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Volumetric Analysis and Enthalpy Change
Specification
c) Describe the techniques used in volumetric analysis and enthalpy change measurements
Volumetric analysis
Volumetric analysis (titration) involves the reaction between two solutions. For one solution, both the volume and the concentration are known; for the other, the volume only is known. Apparatus used includes a burette, a pipette and a volumetric flask.
What is a standard solution?
A solution for which concentration is accurately known. The concentration may have been found by a previous titration or by weighing the solute and making a solution of known volume. Such a solution is a primary standard solution.
How is a 250cm3 standard solution prepared?
- Make sure that the balance is clean and dry. Wipe it with a damp cloth.
- Place the weighing bottle on the pan and tare the balance (i.e. re-zero it)
- Take the bottle off the balance and add solid to it. This ensures that no spillages fall on the pan.
- When you have the required amount, write its value down immediately.
- Replace on balance, and if the required amount is added, withdraw the mass.
- Wash out a 250cm3 volumetric flask three times using pure water.
- Transfer the solid to a 250cm3 volumetric flask using a funnel, and wash out the weighing bottle into the flask through the funnel.
- Add about 100cm3 of distilled water to the flask.
- Stir the solution using a glass rod.
- Wash all remaining apparatus including the glass rod, funnel and transfer the rest of this to the flask.
- Make up to 250cm3 with distilled water so that the bottom of the meniscus just touches the 250cm3 mark.
- Stopper the flask.
- Shake the flask vigorously and/or invert the flask 5 or 6 times to dissolve the solid.
- Concentration of solution = mass of solid used/molar mass of solid x 1000/250 (units moldm-3)
Using the pipette
- A glass bulb pipette will deliver the volume stated on it within acceptable limits only.
- Using a pipette filler, draw a little of the solution to be used into the pipette and use this to rinse the pipette.
- Fill the pipette to about 2-3cm3 above the mark. Pipette fillers are difficult to adjust accurately, so quickly remove the filler and close the pipette with your forefinger (not thumb). Release the solution until the bottom of the meniscus is on the mark.
- Immediately transfer the pipette to the conical flask in which you will do the titration, and allow the solution to dispense under gravity.
Using the burette
- Making sure that the tap is shut, add about 10-15cm3 of the appropriate solution to the burette and rinse it out, not forgetting to open the tap and rinse the jet.
- Close the tap and fill the burette. A small funnel should be used to add the solution but be careful not to overfill the funnel.
- Remove the funnel, because titrating with a funnel in the burette can lead to serious error if a drop of liquid in the funnel stem falls into the burette during the titration.
- Bring the meniscus on to the scale by opening the tap to allow solution to pass through the burette. There is no particular reason to bring the meniscus exactly to the zero mark.
- Make sure that the burette is full to the tip of the jet.
- After a suitable indicator has been added to the solution in the conical flask, swirl the flask under the burette with one hand whilst adjusting the burette tap with your other hand.
- Add the solution in the burette to the conical flask slowly, swirling the flask all the time.
- As the endpoint is approached, the indicator will change colour more slowly. The titrant should be added drop by drop near to the endpoint.
- Repeat the titration until you have three concordant titres, i.e. volumes that are similar. This means within 0.2cm3 or better if you have been careful. Taking the mean of three tires that differ by 1cm3 or more is no guarantee of an accurate answer.
Common indicators
- Methyl orange - yellow in alkali, red in acid (orange at end point)
- Phenolphthalein - pink in alkali, colourless in acid.
Enthalpy change measurements
- Weigh a spirit lamp (containing a liquid alcohol) using a balance accurate to 3 decimal places. Record the mass measured.
- Use a measuring cylinder to put 100 cm3 of distilled water into a small beaker and clamps this at a fixed height above the spirit lamp (about 2 cm).
- Record the initial temperature of the water using a thermometer.
- Light the lamp using a burning splint.
- Heat the water using the spirit lamp until the temperature has gone up by about 10C. Stir the water with the thermometer the whole time.
- Put a cap on the spirit to stop the alcohol burning. The lid stops also stops further evaporation of the liquid alcohol.
- Reweigh the spirit lamp and record the mass.
- Calculate the enthalpy change
Possible sources of error
- There may be heat loss due to the apparatus used and heat may have dissipated through the insulating material --> should use a polystyrene cup and insulation like a lid.
- The specific heat capacity and density of water are used (and not of HCl).
- The masses of solid added to the acid are ignored.
- It is assumed that the specific heat capacity of the polystyrene cup is negligible.
- Some heat is lost when the hydrogen or carbon dioxide are evolved in the reactions.
Also See
Why not read these other revision notes for Edexcel Unit 3B - Laboratory Techniques?
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