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AH Chemistry (short) > Unit 4- Researching chemistry > Flashcards

Unit 4- Researching chemistry Flashcards

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1
Q

Why are conical flasks used in titrations?

A

Their shape allows the swirling of the contents without spillage

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2
Q

In a titration what should be excluded from the calculation of the average value?

A

The rough titre and any rogue points

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3
Q

How accurate are beakers for measuring the volume of liquids?

A

They are of little use in measuring the volume of liquids accurately and only provide a rough guide to the volume

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4
Q

Which is more accurate in measuring volumes of liquids; a measuring cylinder or a pipette?

A

A pipette is more accurate

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5
Q

What are burettes used for?

A

Measuring non-standard volumes of liquid

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6
Q

What does it mean if results are accurate?

A

They are close to the true value

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7
Q

What does it mean if results are precise?

A

They are close to each other

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8
Q

Why would a control experiment be carried out?

A

To validate a technique or procedure as accurate

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9
Q

What is stoichiometry?

A

The study of mole relationships involved in chemical reactions

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10
Q

What is percentage by mass?

A

The mass of a solute made up to 100cm^3 of solution

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11
Q

What is percentage by volume?

A

The number of cm^3 of solte made up to 100 cm^3 of soltion

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12
Q

What is percentage by volume?

A

The number of cm^3 of solute made up to 100 cm^3 of solution

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13
Q

What does the unit ppm stand for and what does it refer to?

A

ppm stands for parts per million and refers to 1 mg per kg or 1 mg per litre

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14
Q

What 4 things can reduce the percentage yield achieved in a process?

A
  • mass transfer or mechanical losses
  • purification of product
  • side reactions
  • equilibrium position
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15
Q

What can gravimetric analysis determine?

A

The mass of an element or compound in a substance

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16
Q

What can an accurate electronic balance be used for?

A

Determining the mass of a substance (the tare function allows the balance to be set to zero)

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17
Q

What are weighing boats used for?

A

Measuring substances on a balance without contamination

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18
Q

How does weighing by difference work?

A

The mass of an empty weighing bottle and stopper is measured and the chemical is added to the bottle and reweighed. The difference between the two measurements is the mass of the chemical

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19
Q

What does “weighing accurately approximately” mean?

A

This is the term used to measure the exact mass of a substance on a balance but the mass is close to a specific mass stated

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20
Q

What does heating to constant mass do?

A

Removes all moisture from a substance which would increase the mass of the substance

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21
Q

What are the 4 steps to heat a substance to constant mass?

A
  • the substance is heated to remove moisture from the substance
  • the substance is allowed to cool in a desiccator to prevent re-absorption of
    water
  • the substance has its mass measured on the balance once cooled
  • repeating the 3 steps until a constant mass is obtained on the balance
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22
Q

During gravimetric analysis the substance is converted into another substance of known of known chemical composition, which can then be readily isolated and purified. What are the two ways in which this conversion can occur?

A

Either through precipitation or volatilisation

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23
Q

What happens after a precipitation reaction has taken place?

A
  • the precipitate is separated from the filtrate
  • 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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24
Q

What happens during a volatilisation reaction?

A
  • the substance is heated and any volatile products (often water) are evaporated
  • the substance is heated to constant mass and the final mass is recorded
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25
Q

What is a standard solution?

A

A solution of accurately known concentration

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26
Q

What are the 4 steps to prepare a standard solution?

A
  • weighing a primary standard (solid) accurately
  • dissolving in a small volume of solvent (usually deionised water) in a beaker
  • transferring the solution and rinsing in a volumetric flask
  • making up to the graduation mark with solvent stoppering and inverting
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27
Q

How can you ensure you are being accurate when carrying out a dilution?

A
  • use a pipette to transfer an appropriate volume of standard solution into a
    volumetric flask
  • make up to the graduation mark with solvent (deionised water), stoppering and
    inverting
28
Q

What 4 things must a primary standard be?

A
  • high state of purity
  • stable when in solid and in solution
  • soluble
  • have a reasonably high GFM
29
Q

What are some examples of primary standards?

A
  • sodium carbonate- Na2CO3
  • hydrated oxalic acid- H2C204*2H20
  • potassium hydrogen phthalate- KH(C8H404)
  • silver nitrate- AgNO3
  • potassium iodate- KIO3
  • potassium dichromate- K2Cr2O7
30
Q

Why can sodium hydroxide not be used as a primary standard?

A
  • it has a relatively low GFM
  • is unstable as a solid
  • is unstable as a solution
31
Q

What are acid-base titrations used for?

A

Volumetric analysis between acids and bases

32
Q

What are redox titrations used for?

A

Volumetric analysis between oxidising and reducing agents

33
Q

What are complexometric titrations based on?

A

Reactions using complexometric reagents like E.D.T.A to form complexes with metal ions to determine the concentration of metal ions

34
Q

What are back titrations used for?

A

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

35
Q

What are the steps in a back titration?

A
  • an excess of a known reactant (hydrochloric acid) is added to the unknown reactant (calcium carbonate) and allowed to fully react
  • the unreacted leftover reactant (hydrochloric acid) is then titrated against another chemical (sodium hydroxide) to calculate the number of moles of known reactant (hydrochloric acid) that was left over
  • the number of moles of known reactant (hydrochloric acid) which reacted with the unknown chemical (calcium carbonate) is calculated by subtracting the number of moles of known reactant (hydrochloric acid)leftover from the number of moles at the start
  • the initial number of moles of the unknown substance (calcium carbonate) is then calculated using stoichiometry in a balanced chemical equation
36
Q

When is a back titration useful?

A

When working out the quantity of a substance in a solid with a low solubility

37
Q

What can colorimetry be used for?

A

Measuring the concentration of a species based on its absorbance of a particular wavelength of light

38
Q

What 3 things does colorimetry involve?

A
  • preparing a series of standard solutions of an 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
39
Q

Colorimetry uses the relationship between between colour intensity of a solution and the concentration of the coloured species present, what is this relationship?

A

The higher the concentration of the coloured species, the higher the absorbance of light

40
Q

What does a colorimeter do?

A

Measures the absorbance of light in a series of standard solutions

41
Q

What is distillation used for?

A

The identification and purification of organic compounds

42
Q

What can be determined through distillation?

A

The boiling point of a substance

43
Q

What happens during distillation?

A

The more volatile (lower boiling point) compound is separated from the less volatile compound, you can then purify the more volatile compound

44
Q

What does heating under reflux allow to happen?

A

Heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances escaping

45
Q

What are the 3 steps in heating under reflux?

A
  • reaction mixture is placed in a round-bottomed flask with anti-bumping granules
  • flask is fitted with a condenser
  • flask is heated using appropriate heat source (heating mantle for flammable liquids)
46
Q

What does vacuum filtration allow for?

A

A filtration to be carried out under reduced pressure and this provides a faster means of separating the precipitate from a filtrate

47
Q

What kind of funnel must be used in vacuum filtration?

A

A Buchner funnel

48
Q

What are the 4 steps in recrystallisation to purify an impure solid?

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 in the solvent
  • filtering, washing and drying the pure crystals
49
Q

What 5 things must be considered when choosing a solvent for recrystalliastion?

A
  • the compound being purified should be completely soluble in the solvent at high
    temperatures and only sparingly soluble at lower temperatures
  • the solvent should be immiscible with the liquid mixture or solution
  • the solute should be more soluble in the solvent than in the liquid mixture or
    solution
  • the solvent should be volatile to allow the solute to be obtained by evaporation of the solvent
  • the solvent should be unreactive with the solute
50
Q

What does solvent extraction involve?

A
  • isolating a solute from a liquid mixture or solution by extraction using a different immiscible solvent in which the solute is also soluble
  • the lower layer is then run off into a container and the upper layer is poured into a second container, this is then repeated to maximise the quantity of solute extracted
51
Q

In solvent extraction , what is the ratio of solute dissolved in each layer determined by?

A

The equilibrium constant K

52
Q

In solvent extraction what can be done to maximise the quantity of solute extracted?

A

Carry out a greater number of extractions using smaller volumes of solvent instead of carrying out a single extraction using a larger volume of solvent

53
Q

What is the melting point of a substance?

A

The temperature range over which the solid first starts to melt, to when all of the solid has melted

54
Q

How can the identity of a pure compound be identified using melting point analysis?

A

The experimentally determined melting point of the pure compound can be compared with a literature or known melting point value

55
Q

What are any differences in a substances melting point from that of its pure form caused by?

A

The presence of impurities

56
Q

What do impurities in a compound do to its melting point and why?

A

They lower the melting point, this is caused by disruption in intermolecular bonding in the crystal lattice

57
Q

How does the technique of mixed melting point analysis work?

A

A small quantity of the product is mixed with some of the pure compound, any differences from the expected melting point are because of impurities in the product

58
Q

What is chromatography used to do?

A

Separate the components present within a mixture

59
Q

In chromatography what are substances separated based on?

A

Differences in polarity or molecular size

60
Q

What does TLC (thin layer chromatography) cause separation based on?

A

The distribution between the stationary phase (solid) and the mobile phase (liquid)

61
Q

How does TLC work?

A
  • different compounds have different solubilities and adsorption to the two phases (stationary and mobile) between which they are partitioned
  • TLC involves spotting the sample to be analysed near one end of a sheet of glass or plastic that is coated with a thin layer of an adsorbent
  • plate is placed on end in a covered jar containing a thin layer of solvent
  • solvent rises by capillary action through the adsorbent and differential partitioning occurs between the components in the mixture
  • the more strongly a component of a mixture is adsorbed onto the stationary phase, the less time it will spend in the mobile phase and the more slowly it will migrate up the plate
62
Q

What 3 things affect how far the compounds on a TLC plate will be carried?

A
  • how soluble the compounds are in the chosen solvent
  • how well they adhere to the plate
  • a developing agent or UV light is normally required to visualise the spots on a TLC chromatogram
63
Q

What is the calculation that can be used to determine the Rf value?

A

Rf= distance travelled by sample/distance travelled by solvent

64
Q

A compound will always have the same Rf value under the same conditions, what are these conditions?

A
  • temperature
  • solvent used
  • saturation levels
65
Q

Using TLC what are the 2 ways in which the identity of z compound be confirmed?

A
  • comparing the experimentally determined Rf value 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 where a co-spot could be used
66
Q

How should a pure substance appear on a TLC plate, once spotted and developed?

A

As a single spot

67
Q

What does the presence of more than one spot in TLC show?

A

That impurities are present

AH Chemistry (short) (4 decks)

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