Acids, Bases and Salts

Singapore Junior Chemistry Olympiad

By the end of this chapter, you can answer

1

Hydrochloric acid and vinegar can have the same concentration. Why is one far more dangerous?

2

Zinc oxide reacts with acids and with alkalis. What does that make it?

3

Ammonia has no OH group. Why is it still a base?

4

Which method makes pure copper(II) sulfate crystals, and which makes barium sulfate?

5

How does a titration measure the amount of acid in a solution, and which indicator should it use?

6

An antacid tablet is insoluble. How can a titration still find how much base it contains?

Recall: Chapters 3 and 4

1Write the ionic equation for AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq).

Ag+(aq) + Cl−(aq) → AgCl(s)

2How many moles are in 25.0 cm3 of 0.200 mol dm−3 solution?

0.200 × 0.0250 = 5.00 × 10−3 mol

3What is a dative bond?

A covalent bond in which both shared electrons come from one atom.
1

Acids

DefinitionAn acid is a substance that produces hydrogen ions, H+, when dissolved in water.

HCl(aq) → H+(aq) + Cl−(aq). Common acids: HCl, HNO3, H2SO4 and CH3COOH (vinegar).

The H+ ions give acids their properties: sour taste, blue litmus turns red, solutions conduct electricity.

Salts are named after the acid: sulfates, nitrates, chlorides. Copper and silver do not react with dilute acids. Lead is coated by insoluble PbCl2 or PbSO4.

The three reactions of acids

1acid + metal → salt + hydrogen

Mg + H2SO4 → MgSO4 + H2

Test: a lighted splint goes out with a ‘pop’.

2acid + base → salt + water

CuO + H2SO4 → CuSO4 + H2O

3acid + carbonate → salt + water + CO2

CaCO3 + 2HCl → CaCl2 + H2O + CO2

Test: carbon dioxide turns limewater milky.

1

Acids

Worked exampleWrite the ionic equation for CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g).

Split HCl(aq) and CaCl2(aq) into ions
the solid CaCO3 stays whole
Cl− appears on both sides
a spectator ion

CaCO3(s) + 2H+(aq) → Ca2+(aq) + H2O(l) + CO2(g)

Your turn(a) Name the salt and gas formed when zinc reacts with dilute hydrochloric acid. (b) Why does solid citric acid not conduct, while its solution does?

(a)
Zinc chloride and hydrogen[1]
(b)
The solid has no free ions. In water the acid ionises, and the ions carry the current.[1]
2

Strong and Weak Acids

DefinitionStrength is how far an acid ionises in water. A strong acid is completely ionised. A weak acid is only partially ionised.

HCl(aq) → H+(aq) + Cl−(aq)

CH3COOH(aq) ⇌ CH3COO−(aq) + H+(aq)

Exam tipStrong is not concentrated. Concentration is how much acid is dissolved in a given volume.
2

Strong and Weak Acids

Quick checkWhich statement about 0.10 mol dm−3 hydrochloric acid and 0.10 mol dm−3 ethanoic acid is correct?

AThey have the same pH.
BHydrochloric acid has the higher concentration of H+ ions.
CEthanoic acid is the more concentrated acid.
DBoth acids are fully ionised.
NoteB: both have the same concentration, but only hydrochloric acid is fully ionised, so it gives more H+ and has the lower pH.
3

Bases and Alkalis

DefinitionA base is a metal oxide or hydroxide. An alkali is a base that is soluble in water, producing OH− ions.

NaOH is a strong alkali (fully ionised). NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH−(aq): ammonia is a weak alkali.

The hydroxide ion, OH−

  • An O atom bonded to an H atom, with one extra electron: charge 1−
  • Negative, with three lone pairs on O, so it strongly attracts H+:

OH− + H+ → H2O

That is why it is a base: it removes the H+ that makes a solution acidic. The oxide ion, O2−, takes two H+.

3

Bases and Alkalis

1base + acid → salt + water: neutralisation

NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

Ionic equation, the same for every acid and alkali:

H+(aq) + OH−(aq) → H2O(l)

  • Na+ and Cl− are spectators, left as the salt.
  • A salt of a strong acid and a strong alkali gives a neutral solution.
3

Bases and Alkalis

2alkali + ammonium salt → salt + water + ammonia

KOH(aq) + NH4Cl(s) → KCl(aq) + H2O(l) + NH3(g)

  • The mixture is warmed.
  • The OH− takes an H+ from the NH4+ ion, leaving NH3.
  • Test: ammonia has a sharp smell and turns damp red litmus paper blue.
3

Bases and Alkalis

Your turnWrite the balanced equation for magnesium hydroxide reacting with hydrochloric acid.

Mg(OH)2(s) + 2HCl(aq) → MgCl2(aq) + 2H2O(l)[1]

Your turnIs every base an alkali? Explain.

No. Only bases that dissolve in water are alkalis. Copper(II) oxide is a base but not an alkali.[1]
4

The pH Scale and Indicators

  • pH < 7: more H+ than OH−, acidic
  • pH = 7: equal, neutral
  • pH > 7: more OH−, alkaline

One pH unit is a tenfold change in [H+]: pH 4 has 1000 times more H+ than pH 7.

4

The pH Scale and Indicators

indicatorin acidchanges over pHin alkali
methyl orangered3 to 5yellow
litmusred5 to 8blue
phenolphthaleincolourless8 to 10pink
thymolphthaleincolourless9 to 10.5blue

Acidic soil is treated with calcium hydroxide (slaked lime).

Your turn(a) P has pH 2 and Q has pH 5. How many times greater is [H+] in P? (b) What colour is methyl orange at pH 9?

(a)
3 units: 1000 times[1]
(b)
Yellow[1]
5

Oxides

5

Oxides

1Basic: most metal oxides

react with acids

MgO + 2HNO3 → Mg(NO3)2 + H2O

Na2O and K2O dissolve to form alkalis.

2Amphoteric: ZnO, Al2O3, PbO

react with acids and alkalis

ZnO + 2HCl → ZnCl2 + H2O

ZnO + 2NaOH → Na2ZnO2 + H2O

3Acidic: most non-metal oxides

react with alkalis

CO2 + 2NaOH → Na2CO3 + H2O

Most dissolve to form acids:

SO2 + H2O → H2SO3

4Neutral: H2O, CO, NO

react with neither acids nor alkalis

5

Oxides

Quick checkWhich oxide reacts with both dilute hydrochloric acid and aqueous sodium hydroxide?

Acopper(II) oxide
Bcarbon dioxide
Caluminium oxide
Dcarbon monoxide
NoteC: aluminium oxide is amphoteric. Copper(II) oxide is basic, carbon dioxide is acidic and carbon monoxide is neutral.
6

Theories of Acids and BasesCore · A Level

theoryacidbase
Arrheniusgives H+(aq) in watergives OH−(aq) in water
Brønsted–Lowryproton donorproton acceptor
Lewiselectron-pair acceptorelectron-pair donor

Arrhenius works only in water, and cannot explain why NH3, with no OH group, is a base.

6

Theories of Acids and BasesCore · A Level

The proton moves to a lone pair on water, forming the hydronium ion, H3O+. H+(aq) and H3O+(aq) mean the same.

Two species that differ by one H+ are a conjugate acid–base pair.

Cl− is the conjugate base of HCl. H3O+ is the conjugate acid of H2O.

6

Theories of Acids and BasesCore · A Level

Worked exampleIdentify the conjugate pairs in NH3 + H2O ⇌ NH4+ + OH−.

NH3 → NH4+
base and its conjugate acid
H2O → OH−
acid and its conjugate base

Water is a base with HCl but an acid with NH3.

HCl and HNO3 are monobasic, H2SO4 is dibasic, H3PO4 is tribasic.

Your turnGive the conjugate base of H2SO4 and the conjugate acid of NH3.

HSO4− and NH4+[1]
6

Theories of Acids and BasesCore · A Level

B is in Group 13: in BF3 it has only six electrons, two short of an octet.

N in NH3 has a lone pair not used in bonding.

N gives the pair to B: a dative bond, and B reaches its octet. NH3 is the Lewis base (donor), BF3 the Lewis acid (acceptor).

No proton moves. Every Brønsted–Lowry base is also a Lewis base.

6

Theories of Acids and BasesCore · A Level

Your turnIn HCl(g) + NH3(g) → NH4Cl(s), which reactant is the Brønsted–Lowry acid? Why is this not an Arrhenius acid–base reaction?

HCl: it gives a proton to NH3.[1]
There is no water, so no H+(aq) or OH−(aq) forms.[1]

Quick checkWhat is the conjugate base of HSO4−?

AH2SO4
BSO42−
CH3O+
DOH−
NoteB: the conjugate base has one H+ fewer. H2SO4 is its conjugate acid.
7

Salts

A salt is an ionic compound formed when the H+ of an acid is replaced by a metal ion or an ammonium ion.

First decide whether the salt dissolves in water.

saltssolubleinsoluble
Na, K and NH4 saltsallnone
nitratesallnone
chloridesmostAgCl, PbCl2
sulfatesmostBaSO4, PbSO4, CaSO4
carbonatesNa2CO3, K2CO3, (NH4)2CO3all others
7

Salts

A second way to make a salt

So far every salt came from an acid. Mix two solutions: if a cation from one and an anion from the other form an insoluble salt, it appears as a solid, a precipitate.

Ba(NO3)2(aq) + Na2SO4(aq) →
BaSO4(s) + 2NaNO3(aq)

Ba2+(aq) + SO42−(aq) → BaSO4(s)

Na+ and NO3− are spectators. Filter off the solid.

7

Salts

Precipitation: Ba(NO3)2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaNO3(aq)

Titration: neither reactant can be filtered off, so find the exact volumes, then repeat without indicator.

Excess solid: the excess makes sure all the acid is used. Filter it off, heat to saturation, cool, filter, wash, dry.

7

Salts

Worked exampleMake lead(II) chloride from lead(II) carbonate, dilute nitric acid and aqueous potassium chloride.

  1. PbCl2 is insoluble: precipitate it. First make a soluble lead salt.
  2. Excess PbCO3 + dilute HNO3, then filter: Pb(NO3)2(aq)
  3. Add excess KCl(aq): Pb(NO3)2(aq) + 2KCl(aq) → PbCl2(s) + 2KNO3(aq)
  4. Filter, wash with distilled water, dry.

Your turnSuggest a method for (a) copper(II) sulfate, (b) potassium nitrate, (c) silver chloride.

(a)
Excess CuO or CuCO3 with dilute H2SO4, filter, crystallise[1]
(b)
Titration of KOH with dilute HNO3, then crystallise[1]
(c)
Precipitation: AgNO3(aq) + NaCl(aq), filter, wash, dry[1]
7

Salts

Quick checkWhich pair could be mixed to make barium sulfate by precipitation?

ABa(NO3)2(aq) and Na2SO4(aq)
BBaCO3(s) and Na2SO4(aq)
CBaCl2(aq) and NaNO3(aq)
DBa(NO3)2(aq) and NaCl(aq)
NoteA: one solution gives Ba2+ and the other SO42−. Barium carbonate is insoluble, so it gives no Ba2+ ions, and C and D contain no sulfate.
8

Acid–Base Titrations

AnalogyA boss fight. The boss's health bar is hidden: you cannot see how much HP he has.

Your gun does exactly 5 HP per shot. Count the shots until he falls.

12 × 5 HP = 60 HP

A titration measures an unknown amount of acid or alkali in the same way.

8

Acid–Base Titrations

Standard solutionconcentration accurately known
Pipette / burettea fixed volume into the flask / added until the colour changes
Equivalence pointreactants have reacted exactly in the ratio of the equation
End-pointthe indicator changes colour
8

Acid–Base Titrations

boss fighttitration
the boss, HP hiddenthe analyte in the flask: amount in mol unknown
exactly 5 HP per shotthe standard solution: known concentration, so each cm3 carries a known amount
shots countedthe titre: volume added from the burette
the boss fallsthe end-point: the indicator changes colour
HP = shots × damagen = c × V

AnalogyCloser to the real thing:

Single shots near the end. Shots fired after he falls still count. Add the titrant drop by drop near the end-point, or the titre is too big.

The equation sets the damage. One H2SO4 neutralises two NaOH, so each "shot" of H2SO4 counts double.

Fight one clone. The pipette takes 25.0 cm3 from the volumetric flask. Find its amount, then scale up to the whole flask.

8

Acid–Base Titrations

Worked example5.00 g of impure NaOH is made up to 1.00 dm3. 25.0 cm3 of it is neutralised by 15.15 cm3 of 0.102 mol dm−3 H2SO4. Find the percentage purity.

  1. 2NaOH + H2SO4 → Na2SO4 + 2H2O
  2. nH₂SO₄ = 0.102 × 0.01515 = 1.545 × 10−3 mol
  3. nNaOH in 25.0 cm3 = 3.091 × 10−3 mol
  1. in 1.00 dm3: × 1000/25.0 = 0.1236 mol = 4.945 g
  2. purity = 4.945 ÷ 5.00 × 100% = 98.9%
8

Acid–Base Titrations

Water of crystallisation: water held in a salt's lattice, as in CuSO4·5H2O.

Worked example8.65 g of Na2CO3·nH2O is made up to 250 cm3. 25.0 cm3 reacts with 24.00 cm3 of 0.125 mol dm−3 H2SO4 (ratio 1 : 1). Find n.

nNa₂CO₃ in 25.0 cm3 = 0.125 × 0.02400 = 3.000 × 10−3 mol

in 250 cm3: 0.03000 mol, so Mr = 8.65 ÷ 0.03000 = 288.3

106.0 + 18.0n = 288.3, so n = 10

Your turn25.0 cm3 of NaOH(aq) is neutralised by 15.00 cm3 of 0.250 mol dm−3 HCl. Find [NaOH].

nHCl = 3.750 × 10−3 mol = nNaOH[1]
[NaOH] = 3.750 × 10−3 ÷ 0.0250 = 0.150 mol dm−3[1]
8

Acid–Base TitrationsCore · A Level

8.1Choosing an indicator

The indicator must change colour within the sudden pH change at the equivalence point.

titrationindicator
strong acid, strong basemethyl orange, phenolphthalein or thymolphthalein
strong acid, weak basemethyl orange
weak acid, strong basephenolphthalein or thymolphthalein
weak acid, weak baseno suitable indicator

Your turnWhich indicator would you use to titrate aqueous ammonia with hydrochloric acid? Why?

Methyl orange[1]
HCl is strong and NH3 is a weak base, so the sudden pH change lies in the acidic range.[1]
8

Acid–Base TitrationsCore · A Level

8.2Back-titration

For an insoluble solid such as CaCO3 or Mg(OH)2, which cannot be titrated directly.

  1. Add a known excess of acid: all the solid reacts.
  2. Titrate the acid left over with a standard alkali.
  3. The acid that reacted with the solid is found by difference.
8

Acid–Base TitrationsCore · A Level

Worked exampleA 1.00 g antacid tablet contains insoluble Mg(OH)2. It is added to 100 cm3 of 0.400 mol dm−3 HNO3 (excess). The leftover acid needs 20.50 cm3 of 0.750 mol dm−3 KOH. Find the percentage of Mg(OH)2.

  1. HNO3 added = 0.04000 mol, left over = nKOH = 0.01538 mol
  2. reacted = 0.04000 − 0.01538 = 0.02462 mol
  3. Mg(OH)2 + 2HNO3: nMg(OH)₂ = 0.01231 mol
  4. mass = 0.01231 × 58.3 = 0.7177 g, so 71.8%

Your turn2.50 g of impure CaCO3 is added to 50.0 cm3 of 1.00 mol dm−3 HCl (excess). The leftover acid needs 20.0 cm3 of 0.500 mol dm−3 NaOH. Find the percentage of CaCO3 (Mr 100.1).

added 0.05000 mol, left over 0.01000 mol, reacted 0.04000 mol[1]
nCaCO₃ = 0.02000 mol, mass = 2.002 g[1]
purity = 80.1%[1]
8

Acid–Base Titrations

Quick check25.0 cm3 of aqueous sodium hydroxide is neutralised by 20.0 cm3 of 0.100 mol dm−3 sulfuric acid. What is the concentration of the sodium hydroxide?

A0.0400 mol dm−3
B0.0800 mol dm−3
C0.160 mol dm−3
D0.200 mol dm−3
NoteC: nH₂SO₄ = 2.00 × 10−3 mol, and each H2SO4 neutralises 2NaOH, so nNaOH = 4.00 × 10−3 mol in 0.0250 dm3. Option B forgets the 2 : 1 ratio.

Exit ticket

1What is the difference between a strong acid and a concentrated acid?

Strong: fully ionised. Concentrated: a lot of acid dissolved per volume.[1]

2Name an amphoteric oxide.

Zinc oxide, aluminium oxide or lead(II) oxide[1]

3Why is excess solid used when making a soluble salt from an acid?

So that all the acid is used up. The excess solid is filtered off.[1]

4Define a Brønsted–Lowry base.

A proton acceptor[1]