Acids
and Bases

IP Chemistry (6092), Sec 3

By the end of this topic, you can answer

1

What do all acids have in common, and what do all alkalis have in common?

2

Why does a piece of magnesium fizz in acid, and how do you prove the gas is hydrogen?

3

Vinegar and hydrochloric acid are both acids. Why is one much stronger than the other?

4

What happens to the ions when an acid neutralises an alkali?

5

What does pH 2 tell you that pH 6 does not?

6

How can you tell whether an oxide is acidic, basic, amphoteric or neutral?

Starter

PredictLemon juice, vinegar and stomach acid all taste sour. Soap and oven cleaner feel slippery.

What could lemon juice and vinegar have in common at the level of particles?

NoteEvery acid releases the same ion in water, and every alkali releases another. This topic is about those two ions and how they react.
1

Acids

1.1What an acid is

DefinitionAn acid is a substance that produces hydrogen ions, H+, in aqueous solution.
AcidFormulaIons in aqueous solution
hydrochloric acidHClH+ and Cl−
nitric acidHNO3H+ and NO3−
sulfuric acidH2SO4H+ and SO42−
ethanoic acidCH3COOHH+ and CH3COO−
  • Acids ionise only in water. Solid citric acid does not conduct, but its aqueous solution does.
  • Acids taste sour, turn blue litmus paper red and have a pH below 7.
1

Acids

1.2Reactions of acids

Acid reacts withProductsExample
a reactive metalsalt + hydrogenMg(s) + H2SO4(aq) → MgSO4(aq) + H2(g)
a basesalt + waterMgO(s) + H2SO4(aq) → MgSO4(aq) + H2O(l)
a carbonatesalt + water + carbon dioxideCaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g)
  • Sulfuric acid forms sulfates, nitric acid forms nitrates and hydrochloric acid forms chlorides.
  • Copper and silver do not react with dilute acids. Lead seems not to, because an insoluble coating of lead(II) chloride or lead(II) sulfate stops the reaction.
1

Acids

1.2Testing the gases

HydrogenA lighted splint at the mouth of the tube is extinguished with a pop.
Carbon dioxideBubbled through limewater, it forms a white precipitate: the limewater turns milky.
1

Acids

Quick checkA green powder is added to dilute hydrochloric acid. It fizzes and the gas turns limewater milky. What type of substance is the powder?

Aa metal
Ba metal oxide
Ca carbonate
Dan alkali
NoteC: the gas is carbon dioxide, which only a carbonate gives with an acid. A metal gives hydrogen, and a metal oxide or an alkali gives no gas.
1

Acids

1.3Strong and weak acids

  • A strong acid is completely ionised in aqueous solution, e.g. hydrochloric, nitric and sulfuric acids.
  • A weak acid is only partially ionised, e.g. ethanoic acid and citric acid.
  • Strength is how much of the acid ionises. Concentration is how much acid is dissolved in a given volume of water.
AnalogyA strong acid is a bag of sweets torn wide open: every sweet falls out. A weak acid is a bag with a tiny tear: only a few get out. Concentration is how many bags are in the room.
1

Acids

1.3Writing the ionisation

HCl(aq) → H+(aq) + Cl−(aq)
CH3COOH(aq) ⇌ CH3COO−(aq) + H+(aq)
  • The ⇌ sign shows a reversible reaction: the ions keep re-forming molecules, so most of a weak acid stays as molecules.
  • A strong acid can be dilute, and a weak acid can be concentrated.
2

Bases and Alkalis

2.1Bases and alkalis

DefinitionA base is a metal oxide or metal hydroxide. An alkali is a base that is soluble in water and produces hydroxide ions, OH−, in aqueous solution.
  • Alkalis taste bitter, feel soapy, turn red litmus paper blue and have a pH above 7.
  • Sodium hydroxide is a strong alkali. Aqueous ammonia is a weak alkali: NH3(g) + H2O(l) ⇌ NH4+(aq) + OH−(aq).
2

Bases and Alkalis

2.2Neutralisation

Neutralisation is the reaction between hydrogen ions and hydroxide ions to produce water:
H+(aq) + OH−(aq) → H2O(l)
Base reacts withProductsExample
an acidsalt + water2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)
an ammonium salt (alkalis, on heating)salt + water + ammoniaKOH(aq) + NH4Cl(s) → KCl(aq) + H2O(l) + NH3(g)
AnalogyAt a dance, every H+ finds an OH− partner and they leave the floor together as a water molecule. The other ions keep dancing: they become the salt.
2

Bases and Alkalis

2.2Testing for ammonia

  • Heat the alkali with the ammonium salt. Ammonia gas is given off.
  • Ammonia has a pungent smell. It turns damp red litmus paper blue.
  • An insoluble base reacts with the H+ ions of an acid, e.g. CuO(s) + 2H+(aq) → Cu2+(aq) + H2O(l).
2

Bases and Alkalis

Worked exampleMilk of magnesia contains magnesium hydroxide. Write the equation for its reaction with hydrochloric acid in the stomach.

Step 1Base + acid → salt + water. The salt is magnesium chloride, MgCl2.

Step 2Balance: two HCl for each Mg(OH)2, giving two H2O.

Step 3Mg(OH)2(s) + 2HCl(aq) → MgCl2(aq) + 2H2O(l)

2

Bases and Alkalis

Quick checkWhich pair of substances gives off ammonia when heated together?

Aammonium chloride and hydrochloric acid
Bammonium sulfate and sodium hydroxide
Csodium chloride and sodium hydroxide
Dammonium nitrate and water
NoteB: an alkali heated with an ammonium salt gives off ammonia. A has no alkali, C has no ammonium salt, and water is not an alkali.
3

The pH Scale

3.1pH and Universal Indicator

  • Below 7: more H+ than OH−, so the solution is acidic. At 7 they are equal: neutral. Above 7: more OH−, so it is alkaline.
  • The lower the pH, the higher the concentration of H+ ions.
3

The pH Scale

3.1Reading the scale

pHIon concentrationsSolutionUniversal Indicator
below 7more H+ than OH−acidicred, orange or yellow
7H+ and OH− equalneutralgreen
above 7more OH− than H+alkalineblue, then violet
  • A solution of pH 4 has 1000 times more H+ ions than one of pH 7.
  • pH is measured with Universal Indicator, a pH meter, or a pH sensor connected to a data logger.
AnalogyEach step down the pH scale is like adding a zero to a price: $1, $10, $100. One pH unit lower means ten times more H+ ions.
3

The pH Scale

3.2Other indicators

IndicatorIn acidChanges at pHIn alkali
methyl orangered3 to 5yellow
screened methyl orangeviolet3 to 5green
litmusred5 to 8blue
thymolphthaleincolourless9 to 10.5blue

3.3Controlling the pH of soil

  • Most plants grow best in soil that is neutral or slightly acidic.
  • Acid rain, fertilisers and microbes can make soil too acidic.
  • Farmers add slaked lime, calcium hydroxide, to neutralise the excess acid. Too much makes the soil too alkaline.
3

The pH Scale

Quick checkSolution P has pH 2 and solution Q has pH 5. Which statement is correct?

AQ is more acidic than P.
BP has a higher concentration of H⁺ ions than Q.
CBoth turn Universal Indicator green.
DQ is alkaline.
NoteB: the lower the pH, the higher the H⁺ ion concentration. Both are acidic, so neither is green or alkaline.
4

Oxides

4.1Four types of oxide

TypeFormed byReacts withExamples
basicmost metalsacids, forming a salt and waterCuO, CaO, MgO, Na2O
amphotericsome metalsboth acids and alkalisZnO, Al2O3, PbO
acidicmost non-metalsalkalis, forming a salt and waterCO2, SO2, SO3, P4O10, SiO2
neutralsome non-metalsneitherH2O, CO, NO
AnalogyAn amphoteric oxide is like someone who speaks two languages: zinc oxide can talk to an acid and to an alkali.
4

Oxides

4.1Oxides in equations

ZnO(s) + 2HCl(aq) → ZnCl2(aq) + H2O(l)
ZnO(s) + 2NaOH(aq) → Na2ZnO2(aq) + H2O(l)
CO2(g) + 2NaOH(aq) → Na2CO3(aq) + H2O(l)
SO2(g) + H2O(l) → H2SO3(aq)
  • Most basic oxides are insoluble. Sodium oxide and potassium oxide dissolve to form alkalis. Most acidic oxides dissolve to form acids.
4

Oxides

4.2Classifying an unknown oxide

Worked exampleWhite solid X is insoluble in water. It reacts with dilute hydrochloric acid and with aqueous sodium hydroxide. Classify X.

Step 1It reacts with both an acid and an alkali, so X is amphoteric. A white metal oxide: X could be zinc oxide or aluminium oxide.

4

Oxides

Quick checkWhich element forms an oxide that dissolves in water to give a solution with a pH below 7?

Acopper
Bmagnesium
Csulfur
Dsodium
NoteC: sulfur is a non-metal, so its oxide is acidic and dissolves to form an acid. Sodium oxide dissolves to form an alkali, and copper and magnesium oxides are basic.

Exit ticket

1Define an acid.

A substance that produces hydrogen ions, H+, in aqueous solution.[1]

2Write the ionic equation for neutralisation.

H+(aq) + OH−(aq) → H2O(l)[1]

3What is the difference between a strong acid and a weak acid?

A strong acid is completely ionised in aqueous solution. A weak acid is only partially ionised.[1]

4Classify aluminium oxide.

Amphoteric: it reacts with both acids and alkalis.[1]