Chemistry S3 HL

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9 Terms

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Mass spectrometry

Mass spectrometry of organic compunds can cause fragmentation of molecules

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Fragmentation pattern

The molecules are vaporised and then bombarded with a beam of high-speed electrons shot from an electron gun. The collisions between the electrons and the molecule are so high that they cause the molecule to break up into molecular ions

The detected ions produce a mass spectrum. The peak with the highest m/z value is the molecular ion (M+ ) peak. This gives the molecular mass of the compound.

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Infrared spectroscopy

It can be used to identify the type of bond present in a molecule

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Wavenumber

The frequency of infrared radiation is measured as the number of waves per cm

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absorption of infrared

If an organic molecule is irradiated with infra-red energy that matches the natural vibration frequency of its bonds, it absorbs some of that energy and the amplitude of vibration increases

This is known as resonance

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modes of vibration

  • symmetric or asymmetric stretching

  • symmetric or asymmetric bending

  • These vibrations have a unique wavelength and frequency, which is used to identify the functional group

<ul><li><p>symmetric or asymmetric stretching</p></li><li><p>symmetric or asymmetric bending</p></li><li><p>These vibrations have a unique wavelength and frequency, which is used to identify the functional group</p></li></ul><p></p>
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condition for IR absorption

A molecule must have a permanent dipole

Symmetric molecules like H2 and O2 are IR inactive

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uses of IR spectroscopy

Pollution

  • Infrared spectroscopy is used to identify pollutants in vehicle emissions in the air

    • Sensors detect and measure the amount of pollutants such as carbon monoxide, carbon dioxide and unburnt hydrocarbons

    • This commonly occurs on motorways and in busy town centres to monitor localised pollution

Breathalysers

  • Infrared spectroscopy can be used to measure alcohol levels using roadside breathalysers

    • A ray of infrared radiation is passed through the breath that is exhaled into the breathalyser chamber

    • The characteristic bonds of ethanol are detected and measured - the higher the absorbance of infrared radiation, the more ethanol in the person's breath

Greenhouse gases

  • Greenhouse gases such as CO2, H2O, and CH4 absorb infrared radiation due to the vibrations of their polar bonds

  • This absorption contributes to the greenhouse effect, as the energy is re-emitted and trapped in the Earth's atmosphere

  • The effectiveness of a greenhouse gas depends on the type of bonds it contains and how strongly they absorb infrared radiation


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Nuclear Magnetic Resonance Spectroscopy

Proton nuclear magnetic resonance (HNMR) spectroscopy gives information on the different chemical environments of hydrogen atoms in a molecule