practicals Flashcards
describe a method for heating in a crucible to measure mass loss in thermal decomposition reactions
- weigh an empty clean, dry crucible lid
- add a mass of metal to the crucible and weigh again
- heat strongly with Busen burner
- lift lid with tongs to allow some oxygen but not to let metal powder escape
- allow to cool
- weight the crucible contents again
- heat crucible again and reweigh until you reach a constant mass
why do you reweigh until you reach a constant mass?
to ensure the reaction is complete
why do you need a lid for heating in a crucible
improves the accuracy of the experiment as it prevents loss of solid from the crucible
list the mistakes and the consequences of the mistakes when heating in a crucible
- using large amounts of hydrated salt because the decomposition is likely to be incomplete
- crucible needs to be dry otherwise a wet crucible would give an inaccurate result as it causes mass loss to be too large since water would be lost when heating
- small amounts of solid should not be used because the percentage uncertainties in weighing will be to high
what are the potential errors in using a gas syringe
- gas escapes before the bung inserted
- syringe sticks
- some gases like CO2 or sulphur dioxide are soluble in water so the true amount of gas is not measured
describe a method of making a solution
- weigh the sample bottle containing the required mass of solid
- transfer to a beaker
- reweigh empty sample bottle
- record the difference in mass
- add distilled water to the beaker, use a glass rod to stir to dissolve the solid
- pour the solution into a volumetric flask
- invert flask several times to ensure uniform solution
describe a method to dilute a solution
- pipette a volume of the original solution into a volumetric flask
- make up to the mark with distilled water using a dropping pipette for the last few drops
- invert several times to ensure uniform solution
describe a method for a simple acid-base titration
- rinse equipment
- pipette 25cm of alkali into a conical flask
- pour acid into the burette below eye level
- make sure the jet space in the burette is filled with acid
- add an 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 drop wise at end point
- note burette reading before and after addition of acid
- repeat titration until 3 concodrant results are obtained
what are concordant results
results within 0.10 within each other
explain the mistakes that can occur in a titration
- burette is not rinsed out with acid/alkali - it may be diluted by residual water in the burette or may react with previous substances left from a previous titration, causes concentration of the substance being lower and a larger titre being delivered
- don’t leave the funnel in the burette because small drops of liquid may fall from the funnel during the titration leading to a false burette reading giving a lower titre volume
- the jet space in the burette is not filled it will lead to errors if it then fills during the titration leading to a larger than expected titre reading
- adding too much indicator affects the titration result
what is the equation of calculating apparatus uncertainties
(uncertainty / measurement made on apparatus) x 100
how do you reduce uncertainty in a titration
- replacing measuring cylinders with pipettes or burettes which have lower apparatus uncertainty will lower the error
- to reduce uncertainty in a burette: increase the volume and concentration fo the substance in the conical flask/decrease the concentration fo the substance in the burette
how do you reduce uncertainty in measuring mass
- using a more accurate balance or a larger mass
- weighing samples before and after addition and then calculating difference will ensure a more accurate measurement of the mass is added
describe a method of measuring enthalpy change
- wash the equipment with the solutions to be used
- dry the cup after washing
- put a polystyrene cup in a beaker for insulation and support
- measure out desired volumes of solutions with volumetric pipettes and transfer them to an insulated cup
- clamp the thermometer into place making sure it is immersed in a solution
- measure the initial temperatures of the solution or both solutions if 2 are used
- transfer the second reagent to a cup, if a solid reagent is used then add the solution to the first cup and then ad the solid weighed out on a balance
- stir the mixture
- record temperature every minute after addition
what are the errors in measuring enthalpy change experimentally
- energy transfer from surroundings
- approximation in specific heat capacity of solution
- ignoring any energy absorbed by the apparatus
- reaction or dissolving may be incomplete/slow
- density of solution is taken to be the same as water
