Chapter 17 - Spectroscopy Flashcards

Mass spectrometry, Infrared spectroscopy.

1
Q

What does the mass spectrometer detect?

A

The mass to charge ratio of the molecular ion, which gives the molecular mass of the compound.

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2
Q

How are molecular ions formed in mass spectrometry?

A

Electron is lost to form a positive ion

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3
Q

How is molecular mass found from a mass spectrum?

A

The molecular ion peak (M+ peak) has to be located.

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4
Q

What is the cause of the M + 1 peak?

A

Some carbon is present as the carbon-13 isotope.

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5
Q

What is fragmentation?

A

The molecular ions break down into smaller pieces called fragments.

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6
Q

What causes the other peaks in a mass spectrum?

A

Fragment ions

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7
Q

How can a mass spectrum differentiate between structural isomers?

A

They will break apart differently and thus will have different mass spectra.

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8
Q

What is the effect on a bond when it absorbs IR?

A

Bend or stretch more as they gain energy.

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9
Q

What does the amount a bond streches or bends depend on?

A

Mass of the atoms in the bond - heavier atoms vibrate more slowly than lighter atoms
Bond strength - Stronger bonds vibrate faster than weaker bonds

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10
Q

What is unique about each bond with regards to IR absorption?

A

Can only absorb radiation that has the same frequency as the natural frequency of the bond.

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11
Q

What scale is used for the frequency values?

A

Wavenumber 200/cm to 4000/cm

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12
Q

How does infrared spectroscopy work?

A

Sample placed inside an IR spectrometer,
A beam of IR radiation in range 200/cm to 4000/cm passed through the sample
Molecule absorbs some of the IR frequencies and the emerging beam is analysed to find the absorbed frequencies.

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13
Q

What is the term given to the dips in the graph?

A

Peaks

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14
Q

What do peaks help us predict?

A

The bond types in the molecules.

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15
Q

What is below 1500/cm known as on the IR spectrum and why?

A

Fingerprint region

The peaks in this region are unique to each molecule.

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16
Q

In an exam question what can be used to determine the molecule in question?

A

IR absorptions on formula sheet

17
Q

What is the typical sequence for identification of an unknown substance?

A

Elemental analysis - determine empirical formula
Mass spectrometry - Determine molecular mass and identify sections in molecule from fragment ions
Infrared spectroscopy - identify bonds and functional groups present
Identity of unknown compound can be worked out now.

18
Q

draw a diagram of a reflux experiment

A

draw a diagram of a distillation experiment

19
Q

What is reflux

A

The continuous heating of a sample where the vapours are condensed and returned to the vessel

20
Q

What is distillation

A

Heat and vapours are condensed and collected immediately