absorbance

Examples of absorbance in the following topics:

• The Greenhouse Effect

  • The greenhouse effect is an elevation in surface temperatures due to atmospheric gases absorbing and re-radiating thermal energy.
  • This thermal radiation from the surface has a much longer wavelength than the solar radiation that was initially absorbed.
  • The majority of gases in the atmosphere, such as nitrogen, oxygen, and argon, cannot absorb this infrared radiation.
  • Gases known as greenhouse gases, including water vapor, carbon dioxide, ozone, and methane, absorb and trap this heat as it tries to escape from the atmosphere.
  • The cloud layer can also absorb infrared radiation and contribute further to the greenhouse effect.

• UV-Visible Absorption Spectra

  • The resulting spectrum is presented as a graph of absorbance (A) versus wavelength, as in the isoprene spectrum shown below.
  • Since isoprene is colorless, it does not absorb in the visible part of the spectrum and this region is not displayed on the graph.
  • Because the absorbance of a sample will be proportional to the number of absorbing molecules in the spectrometer light beam (e.g. their molar concentration in the sample tube), it is necessary to correct the absorbance value for this and other operational factors if the spectra of different compounds are to be compared in a meaningful way.
  • (where A= absorbance, c = sample concentration in moles/liter & l = length of light path through the sample in cm. )
  • Such light absorbing groups are referred to as chromophores.

• Experimental Determination of Reaction Rates

  • The rate of this reaction can be monitored by measuring the absorbance of the solution.
  • The absorbance is given by Beer's law:
  • By Beer's law, the absorbance of the solution is directly proportional to the concentration of the C60O3 in solution, so observing the absorbance as a function of time is essentially the same as observing the concentration as a function of time.
  • Therefore, a plot of the rate versus the absorbance will yield a straight line with a slope of k.
  • The absorbance is proportional to the concentration of the C60O3 in solution, so observing the absorbance as a function of time is essentially the same as observing the concentration as a function of time.

• Exothermic and Endothermic Processes

  • Endothermic reactions absorb energy from the environment, while exothermic reactions release energy to the environment.
  • When a reaction proceeds, it either releases energy to, or absorbs energy from, its surroundings.
  • At constant pressure, the change in enthalpy is equal to the heat given off, or the heat absorbed, in a given chemical reaction:
  • Therefore, the change in enthalpy is positive, and heat is absorbed from the surroundings by the reaction.
  • Paul Andersen explains how heat can be absorbed in endothermic or released in exothermic reactions.

• Complex Ion Equilibria and Solubility

  • The amount of light absorbed by the red complex is measured at 447 nm, the wavelength at which the complex most strongly absorbs.
  • The absorbance (A) of the complex is proportional to its concentration (M) and can be measured directly on the spectrophotometer:
  • The Beer-Lambert Law relates the amount of light being absorbed to the concentration of the substance absorbing the light and the path length through which the light passes:
  • In this equation, the measured absorbance (A) is related to the molar absorptivity constant (ε), the path length (b), and the molar concentration (c) of the absorbing species.
  • The equation shows how the concentration is directly proportional to absorbance.

• Photochemistry

  • The first law of photochemistry, the Grotthuss-Draper law, states that light must be absorbed by a compound in order for a photochemical reaction to take place.
  • The second law of photochemistry, the Stark-Einstein law, states that for each photon of light absorbed by a chemical system, only one molecule is activated for subsequent reaction.
  • Thus, we may define quantum yield as "the number of moles of a stated reactant disappearing, or the number of moles of a stated product produced, per einstein of monochromatic light absorbed

• Measuring Radiation Exposure

  • Radiation dosimetry is the measurement and calculation of the absorbed dose from exposure to indirect and direct ionizing radiation.
  • Radiation dosimetry is the measurement and calculation of the absorbed dose in matter and tissue resulting from exposure to indirect and direct ionizing radiation.

• Nuclear Reactors

  • When a large, fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission.
  • A portion of these neutrons may later be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, and so on.
  • Since moderators both slow and absorb neutrons, there is an optimum amount of moderator to include in a given geometry of reactor core.
  • In the second step, one of those neutrons is absorbed by an atom of uranium-238, and does not continue the reaction.
  • Another neutron leaves the system without being absorbed.

• Heating Curve for Water

  • After all of the solid substance has melted into liquid, the temperature of the liquid begins to increase as heat is absorbed.
  • It is then possible to calculate the heat absorbed by: $q=m\cdot C_{H_2O(l)}\cdot \Delta T$.
  • The liquid will begin to boil when enough heat has been absorbed by the solution that the temperature reaches the boiling point, where again, the temperature remains constant until all of the liquid has become gaseous water.
  • Use the heat of vaporization ($\Delta H_{vap}$ ) to calculate how much heat was absorbed in this process: $q=m\cdot C_{H_2O(g)}\cdot \Delta T$, where m is the mass of the sample of water.
  • After breaking the bonds, heat is then absorbed and converted to increased kinetic energy of the molecules in order to vaporize them.

• Introduction

  • Consequently, virtually all organic compounds will absorb infrared radiation that corresponds in energy to these vibrations.
  • Thus a sample that did not absorb at all would record a horizontal line at 100% transmittance (top of the chart).
  • Liquids are usually examined as a thin film sandwiched between two polished salt plates (note that glass absorbs infrared radiation, whereas NaCl is transparent).