Required Practicals

How do you make a 250 cm³ standard solution of known concentration? (Required Practical 1)

1. Weigh out mass of substance on high precision balance.

2. Transfer to beaker and reweigh weighing boat to determine mass of substance added.

3. Dissolve in deionised water and stir with glass rod.

4. Transfer to volumetric flask.

5. Rinse beaker/rod/funnel and put all washings into flask.

6. Make up to line with deionised water and a dropping pipette up until the meniscus.

7. Stopper the flask and invert it several times to mix.

How do you carry out an acid-base titration? (Required Practical 1)

1. Rinse and fill burette with standard solution.

2. Pipette 25 cm³ unknown into conical flask.

3. Add a few drops of indicator.

4. Record initial burette reading.

5. Slowly add the standard solution from burette into the conical flask swirling constantly.

6. Near the endpoint add the solution dropwise until a permanent colour change occurs.

7. Record final burette reading and calculate final titre volume.

8. Repeat for concordant results (within 0.10 cm³).

Why is a pipette used instead of a measuring cylinder for the unknown solution? (Required Practical 1)

Pipette is more precise (measures fixed volume accurately). Measuring cylinder is less precise for titration.

How do you measure temperature change for anhydrous CuSO₄ dissolving in water?

1. Weigh out a mass anhydrous CuSO₄ using high precision balance.

2. Put 50 cm³ deionised water in polystyrene cup with a lid and thermometer using measuring cylinder.

3. Record the initial temp of the water every minute for 3 minutes.

4. At 4th minute, add CuSO₄ and place the lid

5. Reweigh the weighing boat to establish actual amount of mass added.

6. From 5th to 15th minute, record temp every minute.

7. Plot temp vs time graph.

8. Extrapolate before/after lines to 4th minute to find ΔT.

9. Use the temp change to calculate the enthalpy change of the reaction

Why use a polystyrene cup for enthalpy experiments?

It is a good insulator – reduces heat loss to surroundings.

What is the equation for calculating enthalpy change?

q = m × c × ΔT

Describe the disappearing cross experiment for rate of reaction.

1. Place a set volume (10 cm³) Na₂S₂O₃ sodium thiosulfate in test tube over a cross.

2. Add set volume (1 cm³) HCl, start timer.

3. Record the time until cross disappears.

4. Repeat at different temps keeping the volumes and concentration of the sodium thiosulfate and the hydrochloric acid the same (warm water bath ≤55°C).

5. Post Reaction- Pour the cloudy contents into a sodium carbonate solution to the neutralise the acid and the sulfur dioxide.

Why neutralise the reaction mixture with sodium carbonate?

To neutralise unreacted acid and toxic SO₂ gas.

As temperature increases, what happens to reaction rate and why?

Rate increases – more particles have energy ≥ activation energy, more frequent successful collisions.

What is observed when NaOH is added to Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺?

Mg²⁺: white precipitate; Ca²⁺: white precipitate; Sr²⁺: slight white precipitate; Ba²⁺: no change

What happens when dilute H₂SO₄ is added to Mg²⁺ Ca²⁺ Sr²⁺ and Ba²⁺

Ba²⁺: white precipitate (BaSO₄); Mg²⁺: colourless solution; Ca²⁺ :slight white precipitate; Sr²⁺ : white precipitate

How do you test for ammonium ions (NH₄⁺)?

Add NaOH, warm gently. Test fumes with damp red litmus paper – turns blue. NH₄⁺ + OH^- = NH₃ + H₂O ( the presence of NH₃ will make it go blue)

Why add anhydrous calcium chloride to the distillate?

To remove water (drying agent).

Test for carboxylic acid – reagent and observation?

Add solid Na₂CO₃ – effervescence, CO₂ turns limewater cloudy.

How do you measure rate of reaction using a gas syringe?

1. Add HCl to flask.

2. Attach gas syringe.

3. Add Mg ribbon, start timer.

4. Record H₂ volume every 15 seconds for 2.5 minutes.

5. Repeat with different concentrations.

How do you find initial rate from a volume-time graph?

Draw a tangent at t = 0, calculate its gradient.

Why is initial rate preferred for comparing concentrations?

At t = 0, concentrations are exactly known – no products yet.

How do you set up a Zn/Cu electrochemical cell?

1. Clean the metals that will be used as electrodes with sandpaper to remove oxide layer + propanone to remove grease

2. Add the cleaned metals into a beaker containing their solutions e.g Zn in ZnSO₄, Cu in CuSO₄. Make sure both are 1 mol dm^-3

3. Connect with salt bridge (such as NaCl or potassium nitrate in U-tube) ensure both ends are submerged in the solutions.

4. Connect the half cells using crocodile clips and leads and add in a voltmeter to find the emf of the cell.

Test for carbonate ions?

Add dilute HCl, bubble gas through limewater – turns cloudy.

Test for sulfate ions?

Add dilute HCl + BaCl₂ – white precipitate (BaSO₄).

Test for chloride, bromide, iodide?

Add dilute nitric acid HNO₃ + AgNO₃ silver nitrate. Cl⁻: white ppt (soluble in dilute NH₃). Br⁻: cream ppt (soluble in conc. NH₃). I⁻: yellow ppt (insoluble in NH₃).

How to purify a sample and recrystallise?

1. Dissolve in minimum hot solvent.

2. Filter while hot to remove insoluble impurities.

3. Slowly cool to crystallise (eg an ice bath).

4. Suction filter- filter using a Buchner funnel.

5. Wash with cold solvent and dry.

How do you check purity of a solid?

Measure melting point – pure = sharp, narrow range; impure = lower, broader range.

What happens when NaOH is added to Cu²⁺ solution?

Blue precipitate of Cu(OH)₂.

What happens when NaOH is added to Fe³⁺ solution?

Brown precipitate of Fe(OH)₃.

Why use a pencil for baseline on TLC plate?

Pencil is insoluble in solvent; pen ink would run.

How do you calculate Rf value?

Rf = distance moved by spot / distance moved by solvent front.

Why put a lid on the chromatography tank?

To saturate chamber with solvent vapour – prevents evaporation from plate.

What indicator is used for ethanoic acid – NaOH titration?

Phenolphthalein.

Why keep water bath below 55°C for disappearing cross experiment?

Prevents sodium thiosulfate from decomposing.

Test for water in an organic liquid?

Anhydrous copper(II) sulfate – white to blue; or cobalt chloride paper – blue to pink.

Why add anti-bumping granules before distillation?

Promotes even boiling, prevents violent bumping and prevents the solution jumping into the sidearm condenser if you’re using distillation.