A-level chem RQ practicals

Outline a method to create a standard/volumetric solution:

1. Weigh the sample bottle containing the solid on a (2 dp) balance. 

2. Transfer solid to beaker and reweigh sample bottle. 

3. Record the difference in mass. 

4. Add distilled water and stir with a glass rod until all the solid has dissolved.

 5. Transfer to a volumetric flask with washings.

 6. Make up to the 250cm3 mark with distilled water.

 7. Invert flask several time

Describe a method used for a titration:

• Rinse equipment (burette with acid, pipette with alkali, conical flask with distilled water).

• Pipette 25 cm3 of alkali into conical flask. - Make sure bottom of meniscus is on line on neck of pipette

• Fill burette with acid.

• Make sure the jet space in the burette is filled with acid - would lead to larger than expected titre reading

• Record initial burette reading.

• Add a few drops of indicator.

• Use a white tile underneath the flask to help observe the colour change.

• Add acid to alkali whilst swirling the mixture and add acid near endpoint.

• Stop adding acid when relevant colour change is observed.

• Record final burette reading - read bottom of meniscus

• Repeat titration until at least 2 concordant results are

  obtained (two readings within 0.1 of each other).

Why should the burette be rinsed out with substance that will be put in it?

• acid or alkali added may be diluted by residual water in the burette/may react with substances left from a previous titration

• This would lead to the concentration of the substance being lowered and a larger titre being delivered.

Why is a volumetric pipette more accurate than a measuring cylinder?

• it has a smaller uncertainty

Why only use small drops of indicator?

indicators tend to be weak acids - so adding too much will affect the result of the titration

How do you reduce percentage uncertainty in a titration?

• replacing any measuring cylinders with pipettes and burettes which have a lower apparatus uncertainty.

• increase the vol and conc. of the substance in the conical flask

• decrease the concentration of the sol in the burette so a larger volume will be needed.-

How do you reduce uncertainties in measuring mass?

• Using a more accurate balance or a larger mass will reduce the uncertainty in weighing a solid

• Weighing sample before and after addition and then calculating difference will ensure a more accurate measurement of the mass added

Why may an experimental value for centrally change be different to theoretical value?

• heat loss to surroundings

• Incomplete combustion

• Non-standard conditions

• Evaporation of alcohol

How do you prevent heat loss to surroundings?

• insulate beaker by placing it in a polystyrene cup

How can the accuracy of this experiment be improved?

• read thermometer at eye level to avoid parallax errors

• Stir the solution so the temp is evenly distributed

• Use a digital thermometer for faster and more accurate readings

• Use greater concentrations and masses

Measuring the Enthalpy Change for a Reaction Experimentally

General method

• washes the equipment (cup and pipettes etc) with the solutions to be used

• dry the cup after washing

• put polystyrene cup in a beaker for insulation and support

• Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup

• clamp thermometer into place making sure the thermometer bulb is immersed in solution

• measure the initial temperatures of the solution or both solutions if 2 are used. Do this every minute for 2-3 minutes

• At minute 3 transfer second reagent to cup. If a solid reagent is used then add the solution to the cup first and then add the solid weighed out on a balance.

• If using a solid reagent then use ‘before and after’ weighing method

• stirs mixture (ensures that all of the solution is at the same temperature)

• Record temperature every minute after addition for several minutes

How can the ROR be measured?

Why does ROR increase with changing temperature?

How do you investigate ROR?

What are the issues with this method?

How can you analyse the data?

What are the control variables?

How do you keep the temp constant?

How do you test for group 2 metal cations?

What are the results from this reaction?

How would you distinguish between calcium bromide and strontium chloride?

What are the results from this reaction?

Overall results for group 3

Test for ammonium ions

• Results: alkaline ammonia gas is released which turns the red litmus paper blue

Test for G7

Result for G7 reaction

Test for OH ions

Test for carbonate ions

Test for sulphate ions

• results in this test are an application of the trend that Group II sulfates become less soluble down the group

• magnesium and calcium salts will not form a sulfate precipitate on addition of sulfate ions due to their high solubility.

• Strontium and barium solutions will form white precipitates with addition of sulfate ions

• Full equation : SrCl2(aq) + Na2SO4 (aq)  2NaCl (aq) + SrSO4 (s) Ionic equation: Sr2+ (aq) + SO42-(aq)  SrSO4 (s).

  Ionic equation: Ba2+ (aq) + SO42-(aq)  BaSO4 (s).

Testing for presence of halide ions with silver nitrate.

• This reaction is used as a test to identify which halide ion is present.

• The test solution is made acidic with nitric acid, and then Silver nitrate solution is added dropwise.

• role of nitric acid is to react with any carbonates present to prevent formation of the precipitate Ag2CO3. This would mask the desired observations

  2HNO3 +Na2CO3 2NaNO3 +H2O+CO2

What is the order of testing ions?

What is distillation?

What equipment is used for distillation?

What is reflux, when is it used? Draw the apparatus

• used when heating organic reaction mixtures for long periods.

• The condenser prevents organic vapours from escaping by condensing them back to liquids.

• The reactant vapours of volatile compound are condensed and returned to the reaction mixture

What is an oxidising agent?

Why are bumping granules used?

How do you isolate a liquid product?

Preparation of cyclohexane by the dehydration & distillation of cyclohexanol

*anti bumping granules not chips

Reflux: Full Oxidation of Primary Alcohols

Reaction: primary alcohol —>carboxylic acid

Reagent: potassium/sodium dichromate(VI) solution and sulphuric acid

Conditions: use an excess of dichromate, and heat under reflux: (distil off product after the reaction has finished)

• Measure 5 cm3 of water into a boiling tube. Add 6 g of sodium dichromate(VI), shake and set aside to dissolve.

• Put about 1.5 cm3 propan-1-ol into a 50 cm3 round bottomed flask and add about 5 cm3 of water and two or three anti-bumping granules.

• Put a condenser on the flask for reflux, as shown in figure below.

• Add 2 cm3 of concentrated sulphuric acid down the condenser in drops from a dropping pipette. While the mixture is still warm, start to add your sodium dichromate(VI) solution down the condenser in drops from a dropping pipette.

• The energy released from the reaction should make the mixture boil. Add the solution a drop at a time so that the mixture continues to boil without any external heating.

• When all the sodium dichromate(VI) solution has been added, use a low Bunsen burner flame to keep the mixture boiling for 10 minutes, not allowing any vapour to escape.

• At the end of that time remove the Bunsen burner and arrange the apparatus for distillation. Gently distil 2-3 cm3 of liquid into a test tube.

How do you test for an alcohol?

How do you distinguish between different alcohol

How do you test for an aldehyde?

Test for an Alkene

Test for a COOH

Test for a halogenalkane

Summary of Identification of Functional Groups by test-tube reactions

Required activity 7 measuring the rate of reaction: by an initial rate method

Initial rate can also be calculated from clock reactions where the time taken to reach a fixed concentration is measured.

RP7 Continuous monitoring method

Analysis & Experimental conditions

Required activity 8. Measuring the EMF of an electrochemical cell

• Clean the zinc and copper foils with emery before use. Degrease the metal using some cotton wool and propanone.

• Place the copper strip into a 100 cm3 beaker with about 50 cm3 of 1 mol dm–3 CuSO4 solution.

• Place the zinc strip into a 100 cm3 beaker with about 50 cm3 of 1 mol dm–3 ZnSO4 solution.

• Use a strip of filter paper soaked in saturated potassium nitrate solution for the salt bridge

• Connect the Cu(s)|Cu2+ (aq) and Zn(s)|Zn2+ (aq) half- cells by connecting the metals using the crocodile clips and leads provided to the voltmeter

Method- If one or both of the half cells do not contain a conducting metal, we must use an inert platinum electrode.

• Set up a copper half cell using a similar arrangement to the previous one. Combine it with a Fe2+/Fe3+ half-cell with a platinum electrode.

• The half cell should have a mixture of acidified 1.0M iron(II) sulphate solution and an equal volume of 0.5M iron(III) sulphate solution as the electrolyte. Use a fresh salt bridge.

• platinum electrode is used because it is unreactive and can conduct electricity

Required activity 9: Titration curves

Constructing a pH curve

• Calibrate meter first by measuring known pH of a buffer solution. This is necessary because pH meters can lose accuracy on storage.

• Transfer 25cm3 of acid to a conical flask with a volumetric pipette

• Measure initial pH of the acid with a pH meter

• Add alkali in small amounts (2cm3) noting the volume added

• Stir mixture to equalise the pH

• Measure and record the pH to 1 d.p.

• Repeat steps 3-5 but when approaching endpoint add in smaller volumes of alkali

• Add until alkali in excess

How does separation remove water soluble impurities from a product?

How do you remove water from a purified product?

Recall a method to produce cyclohexanol from cyclohexene. [RP10 w/ alkenes]

• Pour 20cm3 of cyclohexanol into a pear shaped flask

• Use weighing by difference to record mass of cyclohexanol. 

• Add 8cm^3 of H3PO4

• Add anti-bumping granules then assemble distillation apparatus

• Pour distillate into separating funnel then add saturated NaCl solution

• Allow two layers to separate, then run lower layer into beaker

• Transfer upper layer (with the crude cyclohexene) into conical flask

• Add anhydrous CaCl2 to remove water. 

• Stopper the flask, shake until it becomes clear

• Decant the liquid into sample container

How can volatile liquids be purified by distillation?

How can organic solids be purified by recrystallisation?

Why are melting and boiling points good indicators of purity?

Purifying an organic funnel - separating funnel

How do you measure the melting point?

Recrystallisation method

Carry out simple test-tube reactions to identify transition metal ions in aqueous solution

• Test 1a:

• ● Place 10 drops of solution in a test tube.

  ● Add sodium hydroxide solution, shaking gently, dropwise until in excess. Record any observations.

  ● Do not discard this mixture.

• Test 1b:

  ● Half fill a 250 cm3 beaker with the freshly boiled water.

  ● Allow the four test tubes containing the mixtures from Test 1a to stand in the beaker of hot water for about 10 minutes.

  ● Record any observations.

  Test 2:

  ● Place about 10 drops of sodium carbonate solution in a test tube.

  ● Add about 10 drops of solution and shake the mixture gently. Record any observations.

  Test 3:

  ● Place about 10 drops of solution in a test tube.

  ● Add about 10 drops of silver nitrate solution and shake the mixture gently.

  ● Allow the four test tubes to stand for about 10 minutes. Record any observations.

RQP 11 : Results

Required Practical 12

Separation of species by thin-layer chromatography (TLC)

To analyse medicine samples (e.g. aspirin):

1. Crush an aspirin tablet with pestle and mortar.

2. Transfer to a weighing boat or bottle.

3. Dissolve approx 0.1 g of the powdered tablet in 0.5 cm​ o​ f ethanol.

4. Repeat with other painkiller tablets. (caffeine/anadin tablets should be dissolved in 7.0cm ethanol)

Thin layer chromatography method: