Edexcel International A Level Chemistry: Practical Skills in Chemistry I (YCH11) – Revision Notes

Full explanations and longer worked examples are in the Unit 3 study guide; start there if a point below is new to you.

These notes condense Unit 3: Practical Skills in Chemistry I in the YCH11 specification (Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry, Issue 1, September 2017). It is an International AS (IAS) unit. The specification describes a written examination of 1 hour and 20 minutes, out of 50 marks, on the practical skills and techniques from Units 1 and 2, including Core Practicals 1 to 8, in familiar and unfamiliar situations. When you have revised, try the Unit 3 practice questions, tick off each practical on the printable checklist, browse other units via the course hub, and run a free diagnostic. The A2 practical unit follows in Practical Skills in Chemistry II.

What the paper asks of you

• Solve problems and apply chemistry in practical contexts.
• Control variables for validity; present, process and evaluate data; comment on methods.
• Plot and read graphs; use significant figures that suit the data; judge accuracy and precision.
• Recognise, choose and use apparatus; consider range and resolution; deal with hazards.
• Recall or interpret tests for ions, gases and organic functional groups.

Key terms

Word	What it means
Accurate	Close to the true value
Precise	Repeat results close to each other
Random error	Unpredictable scatter; reduced by repeating and averaging
Systematic error	Same shift in every reading; not removed by repeats
Valid	Measures what it claims to, with other variables controlled
Resolution	Smallest change the instrument can show
Concordant	Titres within 0.10 cm³ of each other (the usual working rule)

Uncertainty in steps

1. Find the uncertainty of one reading (given in the question, or half the smallest division as a common convention).
2. Double it if the value comes from two readings: titre, mass by difference, temperature change.
3. Percentage uncertainty = uncertainty ÷ value × 100.
4. Add the percentages for the overall result.
5. To improve, enlarge the quantity with the biggest percentage: more solid, a longer titre, a larger temperature change.

Formulas you use

Quantity	Expression
Amount	n = m ÷ M; n = c × V (V in dm³)
Mass concentration	g dm⁻³ = mol dm⁻³ × M
Gas volume	n = V ÷ molar volume; pV = nRT (Pa, m³, K)
Energy transferred	Q = m × c × ΔT (mass of solution in g)
Enthalpy change	ΔH = −Q ÷ n (kJ mol⁻¹; negative when the temperature rises)
Relative rate	rate = 1 ÷ time
Percentage yield	actual ÷ theoretical × 100

Core Practicals in brief

1. Molar volume (1.11). Weighed metal or carbonate plus excess acid; gas into a syringe. Molar volume = volume ÷ moles; a graph of volume against moles gives it as the gradient. Low values: gas lost before the bung goes on, oxide on the metal, CO₂ dissolving if collected over water.

2. Hess’s Law (6.7, 6.8). Two cup reactions with the same final products, e.g. a hydrogencarbonate and a carbonate each with excess acid, combined to give a decomposition. Plot temperature against time, extrapolate the line after the peak (or trough) to the moment of mixing. Assume solution = water for mass and heat capacity, no heat absorbed by the cup, complete reaction.

3. Concentration of hydrochloric acid (8.20, 8.21). Titrate with a standard alkali. Methyl orange goes from red (acid) to yellow (alkali), with orange marking the end point; phenolphthalein is colourless with acid and pink with alkali.

4. Standard solution (8.22, 8.23). Weigh solid acid by difference and dissolve it in a beaker. Wash every trace into a volumetric flask, fill until the meniscus meets the line at eye level; stopper and invert. Use it to standardise sodium hydroxide.

5. Hydrolysis rates (10.10, 10.11). Halogenoalkane plus silver nitrate in ethanol in a water bath; time to first precipitate. Iodo faster than bromo, faster than chloro (weaker C–X bond). Tertiary faster than secondary, faster than primary. Main error: judging the moment cloudiness first appears; a black cross under each tube makes it consistent.

6. Chlorination of 2-methylpropan-2-ol (10.14). Shake with concentrated HCl in a separating funnel; product is the upper layer; wash with NaHCO₃(aq), venting; wash with water; dry with an anhydrous salt; distil. Measure the product mass for a percentage yield; losses come from transfers, washes and evaporation of the volatile product.

7. Oxidation of propan-1-ol (10.20). Acidified K₂Cr₂O₇: orange to green. Distil as it forms for propanal (ice-cooled receiver); reflux with excess oxidant for propanoic acid. Check purity from the boiling temperature.

8. Unknowns (10.23). Systematic tests for cations, anions, gases and functional groups, below.

Technique checklist (10.19)

• Reflux: vertical condenser, open at the top, anti-bumping granules, water bath or electric heater for flammable liquids.
• Separating funnel: stopper, invert, vent through the tap; let layers settle; drain the lower layer.
• Drying: add anhydrous salt until some moves freely and the liquid is clear; decant or filter.
• Distillation: thermometer bulb at the side arm; water into the condenser at the bottom; collect over a narrow range.
• Boiling temperature: a sharp value near the data value suggests a pure liquid.
• Titration: rinse burette and pipette with their own solutions; fill the jet; white tile; dropwise near the end point.

Test results to know

Species	Test	Positive result
Li⁺, Na⁺, K⁺	Flame test	Red, yellow, lilac
Ca²⁺, Sr²⁺, Ba²⁺	Flame test	Orange-red, red, pale green
CO₃²⁻, HCO₃⁻	Dilute acid	Fizzing; gas turns limewater milky
SO₄²⁻	Acidified BaCl₂	White precipitate
NH₄⁺	NaOH, warm	Gas turns damp red litmus blue
Cl⁻ / Br⁻ / I⁻	HNO₃ then AgNO₃	White / cream / yellow precipitate
AgCl / AgBr / AgI	Ammonia solution	Dissolves in dilute / dissolves in concentrated / does not dissolve
O₂	Glowing splint	Relights
NO₂	Observation	Brown gas, acidic
C=C	Bromine water	Orange to colourless
–OH	PCl₅	Steamy fumes of HCl
Primary or secondary alcohol	Warm acidified K₂Cr₂O₇	Orange to green
Aldehyde	Benedict’s or Fehling’s, warm	Blue to red precipitate
Carboxylic acid	NaHCO₃ or Na₂CO₃	Effervescence

Must-know distinctions

• Reflux vs distillation: reflux keeps volatile material in the flask to finish a reaction; distillation removes a volatile product.
• Accuracy vs precision: tight repeats can all be wrong if a systematic error is present.
• Exothermic vs endothermic in a cup: rise gives negative ΔH; fall gives positive ΔH.
• Methyl orange vs phenolphthalein: both suit strong acid with strong alkali; only methyl orange gives the full end point for a carbonate titrated with acid.
• Mass by difference vs taring: weighing by difference accounts for solid left in the weighing bottle.

Safety pairs

• Concentrated hydrochloric acid: corrosive, irritant fumes. Fume cupboard, gloves.
• Organic liquids and ethanol: flammable. Water bath, no naked flame.
• Potassium dichromate(VI): toxic. Gloves, avoid skin contact.
• Phosphorus(V) chloride: reacts violently with water, gives HCl. Fume cupboard, dry apparatus.

Quick self-test

1. Burette readings are 1.20 cm³ and 24.65 cm³. Give the titre and its percentage uncertainty (±0.05 cm³ per reading).
2. Titres are 22.10, 22.35 and 22.20 cm³. Give the mean titre.
3. 25.0 g of solution warms by 12.4 °C. Calculate the energy transferred.
4. A precipitate appears after 64 s. Calculate the relative rate.
5. Calculate the molar volume of a gas at 25 °C and 100 kPa.
6. ΔT is 6.4 °C, from two readings each ±0.2 °C. Give the percentage uncertainty.
7. Theoretical yield 4.20 g; actual 3.15 g. Give the percentage yield.
8. Which layer does 2-chloro-2-methylpropane form when washed with water?
9. Why is propanal collected in an ice-cooled receiver?
10. Name a solid that colours a flame lilac and, once dissolved, precipitates white with acidified barium chloride.
11. Why is the line extrapolated in a cooling correction?
12. Which C–X bond breaks fastest in Core Practical 5, and why?

Answers

1. 23.45 cm³; 0.10 ÷ 23.45 × 100 = 0.43%.
2. 22.10 and 22.20 are concordant: 22.15 cm³.
3. 25.0 × 4.18 × 12.4 = 1296 J = 1.30 kJ.
4. 1 ÷ 64 = 0.0156 s⁻¹.
5. 8.31 × 298 ÷ 100 000 = 0.0248 m³, so 24.8 dm³ mol⁻¹.
6. 0.4 ÷ 6.4 × 100 = 6.3%.
7. 3.15 ÷ 4.20 × 100 = 75.0%.
8. The upper layer; it is less dense than water.
9. It is volatile, so cooling reduces loss by evaporation.
10. Potassium sulfate.
11. To allow for heat lost while the reaction was still happening.
12. C–I, because it has the lowest bond enthalpy.

Where marks are usually lost

• Including the rough titre in the mean.
• Using one-reading uncertainty for a titre or ΔT.
• Taking ΔT from the highest reading rather than the extrapolated line.
• Forgetting to scale up from a 25.0 cm³ portion to the 250 cm³ flask.
• Quoting a result to more significant figures than the data support.
• Saying “wear goggles” when the hazard is a flammable vapour.
• Drawing a sealed reflux apparatus or a thermometer in the boiling liquid.
• Giving only an end colour for a colour change.
• Mixing up the tests for an aldehyde and a carboxylic acid.

Official syllabus

Source: the YCH11 specification, Issue 1 (September 2017), Pearson Education Limited, for the Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry. Unit 3: Practical Skills in Chemistry I, with Core Practicals 1 to 8 from Units 1 and 2.