7.1.3Additional lessons
Applying Techniques in Chemistry — IB Chemistry HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 160 of 172
Part 1 of 6Quantitative Techniques
These methods turn observations into numbers that can be processed.
Titration
- A solution of known concentration is added from a burette until an indicator shows the end point,
- which allows the concentration of the other solution to be found.
Back titration
- An excess of reagent is added, and the leftover is titrated, which suits insoluble solids or slow reactions.
Gravimetric analysis
- A product is isolated, dried and weighed to constant mass so that its amount can be calculated.
Calorimetry
- The temperature change of a known mass of water or solution is measured to find the energy transferred by a reaction.
Part 2 of 6Separation and Purification
Separation and Purification
Filtration
- An insoluble solid is separated from a liquid by passing the mixture through filter paper in a funnel.
Distillation
- Liquids with different boiling points are separated by heating, condensing the vapour and collecting each fraction in turn.
Recrystallisation
- A solid is dissolved in the minimum amount of hot solvent and crystals form as it cools, leaving impurities in solution.
Chromatography
- Substances move at different rates through a stationary phase, so the mixture separates according to attraction to each phase.
Part 3 of 6Setting Up a Reaction Safely
1
Heating under reflux uses a condenser held vertically so that vapours return to the flask and none of the volatile liquid escapes.
2
Anti-bumping granules are added to the flask so that liquid boils smoothly instead of erupting.
3
A flame is avoided for flammable liquids, and an electric heating mantle or water bath is used instead.
4
For distillation the thermometer bulb sits level with the side arm so that it records the vapour temperature.
5
The apparatus is never sealed at both ends while heating, which prevents a pressure build-up.
Part 4 of 6Choosing a Technique
Match to the aim
- Use titration for concentration, distillation for liquids,
- and filtration for insoluble solids,
- because each technique answers a particular question.
Purity check
- A melting point or boiling point that is sharp and close to the literature value suggests that the sample is pure.
Yield
- Losses on transfer, filtration and drying lower the yield, so the percentage yield rarely reaches 100 %.
Risk
- Assess the hazards of each chemical and technique, and select the control measures such as fume cupboards, eye protection and gloves.
Part 5 of 6Worked Example
Question
- 25.00 cm³ of hydrochloric acid is titrated with 0.100 mol dm⁻³ sodium hydroxide and the mean titre is 22.50 cm³.
- Calculate the concentration of the acid.
- HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l), so the ratio is 1 : 1.
- n(NaOH) = 0.100 × 22.50 ÷ 1000 = 2.250 × 10⁻³ mol.
- n(HCl) = 2.250 × 10⁻³ mol, and concentration = 2.250 × 10⁻³ ÷ 0.02500 = 0.0900 mol dm⁻³.
Answer [HCl] = 0.0900 mol dm⁻³
Part 6 of 6Exam Checklist & Tips
7.1.3 – Key Points for the Exam
- End point The point in a titration at which the indicator changes colour.
- Reflux Heating a mixture with a condenser so that vapour condenses and returns to the flask.
- Distillation Separation of liquids by differences in boiling point.
- Recrystallisation Purification of a solid using its different solubility in hot and cold solvent.
- Titre The volume of solution delivered from the burette to reach the end point.
Common Mistakes
- Placing a stopper in the top of a heated reflux condenser, which creates a dangerous pressure build-up.
- Averaging titres that are not concordant; only titres within about 0.10 cm³ of each other should be averaged.
- Adding too much hot solvent in recrystallisation, which leaves the product dissolved and reduces the yield.
- Placing the thermometer too high in a distillation set-up, which gives a boiling point reading that is too low.
Last updated:
\n