A-Level Chemistry Practicals: Experiments & Safety

• This guide details the required A-level chemistry practicals and provides information on other common A-level experiments.
• You may need to describe experiments or explain the reasons behind individual steps.
• You may also encounter unfamiliar experiments that use the skills and techniques described in the following experiments.

Safety and Hazards

• Irritants, like dilute acids and alkalis, require safety goggles.
• Corrosive substances, such as stronger acids and alkalis, require safety goggles.
• Flammable substances should be kept away from naked flames.
• Toxic substances necessitate gloves, avoiding skin contact, and thorough hand washing after use.
• Oxidizing substances need to be kept away from flammable or easily oxidized materials.
• Hazardous substances in low concentrations or amounts may not pose the same risks as the pure substance.

Heating in a Crucible

• Used for measuring mass loss in thermal decomposition reactions and mass gain when reacting magnesium in oxygen.
• Water of crystallization removal from calcium sulphate is achieved through heating, producing water vapor.
• The equation is: CaSO₄.xH₂O(s) → CaSO₄(s) + xH₂O(g)
• Method includes weighing an empty, clean, dry crucible with a lid, adding about 2g of hydrated calcium sulphate, and reweighing.
• Heat strongly with a Bunsen burner for a couple of minutes, allow cooling, and weigh again.
• Heating and reweighing should continue until a constant mass is achieved, ensuring the reaction is complete.
• A lid improves accuracy by preventing solid loss but should be loose to allow gas to escape.
• The crucible should be dry, as a wet crucible would render inaccurate results.
• Large amounts of hydrated calcium sulphate (e.g., 50g) should not be used.
• Small amounts of the solid (e.g., 0.100 g) should not be used.
• Example Calculation: A 3.51 g of hydrated zinc sulphate yields 1.97 g of anhydrous zinc sulphate upon heating, calculating X as 7 in ZnSO₄.xH₂O.

Using a Gas Syringe

• Gas syringes measure gas volume in various experiments to calculate moles or follow reaction rates.
• Gas volume is directly related to pressure and temperature, therefore, these must be recorded with the volume.
• Moles of gas are calculated from gas volume, temperature, and pressure using the ideal gas equation PV = nRT.
• Potential errors of gas syringes include gas leaks, syringe sticking, or gas solubility in water.
• When drawing a gas syringe, include measurement markings on the barrel.
• Method for using a gas syringe to calculate the Mr of propanone includes:
  • Extracting 0.20 cm³ of propanone into a hypodermic syringe and then measure the mass of this syringe.
  • Remove a gas syringe from the oven and note starting volume as 5cm3 with hand protection.
  • Inject propanone through the self-seal cap into the barrel.
  • Replace gas syringe into oven.
  • Measure the mass of the empty hypodermic syringe immediately.
  • After a few minutes, record the volume, temperature and pressure.
• Example Calculation: 0.150g of volatile liquid injected into a gas syringe at 70°C, 100kPa, and 80cm³ volume; Mr calculated as 53.4 g mol⁻¹.

Making a Solution

• For making a solution, known mass of solid is weighed in a sample bottle using a 2 dp balance.
• The solid is then transferred to a beaker and the empty sample bottle is reweighed to find mass difference
• Add 100cm³ of distilled water to the beaker to dissolve the solid, using a glass rod to stir.
• Heat gently when necessary.
• The solution is poured into a 250cm³ graduated flask via a funnel.
• Rinse the beaker and funnel, adding washings to the volumetric flask.
• Make up to the mark with distilled water, using a dropping pipette for the last few drops.
• Invert flask several times to ensure uniform solution.
• In an alternative, the known mass of solid in the weighing bottle could be transferred to beaker, washed and washings added to the beaker.
• Filling should ensure the bottom of the meniscus sits on the line on the neck of the flask.
• Gradutated volumetric flasks must not be heated or cooled and one mark on on the flask shows the level to fill to.
• In measuring mass accurately, measure mass on 2 or 3d.p. balance of a weighing bottle, empty mass into reaction flask and subtract the mass of the empty weighing bottle.

Dilutions

• To dilute a solution, pipette 25cm³ of original solution into a 250cm³ volumetric flask.
• Make up to the mark with distilled water, using a dropping pipette for the last few drops.
• Invert flask several times to ensure uniform solution.
• A volumetric pipette is more accurate than a measuring cylinder due to smaller uncertainty
• Use a teat pipette to make up to mark in volumetric flask.

Titrations: General Method

• Rinse equipment with appropriate solutions to remove potential contaminants (burette with acid, pipette with alkali, conical flask with distilled water).
• Pipette 25 cm³ of alkali into the conical flask.
• Ensure the correct amount is added when using a bulb pipette.
• Add acid from the burette.
• Ensure the burette jet space is filled with acid before starting.
• Add a few drops of indicator to the flask to discern the endpoint.
  • Phenolphthalein works best with NaOH, and goes from pink to colorless (alkali to acid).
  • Methyl orange works best with HCl, and foes from yellow to red (alkali to acid).
• Use a white tile underneath the flask to sharpen the color change.
• Add acid to alkali, swirling the mixture, and add gradually until the endpoint is made.
• Note burette readings before and after acid addition.
• Repeat titration until at least two readings match within 0.1 cm³.
• A titration determines the concentration of one substance by reacting it with another of known concentration.
• Titrations can be done with neutralization or redox reactions.
• Titrate solution A with solution B means that A is in the conical flask, B is in the burette.
• A conical flask is favored over a beaker because it is easier to swirl the mixture without spilling.

Detailed Method of Titration

• Rinse the pipette with the intended substance (often alkali).
• Pipette 25 cm³ of solution A into the conical flask, ensuring the bottom of the meniscus aligns with the mark.
• Touch the pipette's surface to ensure the correct amount.
• The burette should also be rinsed out with the substance that will be put in it.
• Do not leave the funnel in the burette because small drops of liquid may fall during the titration leading to a false burette reading (would give a lower titre volume).
• The meniscus should be read on the burette's bottom.
• The reading is recorded to 2dp ending in 0.00 or 0.05.
• Add a few drops of indicator to the sample, taking note of color change at the endpoint.

Adding Indicator

• Add a few drops of indicator. Indicators are generally weak acids so only add a few drops of them as if too much is added they will affect the titration result
• Phenolphthalein is used with strong alkalis, turning from pink (alkali) to colourless (acid).
• Methyl orange is used for strong acids, turning from yellow (alkali) to red (acid); the endpoint is orange.
• Use a white tile underneath the flask to more easily observe the colour change.
• Add solution from the burette while swirling the conical flask, and add drop by drop at the endpoint.
• Distilled water washes the sides of the flask, ensuring all acid reacts with the alkali, without affecting the reading.
• It should be noted that only distilled water is used to remove the solution.

Recording Results

• Burette readings before and after solution addition.
• Repeat titration until concordant results, two readings within 0.1.
• Results clearly recorded in a table.
• Include initial and final readings.
• Record titre volumes to 2dp (0.05 cm³).
• Only concordant average titre results.
• Average titre = (24.50+ 24.40)/2 =24.45

Safety Precautions

• Acids and alkalis are corrosive (low concentrations are irritants).
• Eye protection and gloves.
• Spillage: immediately wash affected parts after spillage.
• Unknown substance: potentially toxic, wear gloves.

Testing Batches

• Necessary to titrate multiple samples in quality control.

Common Titration Equations

• Example equations that can be used during the titration experiement, CH3CO2H + NaOH → CH3CO2Na+ + H₂O, etc
• Always consider the mixtures other substances with accid base properties and titrate with confidence.

Manganate Redox Titrations

• Common titration between Fe²⁺ with MnO₄⁻ (purple).
• Self-indicating due to color change.
  • MnO₄⁻(aq) 8H⁺(aq) 5Fe²⁺(aq) → Mn²⁺(aq) 4H₂O(l) 5Fe³⁺(aq)
• Because purple the purple colour of manganate can make it difficult to see the bottom of meniscus in the burette.

Manganate Titration Procedure

• Weigh ferrous sulphate tablets.
• Grind tablets with 1M sulphuric acid using a pestle and mortar.
• Transfer to a 100cm³ volumetric flask with 1M sulphuric acid.
• Add 1M sulphuric acid to make up to 100cm³.
• Stopper and mix the flask to make sure that all the contents are thoroughly mixed
• Titrate 10.0 cm³ portions with 0.0050 M potassium manganate (VII).
• The end-point is detected by a first permanent purple colour.
• Only use dilute sulphuric acid due to the need of supplying of the 8H+ ions, preventing inaccurate titration readings.

Titration Calculation Example

• Using a known concentration of potassium manganate (VII) solution, calculate the percentage of iron by mass in the nail.

Useful Manganate Titrations

• With hydrogen peroxide and ethanediote

EDTA Titrations

• The formation of stable EDTA complex with metal ions is a quantitative titration.
• [Cu(H₂O)₆]²⁺ + EDTA⁴⁻ ⇌ [Cu(EDTA)]²⁻ + 6H₂O
• Always the same ratio with any metal cation.
• Example, a copper(II) polluted river is titrated with EDTA⁴. Calculate the concentration in the water.

Uncertainty: Readings vs Measurements

• Readings: Values from one judgment (e.g., using equipment).
• Measurements: Values from two judgments (e.g., using a burette).

Apparatus Uncertainties

• Each apparatus has a sensitivity uncertainty:
  • Balance: ±0.001 g (3 d.p.).
  • Volumetric flask: ±0.1 cm³.
  • 25 cm³ pipette: ±0.1 cm³.
  • Burette: ±0.15 cm³ (start, end, and endpoint readings).

Calculating Percentage Error

• % uncertainty = (uncertainty / Measurement) * 100
• Add individual uncertainties.

Reducing Uncertainties

• Increase measurement size or use higher resolution apparatus.
• Use pipettes/burettes over graduated cylinders for lower error.

Calculating Percentage Difference

• % = (actual value - calculated value) / actual value * 100
• Discrepancy due to other errors if % difference is larger than % uncertainty, and vice versa.

Measuring Enthalpy Change Using Calorimetry

• Enthalpies of combustion can be calculated by using calorimetry.
• Measured in a water calorimtery are mass of spirit burner, temperature change and volume of water
• Errors include, energy losses, incomplete combustion, heat capacity not included and H₂O is gas instead of liquid.
• Example to calculate, for the combustion propan-1-ol to heat of 150g water with 20.1 to 4.5° calculate enthalpy change as -1470 kJ mol-1

Enthalpy Change of Reaction Calculation

• Enthalpy change, ΔH, can be calculated from experimental data.
• First using q = m x c, x ΔT calculate energy change for quantities used
• Then the moles of the reactant must be extracted.
• Finally divide/add a unit signed with ±
• It must also be noted that all the solutions have density of water and the heat capacity of 4.18
• Example a 25cm³ of 0.20M CuSO4 reacting with excess 0.01mol of zinc will have the enthalpy change of -146 kJ mol-1

Hess's Law

• Hess's law can be used to measure an enthalpy change for a reaction.

Solubility Rules

• All nitrates and Sodium, potassium and ammonium salts are soluble
• Most chlorides, bromides, iodides and sulfates are also soluble
• insoluble ions include, Silver, lead chlorides, bromides iodides, Carbonates Strontium and lead

Making Insolubles

• Insoluble salts made by mixing solutions
• A percipate is a solid.

Filtration

• to remove insoluble salts the salt would be removed by filtration using, use Buchner funnel or Filter paper.
• for for both AQA tests filter paper should be dram.

Acidity and Basicity

• lonic equations only show the ions.
• spectator ions are removed leaving, an Acidity or Bacisity test.

Partial Oxidation of Primary Alcohols

• Reagant used in the reaction is potassium dichromate with sulphuric acid.
• The reaction requires a the limited amounts of dichromate gently warm distilled, to create primary alcohols
• Saftey precautions must be made when handling materials in the experiment.
• General steps that help achieve accurate results is the distillation apparatus set up.

Full Oxidation of Primary Alcohols

• Reagent used in the reaction is potassium dichromate with sulphuric acid is used as a condition.
• Distill off product after reaction has finished if there is an overflow.
• Important steps include that the anti-bumping granules are being added for distillation and reflux, in order to prevent it from bubbling. Make sure you add water to the outter layer of the condenser to maintain a coolant.

A guide to A-level chemistry practicals, common experiments, safety precautions, and potential hazards. Includes information on handling irritants, corrosive, flammable, toxic, and oxidizing substances. Covers techniques like heating in a crucible.