Practical Techniques Flashcards
What are the reactants needed to make aspirin?
Ethanoic anhydride
2- hydroxy benzoic acid
Catalyst (H2SO4)
Describe the process of recrystallisation and purification
Dissolve impure substance in hot solvent, use minimum volume
Filter the hot solution
Allow the solution to cool in ice/cold water. Crystalls form
Filter the crystals out using a buchner filter
Wash the crystals with cold water and leave them to dry
What are the components of a buchner filter?
Flask with a horizontal exit for pressure
Rubber bung
Buchner funnel, with a porous plate (with holes) and moist filter paper
During recrystallisation, why is the minimum amount of hot solvent used?
Minimum amount- so a saturated solution is formed
Hot- to allow the solution to later cool and crystalise
During recrystallisation, why is the hot solution filtered?
To remove insoluble impurities
During recrystallisation, why is the solution placed in cold water or ice?
To produce the largest amount of crystals
During recrystallisation, why are the crystals washed with cold water
To remove soluble impurities
In distillation, why are antibumping granules used?
To reduce vigorous bubbling
Decribe what heat reflux is?
Boiling a mixture continuously, evaporated molecules are condensed and reboiled. Therefore no reactants or products are lost
During recrystallisation, why might percentage yield not be accurate?
Crystals lost when filtering/washing
Not all products crystallise
Side reactions may occur
Describe how to investigate the pH change when an acid is added to an alkali solution
Rinse a burette with acid of known concentration. Fill the burette with the acid.
Transfer 20cm3 of alkali of known concentration to a 100cm3 conical flask
Rinse pH probe with distilled water, clamp to burette so its bulb is in the alkali solution. It is attached to a pH scale
Add equal amounts of acid at a time, stir with a glass rod. Record pH each time.
When closer to equivalence point, reduce the amount added.
Plot a pH curve with results
When drawing a heat reflux diagram, what is important to remember?
Water in at the bottom, water out at the top
The tube must be OPEN at the top
Anti bumping granules may be used
When drawing a fractional distillation diagram, what is important to remember?
Water in at the bottom, water out at the top
The tube must be CLOSED with a bung
A thermometer must be used
Anti bumping granules
Describe the process of producing a standard solution
Weigh the sample in a weighing boat, transfer to a beaker and reweigh the weighing boat. Record the difference in mass
Add distilled/ deionised water to the solid, stir with a glass rod until the solid has completely dissolved
Transfer to a 250cm3 volumetric flask, including washings
Add water until the meniscus sits on the graduated line.
Stopper and invert to mix contents
Describe the experiment you would do to measure enthalpy change of a reaction? What reaction is used for this?
Uses the reaction between water + anhydrous copper (II) sulfate powder —> copper (II) sulfate solution
between Weigh out a known mass of anhydrous copper (II) sulfate in a dry, stoppered weighing bottle.
Place a thermometer in an insulated cup, filled with 25 cm3 deionised water
start a stop watch
Record the temperature every subsequent minute for 3 minutes
At minute 4 start the reaction by adding the anhydrous copper (II) sulfate powder. Do not record the temperature at minute 4
Record the temperature every subsequent minute from minute 5
Plot a graph of temperature against time
Plot line of best fits, extrapolate to the 4 minute line to find the change in temperature
Use q= m x c x change in T
Then
Enthalpy change = q/n
How would you prepare a pure sample of AgBr from a mixture of NaBr and NCl?
Add nitric acid and silver nitrate to the impure solution in a test tube. This will produce precipitates of AgBr and AgCl
Add excess of dilute ammonia to the test tube, the AgCl solid will dissolve, but AgBr will remain a ppt
Filter off the remaining solid (AgBr)
Wash with cold water to remove any soluble impurities
Dry to remove the excess water
During the experiment to measure enthalpy change of a reaction, what are the errors in the experiment. How can you improve accuracy?
Errors:
Heat can be lost to the surroundings
Method assumes all solutions have the specific heat capacity of water
Incomplete reaction may occur
Water could evaporate
Improving accuracy:
Use an electronic temperature sensor and data logging software to plot the graph accurately
Describe an experiment to investigate how temperature effects the rate of reaction, state the equation used in this
Na2S2O3 + 2HCl —> S + SO2 + 2NaCl + H2O
- place 10cm3 of acid into a beaker
- put 1cm3 of sodium thiosulfate into. A test tube, put the test tube in an empty plastic container with cross under
- use thermometer to record initial temp. Add 1cm3 of acid to test tube and record time taken for cross to disappear
- record final temp, use two temps to calculate the mean.
- transfer the solution to a stop bath
- repeat experiment, this time adding 55* water to the plastic container. Leave the sodium thiosulfate for a few minutes in the water so it reaches the temp.
Repeat with at least 5 temps
When investigating the
