What happens to the covalent bonds in molecules when they absorb infrared radiation?

They increase their vibrational motion.

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What happens to the Sun's visible and IR radiation as it reaches the Earth's surface?

Most of it is absorbed by the Earth's surface but some is re-emitted in the form of longer-wavelength IR radiation.

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Why do greenhouse gases absorb longer-wavelength IR radiation?

They absorb longer wavelength IR radiation because it has the same frequency as the natural frequency of their bonds.

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What occurs when greenhouse gases absorb longer-wavelength IR radiation?

Upon absorption, these gases re-emit the energy back to Earth, warming the surface temperature.


The increasing levels of greenhouse gases in the atmosphere are causing the temperature of our atmosphere to increase i.e. global warming. 

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Name 3 greenhouse gases in the atmosphere.

  1. Water vapour (H2O).
  2. Carbon dioxide (CO2).
  3. Methane (CH4).

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What is infrared spectroscopy used for in organic chemistry?

Infrared spectroscopy is used to identify the functional groups present in organic molecules.

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What are the axes labels in an infrared spectrum?

The x-axis is labelled wavenumber, measured in units of cm-1.

The y-axis is labelled transmittance, measured as a percentage.

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What is the fingerprint region in an infrared spectrum?

The fingerprint region is below 1,500 cm-1 and it contains unique peaks which can be used to identify the particular molecule under investigation.

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What is the key absorbance peak in the IR spectrum of an alcohol?

The key absorbance peak in the IR spectrum of an alcohol is within the range 3, 300 - 3,600 cm-1 caused by the O-H bond.

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What is the key absorbance peak in the IR spectrum of an aldehyde or ketone?

The key absorbance peak in the IR spectrum of an aldehyde or ketone is within the range 1650 - 1850 cm-1 caused by the C=O bond.

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What are the key absorbance peaks in the IR spectrum of a carboxylic acid?

The key absorbance peaks in the IR spectrum of a carboxylic acid are within the range 1,650 - 1,850 cm-1 caused by the C=O bond, and a broad peak at 2,400 - 3,300 cm-1 caused by the O-H group.

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Describe two applications of infrared spectroscopy.

  1. Monitoring air pollution - Remote sensors analyse IR spectra of vehicle emissions to detect and measure pollutants like carbon monoxide, carbon dioxide, and hydrocarbons in areas with high traffic.
  2. Breathalysers - IR-based breathalysers pass IR radiation through a breath sample and measure the absorbance, detecting the presence of ethanol based on its characteristic bonds.

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What is the greenhouse effect?

The greenhouse effect is the process by which greenhouse gases in the atmopshere trap heat given off from the Earth's surface and reflect it back towards Earth. 


The increased levels of greenhouse gases in the atmosphere are amplifying the greenhouse effect which is leading to an increase in the temperature of the atmosphere i.e. global warming.

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