Topic 08 · Acids and Bases

Acids and Bases

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 when dissolved in water. A solid acid such as citric acid does not conduct electricity, but its aqueous solution does, because it contains mobile ions.
  • Acids taste sour, turn blue litmus paper red and have a pH below 7.

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)
pop!lighted splintdilute acidmagnesiumhydrogen extinguishes a lighted splint with a popdilute acidcarbonatelimewaterturns milkycarbon dioxide turns limewater milky
  • Sulfuric acid forms sulfates, nitric acid forms nitrates and hydrochloric acid forms chlorides.
  • Unreactive metals such as copper and silver do not react with dilute acids.
NoteLead seems not to react with hydrochloric acid or sulfuric acid. The first reaction forms insoluble lead(II) chloride or lead(II) sulfate, which coats the lead and stops further reaction.
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1

Acids (continued)

1.3Strong and weak acids

DefinitionA strong acid is completely ionised in aqueous solution, e.g. hydrochloric, nitric and sulfuric acids.
DefinitionA weak acid is only partially ionised in aqueous solution, e.g. ethanoic acid and citric acid.
HCl(aq) → H+(aq) + Cl−(aq)CH3COOH(aq) ⇌ CH3COO−(aq) + H+(aq)
  • The ⇌ sign shows a reversible reaction: the ions keep re-forming molecules.
  • Strength is how much of the acid ionises. Concentration is how much acid is dissolved in a given volume of water. A strong acid can be dilute, and a weak acid can be concentrated.
hydrochloric acid: all ionsethanoic acid: mostly molecules−−−−−−−−++++++++−−++
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.
Bases that do not dissolve in waterCuO, ZnO, Mg(OH)2 and most other metal oxides and hydroxides
Alkalis: bases that dissolveNaOH, KOH, Ca(OH)2 and NH3(aq), clear solutions containing OH− ions
  • Alkalis taste bitter, feel soapy, turn red litmus paper blue and have a pH above 7.
  • Sodium hydroxide is a strong alkali: it is completely ionised. Aqueous ammonia is a weak alkali: NH3(g) + H2O(l) ⇌ NH4+(aq) + OH−(aq).

2.2Reactions of bases

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)
damp red litmusturns bluealkali +ammonium saltheat
  • 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).

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

Base + acid → salt + water. The salt is magnesium chloride, MgCl2.Mg(OH)2(s) + 2HCl(aq) → MgCl2(aq) + 2H2O(l)
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3

The pH Scale

3.1pH and Universal Indicator

01234567891011121314acidicneutralalkalinemore H⁺ ions: more acidicmore OH⁻ ions: more alkalinestrong acidweak acidneutralweak alkalistrong alkali
pHIon concentrationsSolutionUniversal Indicator
below 7more H+ than OH−acidicred, orange or yellow
7H+ and OH− equalneutralgreen
above 7more OH− than H+alkalineblue, then violet
  • The lower the pH, the higher the concentration of H+ ions. A solution of pH 4 has 1000 times more H+ ions than one of pH 7.
  • pH is measured with Universal Indicator (paper or solution), a pH meter, or a pH sensor connected to a data logger.

3.2Other indicators

IndicatorColour in acidChanges at pHColour in 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. Soil that is too acidic or too alkaline reduces the nutrients a plant can absorb.
  • Acid rain, fertilisers and microbes in the soil can make it too acidic.
  • Farmers add slaked lime, calcium hydroxide, to neutralise the excess acid. Too much makes the soil too alkaline.
44Topic 08 · Acids and Bases
4

Oxides

4.1Four types of oxide

TypeFormed byReacts withExamples
basicmost metalsacids, forming a salt and waterCuO, CaO, MgO, Na2O
amphotericsome metalsboth acids and alkalis, forming a salt and waterZnO, Al2O3, PbO
acidicmost non-metalsalkalis, forming a salt and waterCO2, SO2, SO3, P4O10, SiO2
neutralsome non-metalsneither acids nor alkalisH2O, CO, NO
  • Most basic oxides are insoluble solids. Sodium oxide and potassium oxide dissolve in water to form alkalis.
  • Most acidic oxides dissolve in water to form acids: SO2(g) + H2O(l) → H2SO3(aq).
ZnO(s) + 2HCl(aq) → ZnCl2(aq) + H2O(l) ZnO(s) + 2NaOH(aq) → Na2ZnO2(aq) + H2O(l) CO2(g) + 2NaOH(aq) → Na2CO3(aq) + H2O(l)

4.2Classifying an unknown oxide

Does it react with acids?Does it react with alkalis?Does it react with alkalis?yesnoamphotericZnO, Al₂O₃, PbObasicCuO, CaO, MgOacidicCO₂, SO₂, P₄O₁₀neutralH₂O, CO, NOyesnoyesno

Worked exampleOxide X is a white solid that is insoluble in water. It reacts with dilute hydrochloric acid and with aqueous sodium hydroxide, and forms a colourless solution with each. Classify X and suggest its identity.

X reacts with both an acid and an alkali, so X is amphoteric.It is a white solid oxide of a metal, so X could be zinc oxide or aluminium oxide.
Exam tipClassify an oxide from the element: a metal oxide is basic (or amphoteric for zinc, aluminium and lead) and a non-metal oxide is acidic (or neutral for water, carbon monoxide and nitrogen monoxide).
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