1.12 Acids and Bases Flashcards

1
Q

Brønsted-Lowry acid

A

Proton (H+) donor

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2
Q

Brønsted-Lowry base

A

Proton (H+) acceptor

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3
Q

Brønsted-Lowry acid-base reaction

A

Reaction involving the transfer of a proton

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4
Q

Monoprotic acid

A

Acid that releases one H+ ion per molecule e.g. HCl

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5
Q

Diprotic acid

A

Acid that releases two H+ ions per molecule e.g. H2SO4

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6
Q

Definition of pH, and useful rearrangement

A

pH = – log [H+]

[H+] = 10^-pH

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7
Q

Ionic Product of Water (Kw)

A

(Kw) H2O ⇌ H+ + OH-
ΔH = endothermic

Kc = [H+] [OH-]/[H2O]
So Kc [H2O] = [H+] [OH-]

so Kc [H2O] = a constant = Kw
Kw = [H+][OH-] where Kw = 1x10^-14

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8
Q

The effect of temperature on the pH of water and the neutrality of water

A

As the temperature increases, the equilibrium moves right to oppose the increase in temperature

Therefore [H+] and [OH-] increase
So Kw increases and therefore pH decreases

However, the water is still neutral as [H+] = [OH-]

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9
Q

Calculating the pH of water

A

In pure water, [H+] = [OH-]
So Kw = [H+]^2
Therefore [H+] = √Kw

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10
Q

pH of a Strong Base

A

Find [OH-] and then substitute into: [H+] = Kw/[OH-]

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11
Q

Mixtures of Strong Acids and Strong Bases

A

1) Calculate moles H+
2) Calculate moles OH-
3) Calculate moles XS H+ or OH-
4) Calculate XS [H+] or XS [OH-]
5) Calculate pH

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12
Q

Weak Acids

A

HA ⇌ H+ + A-

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13
Q

The acid dissociation constant, Ka

A

Ka = [H+] [A-] / [HA]
pKa = -log Ka
Ka = 10^-pKa
These expressions hold for weak acids at all times

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14
Q

The acid dissociation constant, Ka, notes

A

• Ka – has units mol dm-3
• Ka – the bigger the value, the stronger the acid
• pKa – the smaller the value, the stronger the acid

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15
Q

In a solution of a weak acid in water, with nothing else added:

A

a) [H+] = [A-]
b) [HA] ~ [HA]initial

Ka = [H+]^2 / [HA]
This expression ONLY holds for weak acids in aqueous solution with nothing else added

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16
Q

Reactions between weak acids and strong bases

A

When a weak acid reacts with a strong base, for every mole of OH- added, one mole of HA is used up and one mole of A- is formed.

17
Q

Finding the pH in reactions between weak acids and strong bases

A

1) Calculate moles HA (it is still HA and not H+ as it is a weak acid)
2) Calculate moles OH-
3) Calculate moles XS HA or OH-

If XS HA
4) Calculate moles HA left and A- formed
5) Calculate [HA] leftover and [A-] formed
6) Use Ka to find [H+]
7) Find pH

OR

If XS OH-
4) Calculate [OH-]
5) Use Kw to find [H+]
6) Find pH

OR

If mol HA = OH-
4) pH = pKa of weak acid

18
Q

Titration Calculations

A

These ionic equations which help a great deal in titration calculations. H+ + OH- ➔ H2O
2H+ + CO3^2- ➔ H2O + CO2
H+ + HCO3^- ➔H2O + CO2
H+ + NH3 ➔ NH4^+

19
Q

pH Curves and Indicators
What are indicators and how do they work?

A

• Indicators are weak acids where HA and A- are different colours. HA ⇌ H+ + A-
• At low pH, HA is the main species present. At high pH, A- is the main species present.
• The pH at which the colour changes varies from one indicator to another.
• Note that universal indicator is a mixture of indicators and so shows many colours at different pHs.

20
Q

What is a buffer solution?

A

• Buffer solution = solution that resists changes in pH when small amounts of acid or alkali are added.
• Note – the pH does change, just not by much.
• Acidic buffer solutions have a pH less than 7.
• Basic buffer solutions have a pH less than 7.

21
Q

Acidic buffers

A

• Acidic buffer solutions are made from a mixture of a weak acid and one of its salts (i.e. HA and A-) (e.g. ethanoic acid & sodium ethanoate).
• An acidic buffer solution can also be made by mixing an excess of a weak acid with a strong alkali as it results in a mixture of HA and A-.
• The key in an acidic buffer solution is that the [acid] and [salt] are much higher than [H+].

22
Q

Basic buffers

A

• Basic buffer solutions are made from a mixture of a weak alkali and one of its salts (e.g. ammonia & ammonium chloride).
• A basic buffer solution can also be made by mixing an excess of a weak alkali with a strong acid
• The key in a basic buffer solution is that the [base] and [salt] are much higher than [OH-].

23
Q

To work out the pH of an acidic buffer

A

Use: [H+] = Ka x [HA] / [A-]

24
Q

To work out the change in pH of an acidic buffer when acid is added

A

Use: HA (aq) ⇋ A- (aq) + H+ (aq)
Where: H+ added shifts the equilibrium to the left so A- decreases and HA increases
Calculate the original moles and then adjust these values by the moles of the acid added as above.

25
Q

To work out the change in pH of an acidic buffer when alkali is added

A

Use: HA (aq) + OH- ⇋ A- (aq) + H2O(aq)
Where: OH- added reacts with HA which will decrease and produces more A- which will increase.
Calculate the original moles and then adjust these values by the moles of alkali added as above.