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chem spec > chemical analysis > Flashcards

chemical analysis Flashcards

1
Q

define a pure substance in chemistry

A

a single element or compound not mixed with any other substance

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2
Q

define a pure substance in everyday language

A

a substance that has had nothing added to it; unadulterated and in its natural state

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3
Q

what do pure elements and compounds do

A

they melt and boil at specific temperatures - this data can be used to distinguish pure substances from mixtures

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4
Q

easiest method to test if a substance is chemically pure

A

heat it and measure boiling and melting point
- pure substance always melts and boils at fixed temp
- impure melt and boil over a range of temps

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5
Q

define a formulation

A

a mixture that has been designed as a useful product - many products are complex mixtures in which each chemical has a particular purpose

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6
Q

how are formulations made

A

by mixing the components in carefully measured quantities to ensure that the product has the required properties

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7
Q

examples of formulations

A
  • fuels
  • cleaning agents
  • paints
  • medicines
  • alloys
  • fertilisers
  • foods
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8
Q

what is chromatography

A

a physical separation technique that separates different substances in a mixture based on their solubilities

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9
Q

what does separation in chromatography depend on

A

the distribution of substance between the stationary and mobile phase

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10
Q

what is an rf value

A

the ratio of the distance moved by a compound to the distance moved by the solvent

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11
Q

rf value formula

A

distance moved by substance / distance moved by solvent

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12
Q

what is the stationary phase in chromatography

A

the paper

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13
Q

what is the mobile phase in chromatography

A

the solvent

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14
Q

how does paper chromatography separate mixtures

A

different chemicals have different solubilities so are each attracted to the stationary phase to a different extent

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15
Q

chromatography method

A
  • draw pencil line at bottom of chromatography paper
  • place two dots of mixture using capillary tube / draw dot of ink if pen used
  • place bottom of paper into solvent; don’t let touch pencil line
  • solvent will travel up paper and ink dissolve in it and carried up paper with solvent
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16
Q

what can be determined if one spot is formed from chromatography

A

the substance is pure in that solvent

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17
Q

what can be determined if multiple spots are formed from chromatography

A

there are different solvents with different solubilities within it, so an impure mixture

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18
Q

what happens to a more soluble substance in chromatography

A

it will travel further up the paper because it is more attracted to the mobile phase and less attracted to the stationary phase
vice versa for less soluble substance

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19
Q

if only one spot is formed with a solvent, how can we be sure the substance is pure

A

by testing it with a range of solvents, because a pure compound will produce one spot in ALL solvents

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20
Q

what do rf values help with

A

identifying the compounds; look the rf value up in a database and match it to the solvent used to identify the chemical

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21
Q

what happens if an rf value shows several chemicals

A

repeat the experiment using a different solvent

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22
Q

what happens if a chemical has never been analysed before

A

there will not be an rf value on the database

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23
Q

test for hydrogen

A

burning splint held at the open end of a test tube of the gas

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24
Q

positive test for hydrogen

A

hydrogen burns rapidly with a squeaky pop sound

25
Q

test for oxygen

A

glowing splint inserted into a test tube of the gas

26
Q

positive test for oxygen

A

splint relights in oxygen

27
Q

test for carbon dioxide

A

carbon dioxide shaken with or bubbled through an aqueous solution of calcium hydroxide (limewater)

28
Q

positive test for carbon dioxide

A

limewater turns cloudy

29
Q

test for chlorine

A

damp litmus paper is put into chlorine gas

30
Q

positive test for chlorine

A

litmus paper is bleached and turns white

31
Q

what are flame tests used for

A

identifying metal ions

32
Q

what colour flame do lithium compounds produce

A

Li+ ions produce a crimson flame
LiCk

33
Q

what colour flame do sodium compounds produce

A

Na+ ions produce a yellow flame
SoY

34
Q

what colour flame do potassium compounds produce

A

K+ ions produce a lilac flame
PoLl

35
Q

what colour flame do calcium compounds produce

A

Ca2+ ions produce an orange-red flame
CalOR

36
Q

what colour flame do copper compounds produce

A

Cu2+ ions produce a green flame
CoG

37
Q

what happens if a sample contains a mixture of ions

A

some flame colours can be masked

38
Q

issue with flame tests to identify ions in a compound

A

colour of flame is difficult to distinguish esp if low conc of metal compound

39
Q

what can sodium hydroxide solution be used for

A

identifying positive metal ions (cations)

40
Q

what forms white precipitates when sodium hydroxide solution is added

A

aluminium, calcium and magnesium ions

41
Q

what happens when excess sodium hydroxide solution is added

A

aluminium hydroxide precipitate dissolves and goes clear again

42
Q

what forms coloured precipitates when sodium hydroxide solution is added

A

solutions of copper(II), iron(II) and iron(III)

43
Q

what precipitate is formed when copper(II) ions react with sodium hydroxide

A

blue

44
Q

what precipitate is formed when iron(II) ions react with sodium hydroxide

A

green

45
Q

what precipitate is formed when iron(III) ions react with sodium hydroxide

A

brown

46
Q

what is a positive ion

A

a cation

47
Q

what is a negative ion

A

an anion

48
Q

examples of negative ions (anions)

A
  • carbonate ions
  • halide ions
  • sulfate ions
49
Q

how to test for carbonate ions

A
  • add dilute acid to sample
  • acid reacts with carbonate to make carbon dioxide (as well as salt and water)
  • will effervesce
  • limewater test
50
Q

how to test for halide ions

A
  • add dilute nitric acid
  • add dilute silver nitrate solution
  • halide ions produce a precipitate of silver halide

chloride ions: white precipitate of silver chloride
bromide ions: cream precipitate of silver bromide
iodide ions: yellow precipitate of silver iodide

51
Q

how to test for sulfate ions

A
  • add dilute hydrochloric acid to the sample
  • add barium chloride solution
  • white precipitate of barium sulfate forms if sulfate ions present
52
Q

what are ionic compounds made up of

A

positive ions (cations) and negative ions (anions)

53
Q

advantages of instrumental methods

A
  • highly accurate and sensitive
  • quicker
  • enable very small samples to be analysed
54
Q

disadvantages of instrumental methods

A
  • usually very expensive
  • takes special training to use
  • gives results that can often be interpreted only by comparison with data from known substances
55
Q

what are instrumental methods used for

A

detecting and identifying elements and compounds

56
Q

what is flame emission spectroscopy

A

an example of an instrumental method used to analyse metal ions in solutions

57
Q

how does flame emission spectroscopy work

A

sample put into a flame and light given out passed through spectroscope. output is line spectrum that is analysed to identify metal ions in solution and measure their concentrations

58
Q

how do you use the line spectrum from flame emission spectroscopy

A

the position of the lines in the spectrum are specific for a given metal ion

59
Q

as well as identifying a metal ion, what can FES also tell us

A

the conc of the metal ion because the lines become more intense as you increase conc

chem spec (12 decks)

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