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A-level Chemistry > DM - Colorimetry and visible spectroscopy > Flashcards

DM - Colorimetry and visible spectroscopy Flashcards

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1
Q

What are the colours you see from?

A

They are the complements of those that are absorbed.

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2
Q

What happens when white light hits a transition metal ion?

A

One frequency of the light is absorbed. The rest of the frequencies are transmitted and it’s the transmitted frequencies that correspond to the colour that you see.

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3
Q

What are complementary colours?

A

The colours that are opposite of each other on the colour wheel.

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4
Q

What happens if you mix two complementary colours of light together?

A

You get white light.

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5
Q

How do you get white light?

A

By mixing any two complementary colours together.

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6
Q

The colour of the transition metal solution is…

A

…the complement of the colour that is absorbed.

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7
Q

If a colour appears green, what colour must be absorbed?

A

Red

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8
Q

If a colour appears violet, what colour must be absorbed?

A

Yellow

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9
Q

If a colour appears red, what colour must be absorbed?

A

Green

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10
Q

If a colour appears yellow, what colour must be absorbed?

A

violet

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11
Q

If a colour appears orange, what colour must be absorbed?

A

Blue

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12
Q

If a colour appears blue, what colour must be absorbed?

A

orange

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13
Q

What does the colour that a transition metal absorbs depend on?

A

The oxidation state of the ion, the type of ligand and the coordination number/shape of the complex.

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14
Q

What can colorimetry be used for?

A

To find concentrations of transition metal ions in a solution.

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15
Q

What is a colorimeter?

A

A meter to measure colour.

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16
Q

What does a colorimeter measure?

A

The absorbance of a solution.

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17
Q

Why can a colorimeter be used to find the concentrations of transition metal ions in a solution?

A

Because as their concentration increases so will the absorbance.

18
Q

How does a colorimeter work?

A

By filtering a source of white light into monochromatic light.

19
Q

Monochromatic light

A

Has a narrow frequency range, so only has one colour.

20
Q

Explain the process of colorimetry

A

1) Pick a filter that produces a colour that is absorbed by the transition metal you want to look at (otherwise it won’t be absorbed and you can’t measure any change).
2) Set the colorimeter to zero by measuring the absorbance of a blank sample.
3) After passing through the filter, the light goes through the sample where some of it is absorbed. The samples are each placed in a clean cuvette. The remaining light travels to the detector, which compares the absorbance of the sample to the blank.
4) A high absorbance reading means a lot of light has been absorbed and the sample is very concentrated.

21
Q

What is a blank sample?

A

A sample of the solvent your metal ions are dissolved in - usually just water.

22
Q

What does a blank sample do in colorimetry?

A

Tells the colorimeter what zero is, so that it is only measuring the absorbance of the metal ions and not the solvent too.

23
Q

What is a cuvette?

A

A small, rectangular tube with clear sides.

24
Q

What is a cuvette used for in colorimetry?

A

Used to hold liquid samples in experiments involving light.

In colorimetry it is designed not to absorb any of the light.

25
Q

What does a high absorbance in colorimetry mean?

A

That a lot of light has been absorbed and the sample is very concentrated.

26
Q

What does a low absorbance in colorimetry mean?

A

That not a lot of light has been absorbed and the sample is not very concentrated.

27
Q

What can colorimeters be used to find?

A

Concentrations of transition metal ions.

28
Q

What do you need to work out the concentration of a transition metal solution?

A

A calibration graph.

29
Q

How is a calibration graph made?

A

By measuring the absorbance of samples with a known concentration, usually around the expected concentration of your sample.

You can then plot the absorbance of each of these samples against their concentration to get a calibration graph.

30
Q

How can different samples be made to make a calibration graph?

A

These standard samples can be made by dissolving different amounts of the metal ion in a fixed volume of water or by making consecutive dilutions of a single concentrated solution.

They must contain the same metal ion as the unknown sample and be in the same solution or solvent.

You should use a burette/pipette to take accurate volume readings.

31
Q

How do you use a calibration graph to find concentrations of transition metal ions?

A

When you measure the absorbance of the unknown sample you can use the standard curve to read off its concentration.

32
Q

What are the axis for a calibration graph?

A

X-axis = Concentration of ion

Y-axis = Relative absorbance

33
Q

What can visible spectroscopy be used to find?

A

Concentrations of ions.

34
Q

What does visible absorption spectroscopy use?

A

A beam of monochromatic light - light of a single colour.

35
Q

White light

A

A mixture of all the different wavelengths and frequencies of visible light.

36
Q

How can white light be split into monochromatic light?

A

Using a filter.

37
Q

What machine is used in visible spectroscopy?

A

A visible spectrophotometer.

38
Q

What is a visible spectrophotometer used in?

A

Visible spectroscopy.

39
Q

Explain how visible spectroscopy is done using a visible spectrophotometer

A

1) The beam of monochromatic light is passed through a solution of the complex. A detector measures the intensity of light before and after it’s passed through the solution. From this you can then calculate the absorbance (how much light the solution has absorbed).
2) Different frequencies of light are passed through the solution to produce a visible absorption spectrum. The peaks in the graph tell you which colours of light the complex absorbs most strongly.
3) Different complexes absorb different colours, so produce characteristic visible absorption spectra. This allows you to identify the complexes. You can then use a calibration curve to convert absorbance data into concentration data.

40
Q

What is a visible absorption spectrum?

A

A graph of frequency or wavelength against absorbance.

41
Q

What do the peaks in the absorption spectrum tell you?

A

Which colours of light the complex absorbs most strongly.

42
Q

What are the axis of absorbance spectra?

A

X-axis = Either wavelength or frequency.

Y-axis = Absorbance.

A-level Chemistry (51 decks)

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