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AS Chemistry > Practicals > Flashcards

Practicals Flashcards

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1
Q

How do you reduce uncertainty in a burette reading

A
  • decrease the concentration of substance in burette
  • so the titre volume is made larger
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2
Q

Why should you be careful not to “overshoot” when filling flask in a titration

A

So the solution has a known volume

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3
Q

Why should you not leave the funnel in the burette

A

Because some liquid may drip leading to a false burette reading so lower titre volume

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4
Q

What’s the effect of rinsing the burette

A
  • 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
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5
Q

Why can you rinse the pipette and conical flask with water

A

Because adding water doesn’t effect the moles in the flask so it’s fine

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6
Q

How do you carry out a simple laboratory process to show the recrystallised product is pure (aspirin practical)

A
  • measure the melting point (slowly near the melting point)
  • using melting point apparatus
  • melting point should match the data source value
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7
Q

Why would a student use ethanoic anhydride instead of ethanoyl chloride even though it has a slower rate of reaction

A
  • Ethanoyl chloride reacts violently with water
  • and HCL fumes are released
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8
Q

What observations would indicate the sample is not pure

A
  • melting range would be wide
  • melting range would be below the true melting point
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9
Q

In the aspirin practical what’s the purpose of minimum volume and hot water

A
  • to obtain saturated solution
  • to increase yield
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10
Q

In the aspirin practical what’s the purpose of the solution being filtered hot

A
  • To remove insoluble impurities
  • To prevent crystals reforming during filtration
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11
Q

In the aspirin practical what’s the purpose of cooling in ice

A

Increase amount of crystals that are formed

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12
Q

In the aspirin practical what’s the purpose of washing with cold water

A

To remove soluble impurities

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13
Q

Describe what happens when a reaction mixture is refluxed and why it’s necessary

A
  • 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
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14
Q

Why is reflux used

A

Reflux does not allow any reactant vapour to escape

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15
Q

What test tube reaction shows cyclohexanol has been dehydrated (distillation practical)

A

Add bromine water
It would turn from orange to colourless

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16
Q

In distillation practical why is sodium carbonate solution used to wash the distillate

A

It would neutralise the acid

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17
Q

Why is it important to open the tap of the separating funnel

A

Release pressure

18
Q

In the aspirin, practical why is anhydrous calcium chloride a drying agent

A

It Does not react with cyclohexene

19
Q

Describe a reduced pressure apparatus (or draw)

A
  • buchner funnel containing filter paper
  • buchner flask connected to vacuum pump
20
Q

Explain how to decrease % uncertainty in use of thermometer

A
  • Increase magnitude of temperature change
  • by increasing the concentration of the acid
21
Q

What should you make sure before recording a burette reading

A
  • remove funnel
  • ensure no air bubbles
22
Q

Why is a cooled collection vessel needed when obtaining a sample of ethanal

A

To reduce evaporation of ethanal

23
Q

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 ?

A
  • 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
24
Q

RP2 (q= mcAt): Why might an experimental value for enthalpy change be different to the theoretical value?

A
  • Heat loss to the apparatus/ surroundings
  • incomplete combustion
  • Non-standard conditions
  • Evaporation of alcohol/water
25
Q

RP2 (q= mcAt): other than preventing heat loss how can the accuracy of the experiment be improved?

A
  • 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
26
Q

RP2 (q= mcAt): how to improve accuracy

A

use a flame calorimeter:
- flame is enclosed
- fuel burns in pure oxygen rather than air

27
Q

Why is the initial rates method better than the continuous monitoring method?

A

Because the concentrations are known at the start of the reaction

28
Q

What are the issues with the initial rate method

A
  • Some low concentrations may take too long to react
  • Delayed stopwatch reactions
  • Concentrations may not be exact due to measuring apparatus
29
Q

What are the issues with continuous monitoring method?

A
  • 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
30
Q

In the aspirin practical what’s the purpose of vacuum filter with a butchner flask

A

The water pump connected to the flask reduces the pressure and speed up the filtration

31
Q

In the aspirin practical what’s the purpose of drying the crystals at the end

A

To remove excess water, which would affect percentage yield

32
Q

Where is the loss of yield in the aspirin practical

A
  • crystals lost when filtering or washing
  • Some product stays in solution after recrystallisation
  • Side reactions
33
Q

In the distillation, practical why are anti-bumping granules used?

A

To allow more even heating of the mixture

34
Q

How can a student confirm cyclohexane is dry

A

It will go clear

35
Q

What is the colour change seen at the endpoint of a titration using potassium manganate solution?

A

Colourless to purple

36
Q

Why is KCN used instead of HCN

A

HCN is too weak

37
Q

How do you stop a reaction occurring in a sample

A

Cooling it down and heavily diluting it

38
Q

What colour does phenolphthalein go in alkaline

A

Colourless to pink

39
Q

Why is the temperature kept low when making crystals from nitration

A

To prevent multiple substitutions of nitro groups on the benzene ring

40
Q

What happens in recrystallisation if the crystals are not dried properly

A

The mass will be larger than expected which can lower the percentage yield

AS Chemistry (13 decks)

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