point?

A

The volume at this point is the minimum volume of base required to completely react with all of the acid.

43
Q

What is so special at the half-equivalence point?

A

pH = Pka of weak acid.

Ka can be easily calculated from this.

44
Q

Why is the enthalpy change of neutralisation less for a weak acid than a strong acid?

A

Weak acids only partially dissociate. Energy is released when the molecules dissociate, but less thermal energy would be released compared to strong acids, since strong acids fully dissociate and weak acids only partially dissociate.

Therefore less exothermic.

45
Q

Explain why HF, in spite of being a weak acid has an enthalpy of neutralisation which is higher than -57KJmol-1.

A

The enthalpy of hydration of the ions in HF is very exothermic and releases more heat energy than the energy taken in during dissociation of the molecules. Therefore the thermal energy released compensates the energy taken in.

Hence the overall effect is that the standard enthalpy change of neutralisation of HF by NaOH is greater than -57KJmol-1.

46
Q

How many decimal places is pH?

A

2dp ALWAYS

Chemistry organic speedrun v2 + other topics (5 decks)

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