Practical Techniques Flashcards
recapping the methodology and issues with the core practicals and associated techniques
What is the purpose of a bunsen valve?
To prevent suck back of cold water back up the delivery tube when the heat is removed, which can cause the glass the explode.
Describe how to remove an air bubble from the tip of a pipette
Expel some solution (to remove the air bubble and suck up again with the tip of the pipette in the solution
Or fill pipette above the line and expel some solution (or can gently tap pipette to move air bubble to the top and out of solution).
How can you mix a solid with a solution in a conical flask without loss of gas from system?
Seen in core practical 1
Place solid in vial and place in the conical flask. Place bung (attached to delivery tube etc) in conical flask. To mix the two together, shake the conical flask to tip the vial and so the reaction starts.
Describe how to make up a standard solution (10g in 250 cm^3) in a 250 cm3 volumetric flask given the pre-weighed solid in a beaker.
Seen in CP2
- Dissolve solid in distilled water using glass rod
- Pour solution into a volumetric flask using a funnel
- Rinse the beaker, glass rod, and funnel with distilled water and transfer the washings to the conical flask
- Make up to the mark (250cm3)
- Shake the volumetric flask
Describe an experiment to find the concentration of a solution of hydrochloric acid using known conc of sodium hydroxide solution (of approximately equal concentrations)
Seen in CP3
- Rinse burette and pipette with appropriate solution (one going to be in each) after rinsing with distilled water twice.
- remove a fixed amount of HCl in conical flask and transfer using a glass pipette, fill burette with the NaOH solution. [ensure no bubbles in burette and tap filled with solution]
- Add named indicator + colour change (i.e. phenolphthalein going from colourless to pale permanent pink)
- Add solution from burette to flask until indicator changes colour
- Add slowly towards end point, with white tile under conical flask whilst stirring.
- Repeat titrations until concordant results
Devise an experiment, giving outline details only, that would enable the relative rates of hydrolysis of halogenoalkanes to be compared.
CP4
- Use of ethanol as a solvent
- Use of silver nitrate solution
- Equal amount of each halogenoalkane used
- Measure the time taken for the precipitate to form (could be made less subjective by doing time taken for cross to disappear)
- Use a water bath to control a raised temperature
If 10g of your product dissolves in 100cm^3 of hot solvent and the solubility in cold solvent is 1g cm^3, how much product will crystallise?
What happens if you add excess hot solvent so that there is 200cm^3?
9g
Only 8g if you use 200cm3 as 2g of product stays dissolved
Why don’t you use a hot oven (you should use a warm oven or dessicator) to dry your crystals?
Hot oven risks vapourising product or decomposing product so reduces yield.
Devise an experimental procedure to determine the enthalpy change of solution for hydrated magnesium sulfate
- 50 cm3 of water (measure using measuring cylinder) in polystyrene cup.
- Polystyrene cup should have lid with thermometer placed through hole in lid. (can place in beaker to make more stable)
- Measure 2g of magnesium sulfate using a balance.
- Measure temp of solution before adding solid.
- Add magnesium sulfate to polystyrene cup and stir with thermometer.
- Measure temperature every 5 seconds. Plot temperature against time on graph and determine the maximum temperature reached during the reaction (change in temp is ΔT)
- Q = mcΔT and then ΔH = -Q/(1000xn)
Determining max temp change requires extrapolation of the cooling curve as theoretically there is NO loss of thermal energy
What assumptions make determining enthalpy change of solution slightly wrong?
- Solution =/= water so specific heat capacity and density not the same
- heat loss to surroundings
- heat absorbed by metal and thermometer
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