Acids and Bases

Question 1

Arrhenius acid-base theory

Answer 1

● Arrhenius acids give hydronium ions, H3O
+ (aq) [or hydrogen ions,
H+ (aq)] after dissolving in water.
Examples: HCl (aq), HNO3 (aq), CH3COOH (aq) and H2SO4 (aq)

● Arrhenius bases give hydroxide ions, OH-(aq) in water.
Examples: NaOH (aq), KOH (aq), Ca(OH)2 (aq)

Question 2

Acid dissociation

Answer 2

● When an acid dissolves in water, the acid molecule react with H2O molecules by
giving out a hydrogen ion, to form hydronium ions, H3O
+
and a negative ion.
● For example: HCl (aq) + H2O (l) → H3O
+ (aq) + Cl- (aq)

Question 3

what does proticity mean

Answer 3

number of moles of hydrogen ions/ protons produced
per mole of acid molecules when dissolved in water.

Question 4

monoprotic acids

Answer 4

Monoprotic acids produce one mole of H+ (aq) ions per mole of acid molecules.

Examples: HCl (aq), HNO3
(aq) and CH3COOH (aq)

■ CH3COOH (aq) + H2O (l) ⇌ CH3COO- (aq) + H+(aq)

Question 5

polyprotic

Answer 5

• diprotic acids produce two moles of H+ (aq) ions and triprotic
  acids produce three moles of H+ (aq) ions per mole of molecules.

Examples: H2SO4
(diprotic) and H3PO4
(triprotic)

Question 6

Brønsted-Lowry theory

Answer 6

In 1923, Johannes Nicolaus Brønsted (from Denmark) and Thomas Martin
Lowry (from England) published essentially the same theory about how acids
and bases behave independently. Both names have been used for the theory.

● The theory classifies a substance as an acid or a
base in terms of whether it donates or receives
H+ (aq) ions/ protons when dissolved in water or
in a reaction.

In this theory, acid and base must work together so that the protons donated
by the acid can be received by the base.

Question 7

Brønsted-Lowry acid

Answer 7

donates H+ (aq) ions → proton donors
○ Examples: HCl (aq), HNO3 (aq), CH3COOH (aq) and H2SO4 (aq)

Question 8

Brønsted-Lowry base:

Answer 8

receives H+ (aq) ions → proton acceptors
Examples: NaOH (aq), KOH (aq), NH3 (aq), Na2CO3 (aq), NaHCO3 (aq)

Question 9

Brønsted-Lowry theory thinking

Answer 9

● Consider the following generic equation for a reaction between an acid, HA (aq) and a base, B (aq):

HA (aq) + B (aq) → A- (aq) + BH+ (aq)

● When looking at the backward process, BH+
donates H+ ion to A- to form B and at the same time, A-receives H+ ion from BH+ to form HA.

○ BH+ (aq) is a Brønsted-Lowry acid and A- (aq) is a Brønsted-Lowry base (for the
backward reaction).

Question 10

Conjugate acid-base pairs

Answer 10

● In conclusion, two conjugate acid-base pairs can be identified in each
acid-base reaction.

● The identities of ‘acid’ or ‘base’ mainly describe the role of the chemical species as proton donors/ acceptors.

A Brønsted-Lowry acid and its conjugate base always differ by one proton.

A Brønsted-Lowry base and its conjugate acid always differ by one proton.

Question 11

Dissociation of water (1)

Answer 11

● Water has a tendency to dissociate (break up) into ions when in solution:
H2O (l) + H2O (l) ⇌ H3O
+ (aq) + OH- (aq)

● Pure water dissociates only slightly - about one water molecule out of every 10
million dissociates and the rest remain undissociated (in molecular form).

Question 12

Dissociation of water (2)

Answer 12

● Water molecule ionizes (separates into ions) to form a hydronium/ hydrogen ion
and a hydroxide ion:
H2O (l) + H2O (l) ⇌ H3O
+ (aq) + OH- (aq)

● As seen in the equation, water can either → Identify the conjugate acid-base pair in this reaction. donate or receive H+ (aq) ion, which means it can be both a proton donor and a proton acceptor.

○ Water can act as both Brønsted-Lowry acid and base.

○ It is described as being amphoteric

Question 13

Amphoteric species

Answer 13

● Apart from water, the conjugate base of many polyprotic acids are also
amphoteric.

Question 14

Amphoteric species (1)

Answer 14

Example 1: HPO4
2- (aq)
○ It is the conjugate base of
H2PO4- (aq)

Question 15

Amphoteric species (2)

Answer 15

● Example 2: HCO3
- (aq)
○ It is the conjugate base of H2CO3

Question 16

Amphoteric species (3)

Answer 16

HS-
Conjugate base of H2S

Question 17

Properties of acids and bases

Answer 17

● Acids have sour taste, while bases have bitter taste
and slippery feeling.
● They can be distinguished using universal indicator,
pH paper, litmus solution or pH meter.
● They can only show these properties when dissolved
in water.

Question 18

Indicators (1)

Answer 18

Litmus solution
It is just used to identify whether an
aqueous solution is acidic or basic.
Acidic: Red
Neutral: Purple
Basic: Blue

Question 19

Indicators (2)

Answer 19

Universal indicator
It can measure the acidity and the
alkalinity of an aqueous solution by
providing an approximate pH value.

Question 20

Indicators (3)

Answer 20

pH paper, pH meter

Question 21

Indicators (4)

Answer 21

● Methyl orange and phenolphthalein are
indicators mainly used in titration
experiments to identify the end-point and
hence determine the concentration of a
solution.
● These indicators show a sharp colour
change when the acid/ base is neutralised.

Question 22

Acidity and alkalinity of solutions

Answer 22

● In any type of aqueous solutions (acidic, neutral or basic), they all contain both H+
(aq) ions and OH- (aq) ions.

● This is due to the self-ionization / dissociation of water molecules, although the
extent of dissociation is very small.
H2O (l) + H2O (l) ⇌ H3O
+ (aq) + OH- (aq)

Question 23

what is the acidity of a solution dependent on

Answer 23

● The acidity of a solution depends on the relative concentration of H
+ (aq) ions and
OH- (aq) ions in an aqueous solution.

Question 24

what is concentration

Answer 24

● Concentration refers to the amount (number of moles) of solute dissolved in a
unit volume (dm3
) of solution.

● In 1 dm3
solution, if solution A contains larger amount of solute than solution B,
we will describe them as either:
○ solution A is more concentrated than solution B, or
○ solution B is more dilute than solution A.
Note: 1 dm3
= 1000 cm3

Question 25

how is concentration calculated

Answer 25

number of moles of solute (mol)/volume of solution (dm3)

● Unit: mol dm-3 or M
● Example:
Concentration of solution when 0.05 moles of HCl dissolved in 100 cm3
of water
= 0.05 mol ÷ 0.1 dm3
= 0.5 mol dm-3

Question 26

what is pH

Answer 26

pH = -log10 [H3O
+ (aq)] or pH = -log10 [H+ (aq)]

● In chemistry, “p” in pH represents ‘-log10’ and “H” represents [H+ (aq)]. pH can
hence be expressed mathematically.
● On the other hand, as pH = -log10 [H+ (aq)], making [H+ (aq)] the subject,
[H+ (aq)] = 10-pH

Question 27

Relationship between [H+ (aq)],
[OH- (aq)] and pH value

Answer 27

● As pH decreases (down the chart), the acidity of
solution increases, and the solution becomes
more acidic.
● When the pH value of an aqueous solution
decreases by one unit, [H+ (aq)] increases by
10-fold while [OH- (aq)] decreases by 10-fold.

Question 28

When pH increases by 2 units,

Answer 28

[H+ (aq)] decreases by 10^2
times.
[OH-(aq)] increases by 10^2 times

Question 29

when pH increases by x units,

Answer 29

[H+ (aq)] would decrease by
10^x times.

Question 30

when pH decreases by x units

Answer 30

[H+ (aq)] would increase by
10^x times

Question 31

Reaction of acids with metals

Answer 31

● Dilute acids such as HCl (aq) and H2SO4 (aq) react
with reactive metals (those above hydrogen in the
Reactivity Series) to produce a salt and hydrogen
gas, H2 (g).

● The H2 (g) produced can be tested by a burning
splint - a “squeaky pop” sound can be heard.

Question 32

reactivity series

Answer 32

potassium K
sodium Na
calcium Ca
magnesium Mg
aluminium Al
carbon C
zinc Zn
iron Fe
tin Sn
lead Pb
hydrogen H
copper Cu
silver Ag
gold Au
platnium Pt

Question 33

what is the salt

Answer 33

A salt is a compound when the ionizable hydrogen atom(s) of an acid is replaced
by metal ion(s) or other cation(s).

Question 34

give the Reaction of acids with metals (Zn and Mg with HCl and HNO3)

Answer 34

● Reaction:
metal + acid → salt + hydrogen

● Examples:
zinc + hydrochloric acid → zinc chloride + hydrogen
Zn (s) + 2 HCl (aq) → ZnCl2
(aq) + H2 (g)

magnesium + nitric acid → magnesium nitrate + hydrogen
Mg (s) + 2 HNO3 (aq) → Mg(NO3
)2 (aq) + H2 (g)

Question 35

Reaction with metal hydroxides or oxides

Answer 35

● This reaction is known as neutralisation.
● The reaction is exothermic, which means heat is released during the process.
● When an acid reacts with a base, salt and water are produced:
acid + metal oxide/ hydroxide → salt + water
● Example: sulphuric acid + sodium oxide → sodium sulphate + water
H2SO4 (aq) + Na2O (s) → Na2SO4
(aq) + H2O (l)

Question 36

Reaction with metal carbonates and hydrogencarbonates

Answer 36

● Dilute acids react with metal carbonate or hydrogencarbonate to give salt, water
and carbon dioxide gas, CO2 (g).
acid + metal carbonate/ hydrogencarbonate → salt + water + carbon dioxide
● Example:
hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide
2 HCl (aq) + CaCO3 (s) → CaCl2
(aq) + H2O (l) + CO2 (g)

Question 37

Reaction with metal carbonates and hydrogencarbonates (testing of CO2)

Answer 37

● The produced CO2 (g) can be tested with limewater which is
saturated calcium hydroxide solution, Ca(OH)2 (aq).
The colourless solution turns milky.
● The observation can be explained using the following equation:
Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s) + H2O (l)
● The product in this reaction - calcium carbonate, CaCO3 (s) - is
an insoluble white compound

Question 38

Reaction of acids with ammonia

Answer 38

● Acids react with ammonia to produce an ammonium salt:
acid + ammonia → ammonium salt
● Example:
(1) hydrochloric acid + ammonia → ammonium chloride
HCl (aq) + NH3
(aq) → NH4Cl (aq)
(2) sulphuric acid + ammonia → ammonium sulphate
H2SO4 (aq) + 2 NH3 (aq) → (NH4)2SO4(aq)