Practicals Flashcards
How do you reduce uncertainty in a burette reading
- decrease the concentration of substance in burette
- so the titre volume is made larger
Why should you be careful not to “overshoot” when filling flask in a titration
So the solution has a known volume
Why should you not leave the funnel in the burette
Because some liquid may drip leading to a false burette reading so lower titre volume
What’s the effect of rinsing the burette
- you can rinse with whatever solution your going to put in it i.e acid.
- rinsing with water would dilute the concentration making your titre value too high and innaccurate
Why can you rinse the pipette and conical flask with water
Because adding water doesn’t effect the moles in the flask so it’s fine
How do you carry out a simple laboratory process to show the recrystallised product is pure (aspirin practical)
- measure the melting point (slowly near the melting point)
- using melting point apparatus
- melting point should match the data source value
Why would a student use ethanoic anhydride instead of ethanoyl chloride even though it has a slower rate of reaction
- Ethanoyl chloride reacts violently with water
- and HCL fumes are released
What observations would indicate the sample is not pure
- melting range would be wide
- melting range would be below the true melting point
In the aspirin practical what’s the purpose of minimum volume and hot water
- to obtain saturated solution
- to increase yield
In the aspirin practical what’s the purpose of the solution being filtered hot
- To remove insoluble impurities
- To prevent crystals reforming during filtration
In the aspirin practical what’s the purpose of cooling in ice
Increase amount of crystals that are formed
In the aspirin practical what’s the purpose of washing with cold water
To remove soluble impurities
Describe what happens when a reaction mixture is refluxed and why it’s necessary
- a mixture of liquids is heated to boiling point for a prolonged time
- vapour is formed which escapes from the liquid mixture, it’s changed back into liquid and returned to the liquid mixture
- any ethanal and ethanol that initially evaporates can then be oxidised
Why is reflux used
Reflux does not allow any reactant vapour to escape
What test tube reaction shows cyclohexanol has been dehydrated (distillation practical)
Add bromine water
It would turn from orange to colourless
In distillation practical why is sodium carbonate solution used to wash the distillate
It would neutralise the acid
Why is it important to open the tap of the separating funnel
Release pressure
In the aspirin, practical why is anhydrous calcium chloride a drying agent
It Does not react with cyclohexene
Describe a reduced pressure apparatus (or draw)
- buchner funnel containing filter paper
- buchner flask connected to vacuum pump
Explain how to decrease % uncertainty in use of thermometer
- Increase magnitude of temperature change
- by increasing the concentration of the acid
What should you make sure before recording a burette reading
- remove funnel
- ensure no air bubbles
Why is a cooled collection vessel needed when obtaining a sample of ethanal
To reduce evaporation of ethanal
If a student thinks a solution has Cl- or Br- or I- ions in it but doesn’t know which, what further test should they carry out ?
- add dilute ammonia solution followed by concentrated
- if the ppt dissolves then either Cl- or Br- ions are present
- if the ppt does not dissolve then I- ions are present
RP2 (q= mcAt): Why might an experimental value for enthalpy change be different to the theoretical value?
- Heat loss to the apparatus/ surroundings
- incomplete combustion
- Non-standard conditions
- Evaporation of alcohol/water
RP2 (q= mcAt): other than preventing heat loss how can the accuracy of the experiment be improved?
- Read the thermometer at eyelevel
- stir the solution so the temperature is evenly distributed
- use greater concentration and masses so smaller uncertainty
- use a digital thermometer for more accurate readings
RP2 (q= mcAt): how to improve accuracy
use a flame calorimeter:
- flame is enclosed
- fuel burns in pure oxygen rather than air
Why is the initial rates method better than the continuous monitoring method?
Because the concentrations are known at the start of the reaction
What are the issues with the initial rate method
- Some low concentrations may take too long to react
- Delayed stopwatch reactions
- Concentrations may not be exact due to measuring apparatus
What are the issues with continuous monitoring method?
- Some gas may escape before the bung is added
- The magnesium strips may be of different mass and surface area which would affect the rate of reaction
In the aspirin practical what’s the purpose of vacuum filter with a butchner flask
The water pump connected to the flask reduces the pressure and speed up the filtration
In the aspirin practical what’s the purpose of drying the crystals at the end
To remove excess water, which would affect percentage yield
Where is the loss of yield in the aspirin practical
- crystals lost when filtering or washing
- Some product stays in solution after recrystallisation
- Side reactions
In the distillation, practical why are anti-bumping granules used?
To allow more even heating of the mixture
How can a student confirm cyclohexane is dry
It will go clear
What is the colour change seen at the endpoint of a titration using potassium manganate solution?
Colourless to purple
Why is KCN used instead of HCN
HCN is too weak
How do you stop a reaction occurring in a sample
Cooling it down and heavily diluting it
What colour does phenolphthalein go in alkaline
Colourless to pink
Why is the temperature kept low when making crystals from nitration
To prevent multiple substitutions of nitro groups on the benzene ring
What happens in recrystallisation if the crystals are not dried properly
The mass will be larger than expected which can lower the percentage yield
