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CfE AH Chemistry > Unit Three > Flashcards

Unit Three Flashcards

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1
Q

What reagent is used to convert cyclohexanol to cyclohexene and why not concentrated sulfuric acid?

A

Concentrated phosphoric Acid- with sulfuric acid too many side reactions occur.

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2
Q

Why is saturated sodium chloride solution used instead of water to purify cyclohexane distillate?

A

It improves the separation of the 2 phases as it makes the water more polar.

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3
Q

When preparing aspirin how do you check the purity of the final sample?

A

Melting point analysis- the sharper the peak, the more pure the substance is.

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4
Q

What is the catalyst used when preparing aspirin?

A

Concentrated phosphoric acid

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5
Q

How could an unknown ketone be identified?

A

React with Brady’s reagent and measure the melting point of the derivative. The melting point of the derivative is compared with literature values.

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6
Q

What can potassium trioxaloferrate (lll) be prepared from?

A

Ammonium iron (ll) sulphate

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7
Q

What is added to iron (ll) oxalate and potassium oxalate to form potassium trioxaloferrate (lll) ?

A

Hydrogen peroxide

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8
Q

What colour is potassium trioxaloferate (lll)?

A

Green

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9
Q

What can gravimetric analysis be used for?

A

To determine the number of moles of water molecules of crystallisation for a compound such as hydrated barium chloride.

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10
Q

What is a desiccator used for in gravimetric analysis?

A

To cool the heated substance in a dry atmosphere

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11
Q

What are the steps in solvent extraction?

A

1) Separating funnel is used to drain the layers into two separate beakers.
2) The lower aqueous layer is returned to the separating funnel and further solvent is added. The process is repeated.
3) Evaporation / distillation will then be used to separate the solvent from the product you want.

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12
Q

What are methods used to separate substances?

A

Solvent extraction, recrystallisation, chromatography

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13
Q

What properties should the solvent used for your recrystallisation have?

A

The desired product should be readily soluble in the solvent at high temperatures but only sparingly soluble at lower temperatures so that crystals form as the hot mixture cools.

Impurities should either be completely insoluble allowing them to be filtered out.

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14
Q

What does recrystilisation do to the purity and the percentage yield of a product?

A

It increases the purity but decreases the percentage yield.

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15
Q

What would the steps in a recrystallisation be?

A

Add a minimum amount of solvent, the solvent should be hot. The mixture should then be filtered to remove any impurities. The mixture should then be left to cool.

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16
Q

In the gravimetric determination of water in hydrated barium chloride what are the reasons for an inaccurate result?

A
  • errors in balance reading
  • impurities present in the sample of hydrated barium chloride.
  • not all the water being driven off.
  • using a yellow flame instead of a blue flame when heating the crucible as this would result in soot deposits being left on the base of the crucible affecting the masses measured.
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17
Q

When determining the decomposition temperature of sodium hydrogen carbonate why might the theoretical value differ from the experimental value?

A
  • The thermometer used may not have been totally immersed in the solid sodium hydrogen carbonate.
  • If the oil was heated too quickly there could be a temperature difference between the thermometer bulb and the sodium hydrogen carbonate.
  • The gas syringe could be sticky due to friction between the glass surfaces.
  • There may be leaks in the system.
  • Some of the carbon dioxide may have dissolved in the condensed water vapour.
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18
Q

Why can you not simply carry out boiling point analysis on an unknown ketone?

A

Boiling points of liquids vary significantly with atmospheric pressure.

19
Q

How can benzoic acid be prepared?

A

Through alkaline hydrolysis of the ester ethyl benzoate.

20
Q

When preparing benzoic acid why would the percentage yield not be 100%?

A
  • Benzoic acid crystals are lost during their transfer from one container to another.
  • Benzoic acid is slightly soluble in water and in the recrystallisation process some of it would have remained dissolved in the water.
21
Q

When preparing benzoic acid why is sodium hydroxide used not hydrochloric acid

A

It gives a much higher yield as the reaction is reversible but the sodium hydroxide reacts with the benzoic acid produced and pushes the equilibrium to the side of the products.

22
Q

Why can the mass of aspirin not be determined by direct titration?

A

Aspirin is insoluble in water.

23
Q

Why is it important to use a plain aspirin tablet when determining aspirin mass?

A

Other tablets may include other ingredients which may react with the sodium hydroxide.

24
Q

In the complexometric determination of nickel using EDTA what type of ligand is EDTA?

A

It is a hexadentate ligand which binds in a 1:1 ratio with nickel ions forming a stable octahedral complex.

25
Q

What is the indicator used in the complexometric determination of nickel?

A

Murexide

26
Q

What are the sources of error in trying to work out the order of iodine in the propane/iodine reaction?

A
  • Transfer losses
  • Noting time when pipette half empty is difficult
  • Titrations had to be done very accurately as there was no possibility of repeating.
  • Titrations had to be carried out very quickly after sample taken from reaction mixture
27
Q

What is the function of the sodiumhydrogencarbonate in the iodine propanone kinetics reaction?

A

To neutralise the acid catalyst and stop the reaction

28
Q

What are two pieces of equipment needed to carry out a reflux reaction?

A

A condenser and a heating mantle

29
Q

How do you carry out mixed melting point analysis and why?

A

Your sample is mixed with a known pure sample and if the melting point is the same as the pure sample on its own then your sample is pure. It can be used to confirm identity.

30
Q

Why is murexide a suitable indicator for the nickel determination using EDTA?

A

Murexide binds less strongly with metal ions that EDTA and it is a different colour when bound to metal. Colour change is to blue-violet.

31
Q

When preparing benzoic acid from the alkaline hydrolysis of ethyl benzoate what is added to precipitate out the benzoic acid from the sodium benzoate solution?

A

Concentrated hydrochloric acid

32
Q

When extracting manganese from a steel paper clip what happens as the manganese dissolves in the nitric acid?

A

Brown fumes of NO2 form.

33
Q

What is the solvent used when separating cyclohexene?

A

Sodium chloride solution

34
Q

What type of ligand is the oxalate ion?

A

Bidentate ligand

35
Q

How many moles of potassium trioxaloferrate is formed from one mole ammonium iron (11) sulphate?

A

It’s a 1:1 ratio so one mole.

36
Q

How is manganese extracted from a steel paper clip for colorimetric analysis?

A

The paperclip is dissolved in nitric acid which oxidises the metals present to metal ions. A strong oxidising agent is then added which oxidises the Mn2+ ions to purple MnO4- ions. All the solution is washed into a standard flask and made up to the gradation mark.

37
Q

In the propanone iodine reaction what order is the reaction with respect to propanone and the hydrogen ion catalyst?

A

Both are first order

38
Q

What does the graph in the propanone iodine reaction mean and how can we deduce the order of iodine?

A

The graph is linear showing that the rate of the reaction remains constant. This means that as the concentration of iodine decreases the rate of decrease in iodine is not affected. This means the order of the reaction with respect to iodine must be zero order.

39
Q

Why is recrystallisation an important step in identification of a derivative?

A

It is important that the derivative is pure otherwise identification by melting point analysis is difficult.

40
Q

In the preparation of benzoic acid from the ester ethyl benzoate how do you know when the reaction is complete?

A

When the oily layer completely disappears.

41
Q

What does refluxing do?

A

Allow reaction mixture to be heated over a long period of time without volatile reactants and products escaping.

42
Q

How do you improve yield using solvent extraction?

A

The mass of the substance extracted is greater if many small extractions with less solvent are done as opposed to one big extraction.

43
Q

In chromatography are Rf values for the sane substance the same?

A

Under similar conditions yes

44
Q

How many spots will a pure substance show up on a chromatogram

A

One

CfE AH Chemistry (3 decks)

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