Infra Red (IR) Spectroscopy | A-level Chemistry | OCR, AQA, Edexcel

Introduction

This tutorial will guide you through the fundamentals of Infrared (IR) Spectroscopy, a crucial technique in A-level Chemistry. You'll learn about the vibrations of molecules, how to create an IR spectrum, and explore various applications of IR spectroscopy, including its role in environmental monitoring and forensic science. Understanding IR spectroscopy is vital for grasping molecular behavior and chemical analysis.

Step 1: Understand Molecular Vibration and IR Radiation

• Molecules consist of atoms that are constantly vibrating.
• Infrared Radiation is absorbed by these molecules, which increases their vibrational energy.
• Key types of vibrations include:
  • Stretching: Bonds lengthen and shorten.
  • Bending: Bonds change angles.
• Vibration occurs at specific frequencies, typically between 300 cm^-1 and 4000 cm^-1 in the IR region.
• Factors affecting vibration frequency include:
  • Bond Strength: Stronger bonds vibrate at higher frequencies.
  • Bond Length: Longer bonds typically vibrate at lower frequencies.
  • Mass of Atoms: Heavier atoms vibrate more slowly.

Step 2: Explore the Impact of IR Radiation on Global Warming

• Greenhouse gases absorb IR radiation emitted from the Earth's surface, preventing heat from escaping into space.
• This absorption contributes to the greenhouse effect, which results in global warming.
• Increased levels of greenhouse gases exacerbate temperature rise, highlighting the importance of monitoring these gases.

Step 3: Create an IR Spectrum

To create an IR spectrum, follow these steps:

1. Pass a beam of infrared radiation through your sample.
2. The sample's molecules will absorb certain frequencies of the IR radiation.
3. Analyze the beam that passes through the sample.
4. Plot a graph of transmittance against frequency to create the infrared spectrum.
   • Peaks in this spectrum indicate specific vibrations from bonds in the molecule.

Step 4: Examine Applications of IR Spectroscopy

IR spectroscopy has several practical applications:

• Breathalyzers: Used to detect the -OH functional group in alcohol, with absorption levels correlating to blood alcohol concentration.
• Air Quality Monitoring: Measures pollutant levels, including carbon monoxide (CO) and nitrogen oxides (NO).
• Forensic Science: Identifies chemicals present at crime scenes, such as fuels and plastics.

Step 5: Understand the Fingerprint Region

• The fingerprint region is between 500 cm^-1 and 1500 cm^-1.
• Absorptions in this range are primarily due to bending vibrations and are unique to specific molecules.
• This region is valuable for identifying substances based on their unique spectral fingerprints.

Conclusion

Infrared spectroscopy is an essential analytical tool in chemistry, providing insights into molecular structure and behavior. Key takeaways include understanding molecular vibrations, creating IR spectra, and recognizing the practical applications of this technique. As you continue your studies in chemistry, consider exploring more complex spectroscopic methods and their relevance in various scientific fields.