Chemistry Required Practicals

Making up a standard solution

1. Weigh out an accurate mass of a solid in a clean, dry beaker

2. Add enough deionised water to dissolve the solid, stirring with a glass rod 

3. Transfer the solution with rinsing to a 250cm3 volumetric flask using a funnel

4. Rinse the beaker and glass rod with deionised water and add to volumetric flask

5. Make up to the mark by adding deionised water until the bottom of the meniscus is on mark

6. Stopper the flash and invert to mix thoroughly 

(A larger mass of solid dissolved gives a smaller weighing error, so it will be a more accurate solution.)

Uncertainty in mass measurements

percentage error=  (no of measurements taken x uncertainty) / (quantity measured) x100

Apparatus uncertainties:

• Balance: +/- 0.001 g (if 3 d.p balance)
• Volumetric flask: +/- 0.1 cm3
• 25cm3 pipette: +/- 0.1 cm3
• Burette: +/- 0.15 cm3

Reducing uncertainties in a titration:

• Replacing measuring cylinders with pipettes or burettes which have lower uncertainty
• Make titre a larger volume- increasing volume and conc of substance in conical flask or decreasing conc of substance in burette

Reducing uncertainties in measuring mass:

• Use a more accurate balance with a greater resolution or a larger mass of solid weighed

Titration 

1. Rinse burette with solution you're going to fill it with. Discard rinsings and fill burette.

2. Rinse a pipette with solution you're going to pipette into conical flask. Using a pipette and pipette filler transfer 25cm3 of this solution into a conical flask.

3. Add 2-3 drops of a suitable indicator.

4. Add the solution from the burette, with constant swirling, until the indicator just changes colour. This is a trial titration.

5.To reduce the effect of random error on titration results, repeat the titration to achieve 2-3 concordant results, adding the solution dropwise near the end point.

6. Calculate the mean titre from the concordant results.

Titration 

• Pipette and burette should be rinsed first to make sure they won't be diluted by any residual water or react with substances left from a previous titration.
• Only deionised water should be used to wash the conical flask, as it doesn't react with the reagents or change the number of moles of acid used.
• Results should be recorded in a table to 2 d.p.
• If 2 or 3 values are within 0.10cm3 and therefore concordant, we can say results are accurate and repeatable and the titration technique is good and consistent.
• Only make an average of the concordant titre results.
• A conical flask is used in preference to a beaker because it is easier to swirl the mixture without spilling the contents.
• Methyl orange: red in acid and yellow in alkali
• Phenolphthalein: colourless in acid and pink in alkali

Determination of enthalpy change of combustion

1. Accurately measure 100 cm3 of water into a colorimeter/beaker.

2. Weigh a spirit burner containing the liquid to be burnt

3. Measure the initial temperature of the water using a thermometer.

4. Use the spirit burner to heat the water.

5. Stop heating when there is a reasonable temperature rise (15℃). Stir and measure final temp.

6. Reweigh the spirit burner.

7. Calculate the temperature change and heat energy change using q=mcΔT

8. Calculate the mass of fuel used in the burner and calculate number of moles of fuel used.

9. Calculate energy change per mole of fuel used.

Determination of enthalpy of combustion

Value determined is frequently less exothermic than the data book value due to:

• Heat loss to the surroundings from the spirit burner, wick and colorimeter 
• Loss of fuel from the wick or burner, by exaporation
• Loss of water by evaporation
• Incomplete combustion of the fuel, leaving soot on bottom of calorimeter
• Heat used to raise the temperature of calorimeter
• The reaction is unlikely to occur under standard conditions

Experiment can be improved by:

• Using a draught shield to reduce heat loss to surroundings
• Using a lid on the calorimeter to reduce heat loss to surroundings
• Minimising the distance between the flame and calorimeter
• Insulating the calorimeter and spirit burner to reduce heat loss
• Using a top on the spirit burner, with wick protruding, to minimise evaporation
• Burning in a supply of pure oxygeb to prevent incomplete combustion

Determination of enthalpy of neutralisation

Enthalpy of neutralisation is the enthalpy change when an acid and alkali react to produce 1 mole of water

1. Place a polystyrene cup in a glass beaker for support

2. Rinse a measuring cylinder with 1.0 mol dm-3 HCl and measure 25 cm3 of this acid and transfer into the polystyrene cup.

3. Stir the acid with a thermometer and record the temperature

4. Rinse a second measuring cylinder with 1.0 mol dm-3 NaOH and then measure out 25 cm3 of NaOH

5. Add the NaOH to the acid, stir and record the highest temperature reached

6. Calculate temperature change and heat energy change using q=mc T

7. Calculate the moles of acid and water used and the enthalpy of neutralisation

Determination of enthalpy of neutralisation

• An alternative method is to record the temperature against time, and plot a graph, extrapolating the cooling curve after the reaction takes place in order to calculate the temperature change at the point of mixing reactants.
• The extrapolation method is more important when the reaction is highly exothermic as more heat energy is lost at the point of reaction.

Investigation of how the rate of reaction changes with temperature

1. Measure 10 cm3 of 0.2M HCl and 10 cm3 of Na2S2O3 (sodium thiosulfate) in separate measuring cylinders. Put the solutions in separate boiling tubes.

2. Choose a temperature to investigate. Use water baths to get the two solutions to that temperature by placing the boiling tubes in the bath.

3. Place a conical flask on the centre of a large cross drawn on paper. First add the Na2S2O3 to the flask then add the HCl and start the stopwatch and swirl to mix the solutions.

4. Stop the clock when the cross disappears and note the time.

5. Repeat the experiment for four more different temperatures.

Na2S2O3 + 2HCl -------> S + SO2 + H2O + 2NaCl

Carry out simple test-tube reactions to identify cations and anions

Ammonium (NH4+)

• Warm with sodium hydroxide solution in a test tube and test the gas produced with moist red litmus paper
• Gas changes moist red limtus paper blue- gas is alkaline (ammonia), white fumes

Magnesium (Mg2+)

• Add a few drops of sodium hydroxide solution and then excess sodium hydroxide solution
• White precipitate

Calcium (Ca2+)

• Dip a nichrome wire in concentrated hydrochloric acid and then into sample. Place in a blue Bunsen flame and record colour of flame.
• Brick-red precipitate

Carry out simple test-tube reactions to identify cations and anions

Barium (Ba2+)

• Dip a nichrome wire in concentrated hydrochloric acid and then into the sample. Place in a blue Bunsen flame and record the colour of the flame.
• Green flame

Strontium (Sr2+)

• Dip a nichrome wire in concentrated hydrochloric acid and then into the sample. Place in a blue Bunsen flame and record the colour of the flame.
• Red flame 

Chloride (Cl-)

• Add some acidified silver nitrate solution follwed by aqueous ammonia
• White precipitate, which dissolves in dilute ammonia to give a colourless solution

Carry out simple test-tube reactions to identify cations and anions

Bromide (Br-)

• Add acidified silver nitrate solution followed by aqueous ammonia
• Cream precipitate, insoluble in dilute ammonia but soluble in conc ammonia to give a colourless solution

Iodide (I-)

• Add acidified silver nitrate solution followed by ammonia solution
• Yellow precipitate, insoluble in dilute and conc ammonia 

Sulfate (SO42-)

• Add some acidified barium chloride solution
• White precipitate

Carry out simple test-tube reactions to identify cations and anions

Hydroxide (OH-)

• Warm with solid ammonium salt and test the gas produced with red litmus paper 
• Ammonia released which changes red litmus to blue 

Carbonate (CO32-)

• Add some dilute nitric acid and test the gas produced with limewater
• Effervescence, and the gas changes colourless limewater milky

Preparation of an organic liquid

Synthesis of an organic liquid:

1. Preparation- reactants are often heated under reflux

2. Separation of the impure product- liquids usually separated by distillation

3. Purification of product- carried out by solvent extractionto remove impurities

4. Dry- by swirling with a drying agent

5. Final distillation- identity and purity check by working out boiling point

Preparation of an organic liquid 

1. Preparation

• When preparing an organic liquid the reactions are often slow as the organic reactants contain strong covalent bonds. Therefore it is often necessary to heat the reactants under reflux for some time.
• The reactants are often added slowly to the reaction flask, with cooling. This is because the reaction is often exothermic and adding reactants slowly with cooling dissipates the heat, preventing the temp increasing and avoiding dangerous splashing by side-products.
• The condenser prevents vapour escaping and so the reactants can be boiled for a long period without any loss of vapour.
• Anti-bumping granules prevent explosive boiling by providing a large surface area for bubbles to form, ensuring that only small bubbles form.
• Most organic compounds are flammable and so often a flameless method is used to heat them. Instead of a bunsen burner a water bath, sand bath or electric heater can be used.

Preparation of an organic liquid

2. Separation of the crude product

• In general, distillation is used to separate an organic product from its reacting mixture.
• After reflux, allow the apparatus to cool and rearrange for distillation.
• Before carrying out reflux or distillation it is important to make sure that the joint in your apparatus are secure, so that volatile chemicals do not escape during the reaction.
• Anti-bumping granules must also be used for smooth boiling in distillation, because if bumping occurs the liquid can splash over into the condenser, causing an impure product, or can blow the distillation apparatus apart.
• The distillate is the crude product, and is collected over a boiling point range.
• The water goes in the bottom of the condenser to go against gravity. This allows more effectient cooling and prevents back flow of water.

Reflux diagram:

Distillation diagram:

Preparation of an organic liquid

3. Purification of the product

• The crude liquid is often contaminated by products or unreacted reactants. Purification of the liquid can be carried out using solvent extraction in a separating funnel:

1. Place the organic liquid in a separating funnel and add a portion of aqueous solution (most often sodium hydrogen carbonate solution to remove acidic impurities).

2. Stopper and shake, releasing the pressure (often due to carbon dioxide in an acid and carbonate reaction) by inverting and opening the tap.

3. Allow the separating funnel to stand until the layers settle and separate.

4. Remove stopper and open tap to run off both layers in separate beakers.

5. Discard the aqueous layer.

6. Place the organic layer back into separating funnel and repeat process.

Preparation of an organic liquid

4. Drying 

1. Add a spatula of a drying agent, for example anhydrous calcium chloride or anhydrous magnesium sulfate, to the organic liquid in a conical flask.

2. Swirl the mixture.

3. Add more of the drying agent until the liquid changes from cloudy to clear.

4. Filter or decant off the liquid into a clean, dry flask.

Decant means to carefully pour a liquid from one container to another, in order to leave any solid in the bottom of the original container. 

Preparation of an organic liquid

5. Final distillation

• The product can be redistilled. The boiling point measured during the distillation indicates the identity and purity of the liquid purity. If the boiling point is sharp then the product is pure, if it covers a range then impurities may be present. 
• If the boiling points of the reactants and the products are very similar, often fractional distillation can be used instead of simple distillations.

Why percentage yield less than 100%:

• Side reactions occur, so by-products may be produced instead of expected products.
• The reagents may be impure.
• The reaction is incomplete.
• Some product is lost in the purification steps.
• Some product is lost in distillation

Tests for alcohol, aldehyde, alkene and carboxylic acid

Alkene

• Shake with bromine water
• Orange solution changes to colourless solution

Primary/secondary alcohol

• Warm with acidified potassium dichromate 
• Orange solution changes to green solution

Tertiary alcohol

• Warm with acidified potassium dichromate
• Solution remains colourless

Aldehyde

• Warm with Fehling's solution
• Red precipitate

Tests for alcohol, aldehyde, alkene and carboxylic acid

Ketone

• Warm with Fehling's solution
• Solution remains blue

Chloroalkane

• Warm with silver nitrate solution in ethanol
• White precipitate

Bromoalkane

• Warm with silver nitrate solution in ethanol
• Cream precipitate

Iodoalkane

• Warm with silver nitrate solution in ethanol
• Yellow precipitate

Tests for alcohol, aldehyde, alkene and carboxylic acid

Carboxylic acid 

• Add sodium carbonate 
• Effervescence, the gas produced changes colourless limewater cloudy 
• Add magnesium
• Effervescence, a pop is heard when a lighted splint is applied to the gas produced

Ester

• Warm with ethanol and a few drops of conc sulfuric acid
• Sweet smell

Measuring the rate of a reaction by a continuous monitoring method

A gaseous product ---> monitored by measuring volume of gas produced or by loss in mass of the reaction system

A coloured reactant or product --->monitored using calorimetry 

A titratable reactant or product ---> monitored by sampling, quenching and titrating

A directly measurable reactant or product ---> ie. H+ ions or OH- ions, by measuring pH using a pH meter

Measuring the rate of a reaction by a continuous monitoring method

Measuring gas volume

For gases that are not very soluble in water, such as oxygen and hydrogen, the gas may be collected under water in an inverted measuring cylinder or burette.

To start the reaction magnesium is dropped into the flask and the stopper quickly replaced. There are some errors in this method:

• Some gas may escape due to the time lag between adding the magnesium and replacing the bung. To reduce this error, the magnesium could be suspended by a string above the acid and the stopper loosened just enough to release the thread, droping the magnesium into the acid. Alternatively, the magnesium could be placed in a small tube in the conical flask, acid added, bung replaced and then the flask swirled to mix reactants.
• When the stopper is replaced, the volume of the bung displaces the same volume of air into the measuring cylinder, increasing the volume. 

Measuring the rate of a reaction by an intial rate method

Clock reactions

• The time taken to reach a specific point in the reaction soon after the reaction has started can be recorded and the reaction repeated with different concentrations to determine how the time taken to reach this point changes.
• A clock reaction measures the time from the start of the reaction until there is a visual change, such as : appearance of a precipitate, disapearance of a solid or change in colour.

Hydrogen peroxide and iodide ions:

• Hydrogen peroxide and iodide ions are reacted together in the presence of a starch indicator. At first the thiosulfate ions react with any iodine as soon as it is formed, turning it back to iodide ions, so there is no colour change.
• When all the thiosulfate ions have been used up, free iodine is produced and this gives a deep blue-black colour with starch.
• The reaction time for the initial part of the reaction to take place is measured and 1/t calculated as a measure of the intital rate of the reaction. Reaction is repeated with different initial concentrations of the reactants.

Measuring the EMF of an electrochemical cell

• In an electrochemical cell the two half-reactions occur in separate half-cells. The electrons flow from one cell to the other through a wire connecting the electrodes.
• The potential difference between the two half-cells is maximum when no current is flowing  and is called the cell potential. The cell potential can be measured using a high-resistance voltmeter.
• As very little current is drawn by the voltmeter, each electrode is effectively in equilibrium, and the measured cell potential will be close to the standard cell potential.
• The electric circuit is completed by a salt bridge conncenting the two solutions. It allows the ions to flow while preventing the solutions from mixing.
• Often a salt bridge is a piece of filter paper soaked in potassium nitrate solution. Potassium nitrate is used because all potassium salts and nitrate salts are soluble, so the potassium nitrate does not react to produce precipitates with any of the ions in the half-cells.

Investigate how pH changes when a weak acid reacts with a strong base and when a strong acid reacts with a weak base 

• The pH of solutions can be measured with a pH meter, pH probe, data logger or narrow-range pH paper.
• When using a pH meter or probe, it is necessary to calibrate it before use, so that accurate pH values can be obtained. After storage a pH meter does not give accurate readings because the glass electrode in the pH meter does not give a reproducible EMF over longer periods of time.
• Calibration should be performed with a least two standard buffer solutions that span the range of pH vales to be measured. The pH probe should be washed with deionised water before and after being placed in each buffer solution.
• Periodic measurement of the pH of the reaction solution allows plotting of a graph of pH against volume of solution added. This is called a titration curve.

Preparation of a pure organic solid (and test of its purity)

1. Preparation- react suitable quantities of the reactants to produce the product

2. Separation os the crude product- the solid is separated from the reaction mixture by suction filtration.

3. Purification of the product- removes impurities and is carried out by recrystallisation

4. Drying out the product- this is carried out by sucking air over the solid in the Buchner flask, and drying in a low temperature oven.

5. Check the purity- by carrying out a melting point determination

Preparation of a pure organic solid (and test of its purity)

Preparation:

• To prepare an organic solid solutions of the reactants are often added together at room temperature and the product precipitates out. 
• Alternatively, the reactants are refluxed together, the anti-bumping granules decanted off and the solid forms on cooling and crystallising .

Separation of the crude product:

• Suction filtration/filtration under reduced pressure is used to separate the product. It is faster than normal filtration and the solid is left quite dry.
• To set up the apparatus for suction filtration, place a circle of filter paper into a Buchner funnel and place in a stopper in a Buchner flask.
• Connect the Buchner flask to a suction pump and pour the mixture into the funnel. The suction draws the liquid through into the Buchner flask and leaves the crude product in the filter paper.

Preparation of a pure organic solid (and test of its purity)

Purification of the product (recrystallisation) and drying:

1. Dissolve the impure crystals in the minimum volume of hot solvent

2. Filter the hot solution by gravity filtration, using a hot funnel and fluted filter paper, to remove any insoluble impurities.

3. Allow the solution to cool to room temperature and crystallise. Sometimes scratching the sude of the flask with a glass rod or adding a small seed crystal can aid crystal formation.

4. Filter off the crystals using suction filtration.

5. Wash by pouring over some ice-cold solvent, which removes any aqueous impurities.

6. Dry by sucking air over the crystals in the Buchner flask and then in a low-temperature oven. Alternative methods of drying include placing in a desiccator with a drying agent.

Preparation of a pure organic solid (and test of its purity)

Recrystallisation method reasons:

1. Minimum volume of hot solvent- ensures hot solution is saturated so that crystals form on cooling and ensures as much solute is obtained as possible.

2. Hot filter- removes any insoluble impurities and heat prevents precipitation of the solid

3. Cool solution- reduces solubility of crystals so impurities remain in solution. 

4. Suction filtration- the water pump connencted to the Buchner flask reduces the pressure and speeds up the filtration.

5. Wash crystals with ice-cold solvent- removes soluble impurities and cold solvent prevents the crystals from dissolving.

Preparation of a pure organic solid (and test of its purity)

Checking the purity:

1. Place some of the solid in a melting point tube sealed at one end

2. Place in melting point apparatus and heat slowly

3. Record the temperature at which the solid starts to melt and the temperature at which it finishes melting.

4. Repeat and average the temperature.

5. Compare the melting point with known values in a data book

* The greater the range the more impurities are present

* If the melting point is lower than the data book value it contains impurities such as water

Preparation of a pure organic solid (and test of its purity)

Percentage yield less than 100%:

• Crystals lost when filtering or washing
• Some product stays in solution after recrystallisation 
• Some product is lost in transferring between vessels- sometimes rinsing is useful to minimise this loss
• Other side reactions may occur

* If the crystals are not dried properly the mass will be greater than expected which can lead to a percentage yield > 100%

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

Cu 2+

• Adding NaOH until in excess: Blue ppt, insoluble in excess reagent 
• Adding NH3 until in excess: Blue ppt that dissolves in excess to form a dark blue solution
• Adding Na2CO3: Green ppt of copper carbonate 

Fe 2+

• Adding NaOH until in excess: Green ppt, insoluble in excess reagent
• Adding NH3 until in excess: Green ppt, insoluble in excess reagent
• Adding Na2CO3: Green ppt

Fe 3+

• Adding NaOH until in excess: Brown ppt, insoluble in excess reagent
• Adding NH3 until in excess: Brown ppt, insoluble in excess reagent
• Adding Na2CO3: Brown ppt, bubbles of gas

Separation of species by thin-layer chromatography

In thin-layer chromatography, the stationary phase is a thin layer of either aluminium oxide, solicon oxide or silica gel, which is supported on a glass plate. The mobile phase is a solvent.

1. Draw a pencil line 1cm from the bottom of the plate and place two pencil crosses on the line

2. Place a drop of purified solid on a watch glass and dissolve in a few drops of solvent (eg. ethanol). Use a capillary tube to place a spot of solvent on a pencil cross. Allow the spot to dry and repeat 3-4 times, producing a concentrated spot.

3. Place solvent in a beaker to a depth below 1cm and place the TLC plate in the beaker and cover with a lid 

5. Allow the solvent to run up the plate and remove from beaker when it has almost reached the top. Mark the solvent line with a pencil.

6. Place the plate in a fume cupboard until all the solvent has evaporated. Circle the spot using a pencil and calculate the Rf value for each substance visible on the plate. 

Separation of a species by thin-layer chromatography

If the spots are colourless then there are different ways to view them:

• Place the plate under a UV lamp and mark the locations of the substances using a pencil
• In the fume cupboard, place the plate in a beaker containing iodine crystals and cover with a watch glass. The iodine is a locating agent which causes the spots to become brown.
• Spray with ninhydrin developing agent in a fume cupboard.

Practical reasons:

• Wear plastic gloves when holding the TLC plate to prevent contamination of the plate by amino acids on the skin, which would interfere with the results.
• Pencil should be used as it is insoluble and so will not move with the solvent.
• The solvent depth should be below 1cm as if it is greater than this then the mixture will dissolve in it. 
• A lid should be used to stop the solvent evaporating and to allow the solvent to saturate the atmosphere inside the beaker.
• A fume cupboard is used as the solvent can be toxic or flammable.