Acids, bases and pH

Question 1

What is the Arrhenius model of acids and bases?

Answer 1

Acids dissociate and release H+ ions in aqueous solution.
Alkalis dissociate and release OH- ions in aqueous solution.
H+ ions are neutralised by OH- ions to form water.
An alkali is a soluble base.

Question 2

What is the Bronsted Lowry model for acids and bases?

Answer 2

A Bronsted-Lowry acid is a proton donor.
A Bronsted-Lowry base is a proton acceptor.

Question 3

What are conjugate acid-base pairs?

Answer 3

It contains two species that can be interconverted by transfer of a proton.
For the reaction: HCl ⇌ Cl- + H+
In the forward direction, HCl releases a proton to form its conjugate base, Cl-.
In the reverse direction, Cl- accepts a proton to form its conjugate acid, HCl

Question 4

What is the hydronium ion?

Answer 4

H3O+
Acid-base equilibrium for Hydrochloric acid:
HCl + H2O ⇌ H3O+ + Cl-

Question 5

What are the different forms of acid?

Answer 5

Monobasic, dibasic and tribasic acids, which refers to the total number of hydrogen ions in the acid that can be replaced per molecule.
Typically be a replacement of protons by metal ions or ammonium ions to form a salt.

Question 6

What are examples of mono,di and tribasic acids?

Answer 6

HCl, CH3COOH
H2SO4, H2CO3 (carbonic acid)
H3PO4 (phosphoric), H3BO3 (boric acid)

Question 7

What is the rule for writing ionic equations?

Answer 7

Write out the full equation.
Write out the equation in terms of ions.
Cancel out spectator ions (same on both sides).
Do not break down solids or liquids into ions.

Question 8

What is the reaction of zinc with a dilute acid?

Answer 8

2H+(aq) + Zn(s) –> Zn2+(aq) + H2(g)

Question 9

What is the reaction of solid copper (II) carbonate with an acid?

Answer 9

2H+ (aq) + CuCO3 (s) –> Cu2+ (aq) + H2O (l) + CO2 (g)

Question 10

What is the reaction of magnesium oxide with an acid?

Answer 10

2H+ (aq) + MgO (s) –> Mg2+ (aq) + H2O (l)

Question 11

What is Sorensen’s pH scale?

Answer 11

For any solution at 25°C:
pH less than 7 shows increasing acidity.
pH greater than 7 shows increasing alkalinity.
pH 7 is neutral.

Question 12

What is the relationship between pH and [H+]?

Answer 12

A low [H+] means a high value of pH.
A high [H+] means a low value of pH.

Question 13

What are the equations for pH and [H+]?

Answer 13

pH = -log[H+]
10 ^-pH = [H+]
A change in one pH number is equal to a 10x difference in [H+].

Question 14

How can you calculate pH of strong acids?

Answer 14

A strong monobasic acid completely dissociates, so [H+] is equal to the concentration of the acid [HA].
So the pH is directly calculated from the concentration of the acid.

Question 15

What are weak acids?

Answer 15

Weak acids partially dissociate in aqueous solution.
e.g. ethanoic acid,
The further an equilibrium lies to the left, the weaker the acid.
Strong acid examples - HCl, H2SO4, HNO3

Question 16

What is the acid dissociation constant?

Answer 16

Ka = [H+] [A-] / [HA]
Units are mol dm-3

Question 17

What is pKa?

Answer 17

pKa = -logKa
Ka = 10^-pKa
This is for more managable comparibility.

Question 18

What is the relationship between Ka and pKa?

Answer 18

The stronger the acid, the larger the Ka value and the smaller the pKa value.
The weaker the acid, the smaller the Ka value and the larger the pKa value.

Question 19

What are the assumptions for Ka of weak acids?

Answer 19

->HA dissociates to produce equilibrium concentrations of H+ and A- that are equal.
There will also be a small concentration of H+ from water dissociation but this is neglible.
-> The equilibrium concentration of HA is smaller than the undissociated concentration, so there is little decrease in [HA] from start to equilibrium.

Question 20

What are the weaknesses of the assumptions for Ka?

Answer 20

The assumptions only work for weak acids.
Stronger acids dissociate more in solution, so the difference between HA at the start and equilibrium becomes significant, and the assumption that they are equal is no longer valid.

Question 21

What is the simplified Ka expression for weak acids?

Answer 21

Ka = [H+]^2 / [HA]
H+ = √Ka x [HA]
then pH can be found from [H+]

Question 22

What is the ionisation of water?

Answer 22

H2O (l) + H2O (l) ⇌ H3O+ (aq) + OH- (aq)
or H2O ⇌ H+ + OH-
The dissociation of water is very small.

Question 23

What is Kw?

Answer 23

The ionic product of water.
Kw = [H+] [OH-]
At 298k Kw = 1 x 10^-14 mol^2dm^-6.
This sets up a neutral point for pH which controls the [H+] and [OH-] in aqueous solutions.

Question 24

What is the pH of pure water at 298k?

Answer 24

On dissociation, water is neutral - it produces the same number of H+ and OH- ions.
So Kw = [H+]^2
So [H+] = √Kw
pH = -log[H+]

Question 25

How is the number of OH- ions and H+ ions deduced using Kw?

Answer 25

The indices for them add up to 10^-14
So Kw / [OH-] = [H+]
A solution is more acidic when [H+] is more than [OH-].

Question 26

What is the pH of a strong base?

Answer 26

A strong base is an alkali that completely dissociates in solution - releasing OH- ions.
pH of a strong base can be calculated from the concentration of the base and the Kw.
NaOH

Question 27

What is the pH of a weak base?

Answer 27

E.g. Ammonia gas
NH3 + H2O ⇌ NH4+ + OH-
The equilibrium is positioned well to the left.
So the pH is calculated similarly to weak acids.

Question 28

How do Kw and Ka behave like other equilibrium constants?

Answer 28

The value of Kw is fixed at a given temperature - so if the concentration of one thing changes the other thing must change so Kw stays the same.
Dissociation of water is endothermic, so warming the solution shifts equilibrium to the right (more dissociation), and Kw increases.