Topic 12.1 Strong and weak acids

Question 1

Brønsted-Lowry Theory

Answer 1

Proposed the theory into how acids and bases react with each other (in 1923)

Question 2

Conjugate acid-base pairs

Answer 2

Acid + conjugate base
Base + conjugate acid
(strong reactactant forms weak opposite product)

Question 3

Monoprotic

Answer 3

Donates one proton
(HCl is monoprotic/monobasic- as 1 mol of acid can neutralise 1 mol OH- ions)

Question 4

Diprotic

Answer 4

Donates two protons
(Sulfuric acid is diuretic/dibasic)

Question 5

Anphoteric

Answer 5

A substance that can act as both an acid and a base
(water is anphoteric)

Question 6

Ammonia acid-base pairs

Answer 6

-Weak acid
NH3 + H2O ⇌ NH4+ + OH-
Base Acid Acid Base
(NH3 and NH4 conjugate pair)
(H2O and OH- conjugate pair)

Question 7

Strong acids

Answer 7

Strong acids are almost completely dissociated in solution.
–> Reverse reaction doesn’t take place so single direction arrow is used
(HCl –> H+)
–> pH increases by a factor of one for each 10x decrease of conc

Question 8

Weak acids

Answer 8

Weak acids only partly dissociate <10% (a significant amount of undissociated acid is present).
–> Represented using a reversible reaction arrow
(HA ⇌ H+ + A-)–>(CH3COOH ⇌ CH3COO- + H+)
–> pH increases 0.5 for each 10x decrease in concentration.

Question 9

Equilibrium constant Ka

Answer 9

• Ka is the acid dissociation constant.
• Used to quantify the degree to which an acid dissociates.

Question 10

Calculating pH

Answer 10

pH is related to the hydrogen ion concentration (moldm^-3) of a solution.
pH = -log[H+]
(eg: pH = -log0.100 pH = 1)

Question 11

Calculating [H+] from ph

Answer 11

[H+] = 10^-pH

Question 12

Calculating pH of a weak acid

Answer 12

-Need to know the degree of dissociation of the acid
-Use Ka the acid dissociation constant to show this
____________

Question 13

pKa

Answer 13

pKa = -logKa
-Larger Ka = stronger acid
-Smaller Ka = weaker acid

Question 14

Dissociation of water

Answer 14

Water ionises to form some H+ and OH- ions
H2O ⇌ H+ + OH-

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

Question 15

The ionic product of water, Kw

Answer 15

• Kw = [H+][OH-]
• Kw is the ionic product of water.
• It has a constant value at a given temperature (298K).

Question 16

Strong base

Answer 16

• Sodium hydroxide is a strong base.
• It is completely dissociated in water.

Question 17

What is a buffer solution?

Answer 17

• They resist changes in pH
• A buffer solutions pH is almost unchanged on the addition of small amounts of acid or base
• A buffer solution can be made from a weak acid and a salt of the acid.

Question 18

A buffer solution can be made by mixing:

Answer 18

• A weak acid with its conjugate base
• A weak base with its conjugate acid

Question 19

Calculating buffer pH

Answer 19

pH = pka + log [acid]/[base] (?)
–>
pH = (-logKa) + log [salt]/ [acid]
[H+] = Ka x [acid]/[salt]
pH = -log[H+]

Question 20

Ka

Answer 20

Acid dissociation constant.
HA(aq) ⇌ H+(aq) + A-(aq)

Question 21

pH of aqueous solutions of strong bases

Answer 21

When an acid is dissolved in water, it produces so many hydrogen ions that the small contribution of the water is insignificant, unless the acid concentration is very small.
However the fact that water ionises is the reason why even the most alkene solutions contain some hydrogen ions.
Sodium hydroxide is a strong base, so in dilute aqueous solutions we can consider it ions to be completely dissociated.

Question 22

acid

Answer 22

• Proton donor
• Positive charge/becomes more negative
• Higher pH- weaker the acid
• carboxylic- weak acid
• Others strong

Question 23

base

Answer 23

• Proton acceptor
• Lone pair/becomes more positive
• Higher pH- stronger base
• OH- strong base
• NH3- weak base

Question 24

equivalence point

Answer 24

The equivalence point is where the moles of acid and the moles of base would neutralize each other according to the chemical reaction.

(when an acid and base have reacted together in the exact proportions as dictatef by the stoichiometric equation)

Question 25

strong acid strong base

Answer 25

equivalence 7

Question 26

weak acid strong base

Answer 26

equivalence 8-9
conjugate base makes it alkaline

Question 27

strong acid weak base

Answer 27

equivalence <7
- conjugate acid makes it acidic

Question 28

weak acid weak base

Answer 28

point of inflexion

Question 29

making buffer

Answer 29

• To make a buffer with a pH<7: use a mixture of a weak acid and its conjugate base.
  (e.g - sodium hydroxide and ethanoic acid).
• To make a buffer with a pH>7: use a mixture of a weak base and its conjugate acid.

Question 30

phenolphthalein indicator

Answer 30

Acid- colourless
Neutral- pale pink
Base- pink

Question 31

methyl orange

Answer 31

Acid- red
Neutral- orange
Base- yellow
–> Only with strong acids.