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researching chemistry Flashcards

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1
Q

What is stoichiometry?

A

The study of the relationships involved in chemical reactions

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2
Q

How is percentage yield is reduced?

A
  • mass transfer or mechanical losses
  • purification of product
  • side reactions
  • equilibrium position
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3
Q

How to heat to constant mass?

A

-Heat a substance
- allowing to cool in a desiccator to prevent absorption of water
- weighing
- repeating the steps of heating, cooling and weighing until no further changes in mass are observed.

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4
Q

What is gravimetric analysis used for?

A

To determine the mass of an element or compound in a substance.

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5
Q

What happens during a precipitation reaction

A

The substance undergoes a precipitation reaction. The precipitate is separated from the filtrate and the filtrate is tested to ensure the reaction has gone to completion. The precipitate is washed, dried to constant mass and then weighed.

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6
Q

What happens during a volatilisation reaction

A

The substance is heated and any volatile products are evaporated. Heated to constant mass and the final mass recorded.

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7
Q

How can a standard solution be prepared?

A
  • Weighing a primary standard accurately
  • Dissolving in a small volume of solvent in a beaker
  • Transfer the solution and rinsing’s into a volumetric flask
  • Marking up to the graduation mark with solvent
    -Stoppering and inverting
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8
Q

How else can standard solutions be made?

A

Accurate dilution by pipetting an appropriate volume of a standard solution into a volumetric flask, making up to the graduation mark with solvent, stoppering and inverting.

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9
Q

What must a primary standard have?

A
  • Be available in a high state of purity
  • be stable when solid and in solution
  • be soluble
  • have a reasonably high GFM
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10
Q

Why is sodium hydroxide not a primary standard

A

It has a relatively low GFM , unstable as solid and in solution. Must be standardised before being used in volumetric analysis

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11
Q

What is a complexometric titration?

A

Based on reactions in which complexes are formed. EDTA is an important complexometric reagent and can be used to determine the concertation of metal ions in solution.

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12
Q

What is a back titration?

A

Used to find the number of moles of a substance by reacting it with an excess volume of a reactant of known concentration.

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13
Q

What things must you consider with colorimetry?

A
  • Preparing a series of standard solutions of appropriate concentration
  • Choosing an appropriate colour or wavelength of filter complementary to the colour of the species being tested
  • Using a blank
  • Preparing a calibration graph
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14
Q

What does colorimetry do?

A

Used the relationship between colour intensity of a solution and the concentration of the coloured species present.
Colorimeter or spectrophotometer used to measure the absorbance of light of a series of standard solutions and this data is used to plot a calibration graph.

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15
Q

What is heating under reflux?

A

Allows heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances escaping.
The reaction mixture is placed in a round-bottomed flask with anti-bumping granules and the flask is fitted with a condenser

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16
Q

Vacuum filtration

A

Involves carrying out filtration under reduced pressure and provides a faster means of separating a precipitate from a filtrate. Buchner or Hirsch funnel can be used.

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17
Q

Process of recrystallisation

A

Uses and impure solid involving:
- dissolving an impure solid gently into a minimum volume of a hot solvent
- hot filtration of the resulting mixture to remove any insoluble impurities
- Cooling the filtrate slowly to allow crystals of the pure compound to form leaving soluble impurities dissolved in the solvent

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18
Q

What is weighing accurately approximately?

A

weigh out as close as possible to a certain mass but ensure
that you record the actual mass given on the balance.

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19
Q

What is colorimetry?

A

Colorimetry uses the relationship between colour intensity of a solution and the
concentration of the coloured species present

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20
Q

What does a colourimeter allow you to do?

A

A colorimeter or a spectrophotometer is used to measure the absorbance of light of a
series of standard solutions, and this data is used to plot a calibration graph.

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21
Q

What is distillation?

A

Distillation is used for
identification and purification of organic compounds.

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22
Q

What is determined by distillation?

A

The boiling point of a compound, determined by distillation, is one of the physical properties that can be used to confirm its identity.

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23
Q

How can distillation purify a compound?

A

by separating it from less volatile substances
in the mixture.

24
Q

What is heating under reflux?

A

Heating
under reflux allows heat energy to be applied to a chemical reaction mixture over an
extended period of time without volatile substances escaping

25
Q

What is the process of heating under reflux?

A

the reaction mixture is placed in a round-bottomed flask with anti-bumping granules and the flask is fitted with a condenser. The flask is then heated using an appropriate source of heat

26
Q

What is vacuum filtration?

A

Vacuum
filtration involves carrying out a filtration under reduced pressure and provides a faster
means of separating a precipitate from a filtrate. A Büchner, Hirsch or sintered glass funnel
can be used during vacuum filtration.

27
Q

What is the process of recrysalisation?

A

♦ dissolving an impure solid gently in a minimum volume of a hot solvent
♦ hot filtration of the resulting mixture to remove any insoluble impurities
♦ cooling the filtrate slowly to allow crystals of the pure compound to form, leaving
soluble impurities dissolved in the solvent
♦ filtering, washing and drying the pure crystals

28
Q

Why is a solvent for recrylsatlisation required?

A

so that the compound being purified is
completely soluble at high temperatures and only sparingly soluble at lower temperatures.

29
Q

What is solvent extraction?

A

Solvent extraction involves isolating a solute from a liquid mixture or solution by extraction using an
immiscible solvent in which the solute is soluble

30
Q

What happens during solvent extraction?

A

When carrying out a solvent extraction, the two immiscible solvents form two layers in the
separating funnel. The solute dissolves in both solvents and an equilibrium establishes
between the two layers. The ratio of solute dissolved in each layer is determined by the
equilibrium constant, K . The lower layer is run off into a container and the upper layer is
poured into a second container. This process is repeated to maximise the quantity of
solute extracted.

31
Q

What must a solvent be?

A

♦ immiscible with the liquid mixture or solution (usually water)
♦ one in which the solute is more soluble in than the liquid mixture or solution
(usually water)
♦ volatile to allow the solute to be obtained by evaporation of the solvent
♦ unreactive with the solute

32
Q

What is melting point?

A

. The melting point of a substance is the temperature range over which
the solid first starts to melt, to when all of the solid has melted

33
Q

How can the identity of a pure compound be confirmed?

A

by melting point analysis and a
comparison of the experimentally determined melting point with a literature or known
melting point value.

34
Q

What does the determination if the melting point of a compound indicate?

A

can give an indication of the purity of a
compound. The presence of impurities in the compound lowers the melting point and
broadens its melting temperature range due to the disruption in intermolecular bonding in
the crystal lattice.

35
Q

What does the determination of a mixed melting point involve?

A

involves mixing a small quantity of the product with
some of the pure compound and determining the melting point. The melting point value
and the range of the melting temperature can be used to determine if the product and the
pure compound are the same substance.

36
Q

What is chromatography?

A

Chromatography is a technique used to separate the components present within a mixture.
Chromatography separates substances by making use of differences in their polarity or
molecular size

37
Q

What happens during thin layer chromatography?

A

Thin-layer chromatography (TLC) uses a fine film of silica or aluminium oxide spread over
glass, aluminium foil or plastic. A small sample of the mixture being tested is spotted onto
the base (pencil) line of the chromatogram. A solvent dissolves the compounds in the spot
and carries the compounds up the chromatogram. How far the compounds are carried
depends on how soluble the compounds are in the chosen solvent and how well they
adhere to the plate. A developing agent or ultraviolet light is normally required to visualise
the spots on the chromatogram.

38
Q

How is Rf value calculated?

A

distance travelled by the sample/ distance travelled by the solvent

39
Q

How can the identity of a compound be confirmed?

A

♦ comparing the experimentally determined Rf values with a literature or known value
determined under the same conditions
♦ making a direct comparison on a TLC plate between the compound being tested and
the pure substance — a co-spot could be used

40
Q

When will a compound have the same Rf value?

A

Under the same conditions (temperature, solvent, and saturation levels) a compound
always has the same Rf value (within experimental error).

41
Q

What is TLC used for?

A

TLC is used to assess the purity of substances. A pure substance, when spotted and
developed on a TLC plate, should appear as a single spot (some impurities may not be
visible by TLC analysis). The presence of more than one spot shows that impurities are
present.

42
Q

What is ppm?

A

parts per million or 1mg per kg or 1mg per litre

43
Q

What is the percentage by mass with volume?

A

mass of solute made up to 100 cm3 of solution.

44
Q

What is the percentage by volume?

A

Percentage by volume is the number of cm3 of solute made up to 100
cm3 of solution.

45
Q

What is ppm?

A

parts per million and refers to 1 mg per kg or 1 mg per litre

46
Q

What is the process of weighing by difference?

A
  • Measure the mass of the empty weigh boat
  • Measure out the solid in the weigh boat
  • Take note of the mass of the weigh boat + solid and use this to work out how much solid has been measured out (mass of weigh boat + solid - mass of empty weigh boat)
  • Transfer solid to beaker
  • Reweigh the empty weigh boat to calculate total solid transferred to beaker
47
Q

What is weighing accurately approximately?

A

This means the exact mass should be known and should be close to the true value stated.

48
Q

What happens in gravimetric analysis?

A

The substance is converted into another substance of known
chemical composition, which can be readily isolated and purified.
The conversion can occur either through precipitation or volatilisation

49
Q

What are the examples of primary standards?

A

♦ sodium carbonate,
♦ hydrated oxalic acid,
♦ potassium hydrogen phthalate,
♦ silver nitrate,
♦ potassium iodate,
♦ potassium dichromate,

50
Q

What happens to a resulting mixture in a back titration?

A

The resulting mixture is then titrated to work out
the number of moles of the reactant in excess. From the initial
number of moles of that reactant, the number of moles used in
the reaction can be determined. The initial number of moles of the
substance being analysed can then be calculated.

51
Q

When is a back titration useful?

A

A back titration is useful when trying to work out the quantity of substance in a solid with a low solubility

52
Q

What does a colorimeter measure?

A

A colorimeter or a spectrophotometer is used to measure the
absorbance of light of a series of standard solutions, and this data is
used to plot a calibration graph

53
Q

What must the concentration of the coloured species do?

A

The concentration of coloured species in the solution being tested
must lie in the straight line section of the calibration graph.

54
Q

When will the quantity of the solute extracted be greater?

A

if a number of extractions
using smaller volumes of solvent are carried out rather than a single
extraction using a large volume of solvent.

55
Q

What is the function of melting point analysis?

A

The identity of a pure compound can be confirmed by melting point
analysis and a comparison of the experimentally determined melting
point with a literature or known melting point value.

Advanced Higher Chemistry (14 decks)

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