ACIDS AND BASES (LIVE)
26 MAY 2015
Section A: Summary Notes
Definitions of acids and bases:




+
Arrhenius acid – a substance that releases H ions in an aqueous solution
Arrhenius base – a substance that releases OH ions in an aqueous solution
Brønsted-Lowry acid – a substance that is a proton donor
Brønsted-Lowry base – a substance that is a proton acceptor
+
Protolytic reaction is an acid-base reaction in which there is a transfer of protons (H ions) from an
acid to a base. Monoprotic acids can donate only one proton e.g. HCl, HNO 3.
Polyprotic acids can donate more than one proton. Diprotic acids donate two protons e.g. H2SO4.
Triprotic acids donate three protons e.g. H3PO4.
An ampholyte is a substance that has the ability to act as either an acid or a base, depending on the
other reactant.
Hydrolysis is the reaction between water and the ions in a salt solution. Hydrolysis leads to the
development of an acidic or basic solution.
A standard solution is a solution of which the concentration is exactly known and remains constant for
a certain period of time, especially during a reaction.
General Acid – Base Reactions





Acid + metal  salt + hydrogen
Acid + metal oxide  salt + water
Acid + metal hydroxide  salt + water
Acid + metal carbonate  salt + water + carbon dioxide
Acid + metal hydrogen carbonate  salt + water + carbon dioxide
Conjugate Acid – Base pairs
When an acid donates a proton, during a chemical reaction, it forms a conjugate base. When a base
accepts a proton, during a chemical reaction, it forms a conjugate acid. In acid – base reactions the
acid and the conjugate base it forms as well as the base and the conjugate acid it forms are known as
conjugate acid-base pairs.
Strength of Acids and Bases
A strong acid is an acid which ionises almost completely in water; it has a high percentage of
+
+
ionisation. This means that the concentration of the H ion (H3O ) is high. (A strong acid will have a
weak conjugate base)
A weak acid is an acid which ionises only slightly in water; it has a low percentage of ionisation. This
+
+
means that the concentration of the H ion (H3O ) is low. (A weak acid will have a strong conjugate
base)
A strong base is a base which dissociates almost completely in water, whereas a weak base is a base
which dissociates or ionises only slightly in water. (A strong base will have a weak conjugate acid
and a weak base will have a strong conjugate acid)
Auto Ionisation of water
In auto-ionisation of water, a proton is transferred between two molecules of water.
H2O (l) + H2O (l)⇌ H3O
+
-
(aq)
+ OH (aq).
+
-
The equilibrium constant (Kw) for this reaction is Kw = [H3O ].[OH ]
Kw is the ionic product of water. Kw = 1 x 10
–7
+
–14
at 25°C.
-3
-
–7
In pure water,[H3O ] = 1 x 10 mol.dm and [OH ] = 1 x 10 mol.dm
-3
pH and concentration of solutions
+
-
In acidic solutions, [H3O ] > [OH ] and pH < 7
+
-
In neutral solutions, [H3O ] = [OH ] and pH = 7
+
-
In alkaline solutions, [H3O ] < [OH ] and pH > 7
(note: an alkali is a soluble base)
The pH of a solution is an indication of the acidity or alkalinity of a solution. It is the negative
logarithm of the hydronium ion concentration in a solution.
𝑝𝐻 = log 𝐻 +
+
+
+
[H ] = concentration of H or H3O ions
𝐻 + = 10−𝑝𝐻
+
If the pH is known, [H ] is found by:
Section B: Practice Questions
Question 1
3
In an acid-base reaction, 500 cm of a solution of sodium hydroxide is completely neutralised by 680
3
cm of a solution of sulphuric acid. The equation for the reaction is:
𝐻2 𝑆𝑂4 𝑎𝑞 + 2𝑁𝑎𝑂𝐻 𝑎𝑞 ⟶ 𝑁𝑎2 𝑆𝑂4 (𝑎𝑞) + 2𝐻2 𝑂(ℓ)
o
The pH of the base solution before any acid is added is 13,80 at 25 C.
-3
1.1.
Show by calculation that the concentration of the NaOH solution will be 0,631 mol.dm .
(4)
1.2.
What is the difference between a strong base and a concentrated base?
(2)
1.3.
Calculate the mass of salt used to prepare the base solution.
(4)
Question 2
3
-3
3
When 500 cm diluted hydrochloric acid of concentration 0,25 mol.dm is added to 500 cm of sodium
hydroxide, the temperature of the solution rises and the pH changes to 2.3.
2.1.
Classify this reaction as exothermic or endothermic.
2.2.
Calculate
2.2.1.
2.2.2.
(1)
3
The mass of hydrochloric acid in the 500 cm container.
The final concentration of the hydrochloric acid
(3)
(5)
Question 3
3.1.
Consider the following two equations:
−
A:
𝐻𝐶ℓ 𝑎𝑞 + 𝐻2 𝑂 ℓ ⇌ 𝐻3 𝑂+ 𝑎𝑞 + 𝐶ℓ (𝑎𝑞)
B:
𝑁𝐻3 𝑔 + 𝐻2 𝑂 ℓ ⇌ 𝑁𝐻4+ 𝑎𝑞 + 𝑂𝐻− (𝑎𝑞)
3.1.1.
Which chemical substance above acts as an ampholyte? Explain your answer. (3)
3.1.2.
Which solution described by A or B will conduct an electric current the best?
Explain your choice.
(3)
Question 4
4.1.
The ionisation processes for two different acids are shown in equation I and II
I:
𝐶𝐻3 𝐶𝑂𝑂𝐻 ℓ + 𝐻2 𝑂 ℓ ⇌ 𝐻3 𝑂+ 𝑎𝑞 + 𝐶2 𝐻3 𝑂2− (𝑎𝑞)
II:
𝐻𝑁𝑂3 𝑔 + 𝐻2 𝑂 ℓ ⇌ 𝐻3 𝑂+ 𝑎𝑞 + 𝑁𝑂3− (𝑎𝑞)
4.1.1.
4.1.2.
4.1.3.
4.1.4.
Define the term weak acid.
Which of the two acids is considered a weak acid? Explain your answer.
Write down the conjugate acid-base pairs for equation I
Write an equation to show the anion in equation I reacts with water
(2)
(2)
(2)
(3)
Section C: Solutions
Question 1
1.1.
𝐾𝑤 = 𝐻 + 𝑂𝐻 − = 10−14 
𝐻 + = 10−𝑝𝐻
1,585 × 10−14 𝑂𝐻 −  = 10−14
= 10−13,80 
= 1,585 × 10−14
∴ 𝑂𝐻 − = 0,631
𝑁𝑎𝑂𝐻 = 𝑂𝐻 − = 0,631 𝑚𝑜𝑙. 𝑑𝑚−3 
1.2.
A strong base is a base that has a high percentage ionisation in water
A concentrated base contains a large number of moles of base per unit volume of
solution. 
1.3.
(4)
M(NaOH) = 23 + 16 + 1
-1
= 40 g.mol
𝑐=
(2)
𝑚

𝑀𝑉
0,631  =
𝑚

40 0,5
∴ 𝑚 = 0,631 40 0,5
= 12,62 𝑔 
(4)
Question 2
2.1.
2.2.
Exothermic 
(1)
𝐻 + = 10−𝑝𝐻 
= 10−2,3 
= 5,01 × 10−3 𝑚𝑜𝑙. 𝑑𝑚−3 
(3)
Question 3
Water
In reaction A it acts a base and in reaction B it acts as an acid
(3)
A
HCl is a strong acid which has a high percentage of ionization therefore it is a good
conductor. 
NH3 is a weak base which has a low percentage of ionization therefore it is not a good
conductor
(3)
Question 4
4.1.1.
A weak acid is an acid which has a low percentage ionization
4.1.2.
CH3COOH
(2)
It only ionises slightly in water
(2)
 for each conjugate acid − base pair
(2)
4.1.3.
4.1.4.
𝐶2 𝐻3 𝑂2− + 𝐻2 𝑂  ⟶ 𝐶𝐻3 𝐶𝑂𝑂𝐻 + 𝑂𝐻 −  balancing
(3)