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Chemistry A > Seneca C8 (Chromatography onwards) > Flashcards

Seneca C8 (Chromatography onwards) Flashcards

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1
Q

What is Chromatography

A

Chromatography is a process that separates a mixture into its different components.

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2
Q

What are the two phases in chromatography

A

. Mobile phase
. Stationary phase

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3
Q

Mobile phase

A

Substances are picked up and carried by a mobile phase (liquid or gas).

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4
Q

Stationary phase

A

The mobile phase then moves through a stationary phase (solid or viscous liquid).

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5
Q

Separation of substances

A

A substance moves far if it’s more attracted to the mobile phase.

A substance doesn’t move far if it’s more attracted to the stationary phase.

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6
Q

Strength of attraction

A

Different components can sometimes be equally attracted to a solvent.

So, the number of spots a mixture produces can vary depending on the solvent used.

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7
Q

In paper chromatography, a solvent represents which phase?

A

Mobile phase

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8
Q

What are the steps of paper chromatography

A

. Dip the chromatography paper
. Solvent movement
. Distance travelled

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9
Q

Dip the chromatography paper

A

Dip the bottom of the chromatography paper into the solvent

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10
Q

Solvent movement

A

The solvent travels up the chromatography paper.
As the solvent moves, it picks up and transports the substances being tested up the chromatography paper.

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11
Q

Distance travelled

A

The components that travel furthest are highly soluble (dissolve easily) in the solvent. They are also minimally attracted to the chromatography paper

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12
Q

Chromatogram

A

Chromatography produces chromatograms. We use chromatograms to identify compounds in mixtures by calculating Rf values

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13
Q

Rf values

A

Rf = distance travelled by substance / distance travelled by solvent.
We can work this out for both reference substances and tested substances

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14
Q

Solvents

A

Rf values depend on the solvent. We can learn more about the identity of the components by testing any references and the unknown mixtures in a range of solvents

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15
Q

Reference substances

A

A reference substance is a pure sample that’s run next to the tested substances to see if it’s a component in the mixture. These substances provide valuable evidence, but not proof

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16
Q

Stages of chromatography

A

. The mobile phase moves through the stationary phase
. The substance is picked up by the mobile phase

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17
Q

The phases in paper chromatography

A

Mobile phase - The solvent
Stationary phase - Chromatography paper

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18
Q

Which state of matter can a mobile phase in chromatography not be?

A

Solid

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19
Q

In paper chromatography, the componenets that travel the furthest are

A

. minimally attracted to the chromatography paper
. highly soluble in the solvent

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20
Q

What is the process of paper chromatography in detail

A
  1. Use a pencil to draw a horizontal line near the bottom of the chromatography paper.
  2. Place samples of known food colorings (A-D) next to an unknown substance (X) on this ‘start line’.
  3. Place the paper in a beaker containing a small volume of solvent.
  4. Let the solvent move to a few cm below the end of the paper.
  5. By comparing spots produced by X with those produced by A-D, you will be able to identify the unknown substance.
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21
Q

Testing for hydrogen

A

. Place a lit splint close to the mouth of a test tube that contains a gas.

. If the gas is hydrogen, a “squeaky pop” sound will be produced.

. This noise is generated because hydrogen burns rapidly in the presence of oxygen to give H2O.

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22
Q

Testing for oxygen

A

Insert a glowing splint into a test tube that contains a gas.
If the gas is oxygen, the splint will relight.

23
Q

Testing for Carbon Dioxide

A

Take an aqueous solution of limewater (calcium hydroxide) and bubble through the gas.
. Shake CO2 with limewater
If the gas is carbon dioxide, the limewater solution will turn from clear to cloudy.

24
Q

Testing for Chlorine

A

Insert damp litmus paper into a test tube that contains a gas.

If the gas is chlorine, the litmus paper will bleach and change color from red to white.

25
Q

What color will damp litmus paper turn if it comes into contact with chlorine?

A

White

26
Q

Flame tests

A

Some metal ions can be identified using flame tests

27
Q

The following positive metal ions all produce a characteristic flame color when burned:

A

. Copper - Green flame
. Lithium - Crimson flame
. Calcium - Orange - Red flame
. Potassium - Lilac flame
. Sodium - Yellow flame

28
Q

What are the steps of a flame test

A
  1. Submerge a nichrome wire loop in dilute hydrochloric acid to make sure that it is clean.
  2. Place the nichrome wire loop into the sample to be tested.
  3. Hold the nichrome wire loop in a Bunsen burner’s blue flame.
  4. Observe any change in color of the Bunsen flame.
29
Q

Metal ions in metal compounds

A

Metal ions can be found in aqueous solutions of metal compounds. These ions can precipitate out of solution when they react with a sodium hydroxide solution (NaOH). This produces insoluble solids (don’t dissolve in water) called precipitates.

30
Q

Calcium hydroxide

A

Calcium ions (Ca2+) form the white precipitate calcium hydroxide.

Ca2+(aq) + 2OH-(aq) → Ca(OH)2(s)

31
Q

Aluminium hydroxide

A

Aluminium ions (Al3+) form the white precipitate aluminium hydroxide.

Al3+(aq) + 3OH-(aq) → Al(OH)3(s)

Unlike the other two white precipitates, aluminium hydroxide can be dissolved in excess NaOH. This produces a colourless solution.

32
Q

Magnesium hydroxide

A

Magnesium ions (Mg2+) form the white precipitate magnesium hydroxide.

Mg2+(aq) + 2OH-(aq) → Mg(OH)2(s)

33
Q

Here are some more examples of the formation of precipitates:

A

Copper ions (Cu2+) form the blue precipitate copper(II) hydroxide.

Cu2+(aq) + 2OH-(aq) → Cu(OH)2(s)

Iron(III) ions (Fe3+) form the brown precipitate iron(III) hydroxide.

Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)

Iron(III) ions (Fe3+) form the brown precipitate iron(III) hydroxide.

Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)

34
Q

Testing for carbonates

A

CO32- ions are characteristic of all carbonates.
A reaction between a carbonate and a dilute acid gives a salt, carbon dioxide and water.
E.g. Calcium carbonate + hydrochloric acid → calcium chloride + carbon dioxide + water
As carbon dioxide is produced, we can use the standard carbon dioxide test to work out if an unknown substance is a carbonate.
To recap, this test involves bubbling the gas through limewater. If the solution turns cloudy, the gas is carbon dioxide.

35
Q

What are halides

A

Halide ions are negatively charged (they are anions)

36
Q

What are the two steps to halide testing

A

. Add dilute nitric acid
. Add silver nitrate

37
Q

Adding dilute nitric acid

A

Add dilute nitric acid to an unknown solution.
This removes carbonate ions. These could disrupt test results by forming a precipitate with the silver ions added in step 2.

38
Q

Adding silver nitrate

A

dd silver nitrate to the same solution.
If halide ions are present, they will form a precipitate with the silver ions:

Ag+(aq) + Cl-(aq) → AgCl(s) (silver chloride is white).
Ag+(aq) + Br-(aq) → AgBr(s) (silver bromide is cream).
Ag+(aq) + I-(aq) → AgI(s) (silver iodide is pale yellow).

39
Q

What are sulfates

A

Sulfates are compounds that contain SO42- ions

40
Q

What are the two steps to sulfate testing

A

. Add dilute hydrochloric acid
. Add barium chloride

41
Q

Adding dilute hydrochloric acid

A

Add dilute hydrochloric acid to an unknown solution.
This removes carbonate ions. These could disrupt test results by forming a precipitate with the barium ions added in step 2.

42
Q

Adding barium chloride

A

Add barium chloride to the same solution.
If sulfates are present, the white precipitate barium sulfate will form:
Ba2+(aq) + SO42-(aq) → BaSO4(s)

43
Q

What are the advantages of instrumental methods of chemical analysis

A

. Greater sensitivity and accuracy
. Quicker at producing results
. Able to analyze tiny samples

44
Q

What are the disadvantages of instrumental methods

A

. The instruments are often expensive.
. You need to go through special training to operate the instruments.
. The results are often only useful when compared to data from known substances.

45
Q

What is flame emission spectroscophy

A

Flame emission spectroscopy is an instrumental method used to work out the identity and concentration of the metal ions present in a solution

46
Q

What is the process of flame emission spectroscophy

A

. Place a sample of the metal solution being tested into a flame. Light will be given off
. The light that is given off is captured by a spectroscope
. This instrument generates a line spectrum by distinguishing between light with different wavelengths

47
Q

Metal solution

A

A sample of the metal solution being tested is placed into a flame resulting in the emission of light

48
Q

Emitted light

A

The light that is given off is captured by a spectroscope

49
Q

Spectroscope

A

This instrument generates a line spectrum by distinguishing between light with different wavelengths

50
Q

Flame Emission Spectroscopy Analysis

A

. Identity of metal ions
. Concentration of metal ions
. Reference Spectra

51
Q

Identity of metal ions

A

Each metal ion will generate a new line spectrum. This means that we can identify all ions present in a solution.

52
Q

Concentration of metal ions

A

The concentration of an ion is indicated by the intensity of line spectra.

53
Q

Reference spectra

A

Unlike flame tests, flame emission spectroscopy allows us to analyse mixtures by comparing them with reference spectra.

Chemistry A (25 decks)

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