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2.6Inorganic Chemistry

Acids, Alkalis and Titrations — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 8 parts Lesson 13 of 23 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
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Lesson summary

Acids, Alkalis and Titrations — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Describe the properties of acids and alkalis, use the pH scale and indicators, and carry out and calculate from an acid-alkali titration.An acid is a substance that produces hydrogen ions, H+, when dissolved i…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Neutralisation
  • Concordant results

Part 1 of 6Acids, alkalis and the pH scale

An acid is a substance that produces hydrogen ions, H+, when dissolved in water; an alkali is a soluble base that produces hydroxide ions, OH-.

Acids, alkalis and the pH scale
The pH scale runs from 0 to 14: below 7 is acidic, 7 is neutral and above 7 is alkaline.
A pH of 1 contains ten times more H+ than pH 2.
  • Universal indicator gives a range of colours: red (strong acid), orange/yellow (weak acid), green (neutral), blue/purple (alkali).
IndicatorIn acidIn alkali
LitmusRedBlue
Methyl orangeRedYellow
PhenolphthaleinColourlessPink

Strong acids (hydrochloric, sulfuric, nitric) ionise completely in water; weak acids (ethanoic, citric) only partly ionise. Strength is about ionisation, whereas concentration is about the amount dissolved. Concentration is measured in mol/dm3 or g/dm3.

Part 2 of 6Neutralisation

When an acid reacts with an alkali, H+ and OH- ions combine to form water: H+(aq) + OH-(aq) → H2O(l).

1
This is the ionic equation for all neutralisation reactions.
2
The reaction is exothermic.
3
Example: HCl + NaOH → NaCl + H2O.
4
Uses of neutralisation include treating acidic soil with lime, indigestion tablets and neutralising wasp stings.

Part 3 of 6Titration method

A titration finds the exact volumes of acid and alkali that react together. Procedure:

1
Use a pipette and filler to transfer 25.0 cm3 of the alkali into a conical flask; add a few drops of indicator (methyl orange or phenolphthalein).
2
Fill the burette with acid, clamp it vertically and record the initial reading (read at eye level at the bottom of the meniscus).
3
Add the acid slowly, swirling, until the indicator just changes colour (the end point); record the final reading.
4
The titre is final minus initial volume.
  • Do a rough titration first, then repeat accurately until two or three results are within 0.10 cm3 and calculate the mean.
    • (concordant results)

Rinsing glassware with the solution it will hold avoids dilution; adding drops near the end point improves accuracy. A white tile under the flask helps to see the colour change.

Part 4 of 6Titration calculations

Use moles = concentration (mol/dm3) × volume (dm3), and the balanced equation ratio.

Example25.0 cm3 of NaOH required 20.0 cm3 of 0.100 mol/dm3 HCl.

1
Moles HCl = 0.100 × 0.0200 = 0.00200 mol.
2
The ratio HCl : NaOH is 1 : 1, so moles NaOH = 0.00200 mol.
3
Concentration NaOH = 0.00200 ÷ 0.0250 = 0.0800 mol/dm3.
4
To convert to g/dm3, multiply by Mr (NaOH = 40): 0.0800 × 40 = 3.20 g/dm3.
5
When the ratio is not 1:1, as in H2SO4 + 2NaOH → Na2SO4 + 2H2O, multiply or divide moles accordingly.

Part 5 of 6Worked titration with a sulfuric acid ratio

When the acid is dibasic, use the ratio from the equation.

1
Example: 25.0 cm3 of 0.100 mol/dm3 sodium hydroxide neutralises 12.5 cm3 of sulfuric acid.
2
Moles NaOH = 0.100 × 0.0250 = 0.00250 mol.
3
H2SO4 : NaOH = 1 : 2, so moles acid = 0.00125 mol.
4
Concentration = 0.00125 ÷ 0.0125 = 0.100 mol/dm3.

Method to remember

  • (1) moles of the known solution;
    • (2) apply the ratio;
    • (3) divide by the volume in dm3 of the unknown;
    • (4) convert to g/dm3 with Mr if asked.
  • Sources of error include overshooting the end point, bubbles in the burette tip, and failing to rinse the glassware.

Part 6 of 6Worked examples

  1. 25.0 cm3 of sodium hydroxide is neutralised by 22.5 cm3 of 0.200 mol/dm3 hydrochloric acid. Calculate the concentration of the alkali. [4]4 marks

  2. Why is a rough titration carried out first? [1]1 mark

  3. State the ionic equation for neutralisation. [1]1 mark

  4. A solution of pH 3 is compared with one of pH 5. Compare their hydrogen ion concentrations. [2]2 marks

Key terms

TermMeaning
AcidProduces H+ ions in water
AlkaliSoluble base producing OH− ions
NeutralisationReaction of H+ with OH− to form water
TitreVolume of solution added from the burette
End pointPoint where the indicator changes colour
Concordant resultsTitres agreeing within 0.10 cm3
pHScale measuring acidity from 0 to 14

Avoid these mistakes

1
Confusing strong with concentrated — strength refers to ionisation.
2
Reading the burette from the top of the meniscus.
3
Forgetting to convert cm3 to dm3 (divide by 1000).
4
Using the first (rough) titre in the mean.

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