Practical techniques Flashcards
How can you determine the formula of a hydrated salt?
- Weigh empty crucible
- Add the hydrated salt into the weighed crucible. Weigh the crucible containing the hydrated salt.
- Using a pipe-clay triangle, support the crucible containing the hydrated salt on a tripod. Heat the crucible and contents gently for about 1 minute. Then heat it strongly for a further 3 minutes.
- Weigh the crucible.
- Re heat until there are consistent mass readings. You are then left with the anhydrous salt.
How do you identify an unknown group 2 metal?
- Set up a conical flask with a bung in and a gas syringe attached.
- Weigh a sample of the metal and add to the flask.
- Using a measuring cylinder, add 25cm3 1moldm-3 HCl to the flask and quickly replace the bung
- Measure the max volume of gas in the syringe.
How do you prepare standard solutions?
- The solid is first weighed accurately.
- The solid is dissolved in a beaker using less distilled water than will be needed to fill the volumetric flask to the mark.
- This solution is transferred to a volumetric flask. The last traces of the solution are rinsed into the flask with distilled water.
- The flask is carefully filled to the graduation line by adding distilled water a drop at a time until the bottom of the meniscus lines up exactly with the mark.
- Finally the volumetric flask is slowly inverted several times to mix the solution through thoroughly. If this step is missed, the titration results are unlikely to be consistent.
How do you carry out an acid base titration?
- Add a measured volume of one solution to a conical flask using a pipette.
- Add the other solution to a burette and record the initial burette reading to the nearest 0.05cm3
- Add a few drops of indicator to the solution in the conical flask.
- Run the solution in the burette into the solution in the conical flask, swirling the conical flask throughout to mix the 2 solutions. Eventually the indicator changes colour at the end point of the titration.
- Record final burette reading. The volume of solution added from the burette is the titre.
- A trial titration is carried out- rough titration
- The titration is then repeated accurately, adding the solution dropwise as the end point is approached. Further titrations are carried out until 2 accurate titres are concordant- agreeing within 0.1cm3
How do you determine the enthalpy change of combustion?
- Using a measuring cylinder, measure out 150cm3 of water. Pour the water into the beaker. Record the initial temperature of the water to the nearest 0.5
- Add the alcohol to the spirit burner and weigh the burner
- Place the spirit burner under the beaker. Light the burner and burn the alcohol whilst stirring the water with the thermometer.
- After about 3 mins, extinguish the flame and immediately record the max temperature reached by the water.
- Re-weigh the spirit burner containing the alcohol. Assume that the wick hasn’t burnt.
How do you calculate heat loss?
- Pipette 25 cm3 of 1moldm-3 CuSO4 into a polystyrene cup. Weigh out an excess of zinc powder.
- Start a stop clock and take the temperature of the solution every 30s until the temperature stays constant.
- Add the zinc to the solution and stir the mixture. Record the temperature every 30s until the temperature has fallen for several mins.
- Plot graph of temperature against time.
How do you monitor the production of gas using gas collection?
Decomposition of H2O2
1. The hydrogen peroxide is added to the conical flask and the bung is replaced.
2. The initial volume of gas in the measuring cylinder is recorded.
3. Manganese dioxide catalyst is then quickly added to the conical flask and the bung is replaced. A stop clock is stared.
4. The volume of gas produced in the measuring cylinder is recorded at regular intervals until the reaction is complete.
5. The reaction is complete when no more gas is produced.
How do you test the hydrolysis of haloalkanes?
- Set up 3 test tubes;
Tube 1; add 1cm3 of ethanol and 2 drops of 1chlorobutane
Tube 2; add 1cm3 of ethanol and 2 drops of 1bromobutane
Tube 3; add 1cm3 of ethanol and 2 drops of 1idobutane - Stand tubes in water bath at 60c
- Place tube containing 0.1moldm-3 silver nitrate in the waterbath and allow all tubes to reach a constant temp.
- Add 1cm3 of the silver nitrate quickly to each of the test tubes and start a stop clock.
- Observe tubes for 5 mins and record time taken for the precipitate to form.
Results; 1chlorobutane, a white precipitate forms very slowly
1bromobutane, a cream precipitate forms slower than with 1idobutane but faster than 1chlorobutane
1idobutane, a yellow precipitate forms rapidly.
Why is reflux used?
So that the solvents, reactants or products are boiled off.
How do you put together the reflux equipment?
- Flask clamped at neck
- Before fitting the condenser, you need to add the reaction mixture and anti-bumping granules to the flask. These are added to the liquid before the flask is heated so that the contents will boil smoothly. If the granules aren’t used, large bubbles will form at the bottom of the liquid and make the glassware vibrate or jump violently.
- Apply a thin layer of grease to the round-glass joint on the condenser. Place the condenser carefully into the flask and gently rotate the condenser back and forth to provide a good seal and ensure that the apparatus comes apart easily at the end of the experiment.
How do you prepare the equipment for a distillation?
- Flask clamped by it’s neck and the still head is connected to the flask. The still-head adaptor is T shaped and has 2 round glass joints, one to fit the screw-cap adaptor and one to fit the condenser. Grease the joints
- The second clamp is placed around the receiver adaptor at the point at which it is attached to the condenser. The removes the need to clamp the condenser.
- rubber tubing is used to connect the inlet of the condenser to the tap and the outlet to the sink.
- The flask is heated and the mixture in the flask will start to boil. The different liquids in the mixture will have different boiling points. The liquid with the lowest bp is the most volatile and will boil first.
How do you purify an organic liquid?
The organic layer is the aq layer. It is easy to separate the 2 layers although it is important that you know which one is your organic product. Easiest way to tell which one is which is to add some water and the layer that gets bigger is the aq layer.
1. Ensure that the tap of the separating funnel is closed.
2. Pour the mixture of liquids into the seperating funnel and place a stopper in the top of the funnel and invert to mix the contents.
3. Allow the layers to settle
4. Add some water to see which layer increases in volume- this is the aq layer.
5. Place a conical flask under the separating funnel, remove the stopper and open the tap until the whole of the lower layer ahs left the funnel.
6. Place a second conical flask under the separating funnel to collect the other layer.
7. You will have one conical flask containing the organic layer and another containing the aqueous layer. Label them.
In prep using acids, your impure product may contain acid impurities. These can be removed by adding aq sodium carbonate and shaking the mixture in the separating funnel.
Any acid present will react with the sodium carbonate releasing CO2. The tap needs to be slowly opened, holding the stoppered separating funnel upside down, to release any gas pressure that may have built up.
Finally, the aq sodium carbonate layer is remover and the organic layer washed with water before running both layers off into separate flasks.
How do you dry an organic product and what are common inorganic drying salts?
CaCl2- drying hydrocarbons
CaSO4 - General drying
MgSO4 - General drying
1. Add the organic liquid to a conical flask
2. Using a spatula, add some of the drying agent to the liquid and gently swirl the contents to mix together
3. Place stopper on the flask to prevent products from evaporating away. Leave for 10 mind
4. If the solid has all stuck together in a lump, there is still some water present. Add more drying agent until some solid is dispersed in the solution as a fine powder.
5. Decant the liquid from the solid into another flask. If the liquid is dry it should be clear.
How do analyse by colorimetry?
- Prepare the standard solutions of know conc of the coloured chemical e.g. iodine.
- Select a filter with the complementary colour of the chemical. For iodine= green/blue, but colourimeter will usually tell you which setting to use.
- Zero the colourimeter with water.
- Measure the absorbance readings of the standard solutions of iodine.
- Plot a calibration curve of absorbance against iodine concentration. Now have a way of converting an absorbance reading into a conc. of iodine.
- Carry out the reaction. Take absorbance readings of the reaction at measured time intervals.
- Use calibration curve to measure the conc. of iodine at each absorbance reading.
- Plot a second graph of conc. of iodine against time. From the conc-time graph, you can determine the order of reaction with respect to coloured chemical i.e. iodine.
How do you determine Kc from experimental results?
- In a conical flask, mix together reactants and and acid catalyst.
- Add acid catalyst to another conical flask as a control.
- Stopper both flasks and leave for a week to reach equilibrium.
- Carry out a titration on the equilibrium mixture using a standard solution of NaOH.
- Repeat titration with the control to determine the amount of acid catalyst that has been added.
How do you determine the enthalpy change of solution?
- Weigh out a sample of of salt.
- Using a measuring cylinder, pour 25cm3 of distilled water into the plastic cup in the beaker. Measure the temperature of the water, to the nearest 0.5c.
- Quickly tip all of the salt into the water in the plastic cup. Stir the mixture with the thermometer until all of the salt has dissolved and the temperature no longer changes.
- Record the value to the nearest 0.5.
How do you carry out a manganate titration?
- A standard solution of potassium manganate is added to the burette.
- Using a pipette, add a measured volume of the solution being analysed to the conical flask. An excess of dilute sulfuric acid is also added to provide the H+ ions required for the reduction of MnO4- ions. You don’t need to add indicator, as the reaction is self indicating.
- During the titration, the manganate solution reacts and is decolourised as it is being added. The end point of the titration is judged by the first permanent pink colour, indicating when there is excess of MnO4- ions present. In titrations, this end point is one of the easiest to judge.
- Repeat the titrations until you obtain concordant titres.
How do you analyse the percentage purity of an iron(ll) compound?
- Prepare a 250 cm3 solution of impure FeSo4 7H2O in a volumetric flask
- Then using a pipette, measure 25cm3 of this solution into a conical flask. Then add 10cm3 of 1moldm-3 H2SO4
- Using a burette, titrate this solution using a standard 0.02 moldm-3 solution of potassium manganate(VII).
- Finally analyse your results to determine the percentage purity.
How do you analyse oxidising agents?
- Add a standard solution of Na2S2O3 to the burette.
2, Prepare a solution of the oxidising agent to be analysed. Using a pipette, add this solution to a conical flask. Then add an excess of potassium iodide. The oxidising agent reacts with iodide ions to produce iodine, which turns the solution a yellow-brown colour. - Titrate this solution with the Na2S2O3. During the titration, the iodine is reduced back to I- ions and the brown colour fades quite gradually, making it difficult to decide an end point. This problem is solved by using starch indicator. When the end point is being reached, the iodine colour has faded enough to become a pale straw colour.
- A small amount of starch indicator is added. A deep black-blue colour fades. At the end point, all the iodine will have just reacted and the blue-black colour disappears.
How do you analyse household bleach?
- Using a pipette, add 10cm3 of the bleach into a 250cm3 volumetric flask and add water to prepare 250cm3 of solution
- Using a pipette, measure 25cm3 of this solution into a conical flask. Then add 10cm3 of 1moldm-3 potassium iodide followed by sufficient 1moldm-3 HCl to acidify the solution. The HCl provide H+ ions for the reaction.
- Using a burette, titrate this solution using a standard 0.05moldm-3 solution of sodium thiosulfate.
- Repeat the titration to obtain concordant results.
- Finally analyse the results to determine the conc. of chlorate ions in the bleach.
How do you measure standard cell potentials?
- Prepare 2 standard half cells;
- for a metal/metal ion half-cell, the metal ion must have a conc. of 1moldm-3.
- For an ion/ion half cell, both metal ions present in the solution must have the same conc. There must be an inert electrode, usually platinum.
- For a half-cell containing gases, the gas must be at 100kps in contact with a solution, with an ionic conc. of 1moldm-3. There must be an inert electrode e.g. platinum.
- for all half-cells, the temperature must be at 298k - Connect the metal electrodes of the half-cells to a voltmeter using wires.
- Prepare a salt bridge by soaking a strip of filter paper in a saturated aq solution of potassium nitrate.
- Connect the 2 solutions of the half-cells with a salt bridge.
- Record the standard cell potential from the voltmeter.
How do you test for the carbonyl group in aldehydes and ketones?
- Add 5cm depth of a solution of 2,4DNP to a clean test tube. This is in excess.
- Using a dropping pipette, add 3 drops of the unknown compound. Leave to stand.
- If no crystals form, add a few drops of sulfuric acid.
- A yellow/orange precipitate indicates the presence of an aldehyde or a ketone.
How do you test for the aldehyde group?
To make tollens reagent;
- In a clean test tube, add 3cm depth of aq silver nitrate
- add aq sodium hydroxide to the silver nitrate until a brow precipitate of silver oxide is formed.
- Add dilute ammonia solution until the brown precipitate just dissolves to form a colourless solution.
To carry out an aldehyde group test;
- Pour 2cm depth of the unknown solution into a clean test tube.
- Add an equal volume of freshly prepared tollens reagent.
- Leave the test tube to stand in a beaker of warm water at about 50 degrees for 10-15 mins and then observe whether any silver mirror is formed.
How do you identify an aldehyde or ketone by melting point?
- Carry out the 2,4 DNP test
- Filter out the impure yellow/orange solid, to separate it from the solution
- the solid is then recrystalised to produce a pure sample of crystals
- Measure the melting point of the purified 2,4 DNP and record it.
- The melting point is then compared to a database or data table of melting points to identify the original carbonyl compound.
How do you test for the carboxyl group?
Neutralisation reaction of carboxylic acids with carbonates, e.g. sodium carbonate can be used to distinguish carboxylic acids from any other organic compounds.
- Carboxylic acids are the only common organic compounds, sufficiently acidic to react with carbonates. It is useful for distinguishing them from phenols. Phenols aren’t acidic enough to react with carbonates
