Light and Matter Main Concept: The interaction of electromagnetic waves or light with matter is a powerful means to probe the structure of atoms and molecules, and to measure their concentration.  

Energy, Wavelength, and Frequency Equation How Light Affects Matter Light and Matter Energy, Wavelength, and Frequency Equation E, h, v, c, λ How Light Affects Matter and What We Learn In general… X-rays to ultraviolet… Infrared… Ultraviolet to visible… Beer’s Law What it says and how we can use it Absorbance vs Transmittance Concentration

- Planck’s equation: E=hv - When photons are absorbed/emitted by molecules, energy of molecules is increased/decreased by same energy amount of photon - relates energy of a photon (E) to frequency of the electromagnetic wave (v), “h” is Planck’s constant

- v = λ/c or c = λv - c = speed of light (constant) - λ = wavelength - v = frequency

Photoelectron Spectroscopy (PES) utilizes x-rays to high ultraviolet light to analyze the electronic structure of the atom

- Different types of molecular motion lead to absorption/ emission of photons in different spectral regions - Infrared radiation (lower energy)  transitions in molecular vibrations - detect the presence of different types of bonds (usually covalent b/w H, C, N, O; single, double, triple; based on stretching of bonds; different bonds give different amts of energy)

- Ultraviolet/visible radiation (higher energy)  transitions in electronic energy levels - by exciting electrons  probe electronic structure (often for number of double bonds [pi bonds] or number of lone pairs) and metal cations

Question: N2 molecules absorb ultraviolet light but not visible light. I2 molecules absorb both visible and ultraviolet light. Which of the following statements explains the observations? (A) More energy is required to make N2 molecules vibrate than is required to make I2 molecules vibrate. (B) More energy is required to remove an electron from an I2 molecule than is required to remove an electron from a N2 molecule. (C) Visible light does not produce transitions between electronic energy levels in the N2 molecule but does produce transitions in the I2 molecule. (D) The molecular mass of I2 is greater than the molecular mass of N2.

Question: UV-vis spectroscopy would be ideal to determine the concentration of aqueous solutions of which of the following? (A) II only (B) I and III only (C) I, II, and IV (D) I, II, III, and IV

- amount of light absorbed by solution can be used to determine concentration of the absorbing molecules via the Beer-Lambert law (or Beer’s Law) - Beer’s law relates measured absorbance to molar absorptivity, path length, and analyte concentration

- Beer’s Law: A = a b c - A = absorbance - a = molar absorptivity (a constant depending on substance, assumes constant wavelength and temperature) - b = path length - c = concentration path length

- molarity is most common method used in lab to express concentration - solute: substance being dissolved (lesser amt) - solvent: substance solute is dissolved in (greater amt) - Molarity is defined as the number of moles of solute per liter of solution. - Molarity, M = moles of solute per liter of solution

- To prepare a solution directly: - prepare mass amt of solute as required - add distilled water to solute until desired solution volume is reached (as needed for molarity) - To prepare a solution via dilution: - use M1V1 = M2V2 - M1 = molarity of desired solution - V1 = volume of desired solution - M2 = molarity of stock solution - V2 = volume of stock solution you need (unknown) - measure out volume of stock solution and add distilled water until desired volume is reached

- Transmittance: how much light is emitted by a sample; generally, higher concentrations transmit less light - Absorbance: how much light is absorbed by a sample; generally, higher concentrations absorb more light and used most often - Both are dependent on wavelength used