Andrew VoylesIB Chemistry Assessment Statements

A.1.1: The need for analytical techniques has become increasingly important in past years, due to the need to understand the mechanisms and products of chemical reactions at a molecular level. These techniques allow us to determine structure, reaction mechanisms, substance composition, substance purity, and to identify and separate substances.

A.1.2: None of the different analytical techniques can, by itself, determine the structure of the compound. More than one must be used in conjunction for this to be possible. UV spectroscopy is used in identifying metal ions and conjugated pi bonds. Infrared spectroscopy is used in determining organic structure of functional groups, as well as bond length and strength. Mass spectrometry is used in determining the % abundance of isotopes, as well as organic structure, through any molecular groups that have been disassociated from the molecular ion. Chromatography is used in separating the components of compounds. Nuclear magnetic resonance is used in determining organic structure, through carbon framework and proton environments.

A.2.1 The electromagnetic spectrum is a spectrum of all electromagnetic radiation. EM radiation is the transfer of energy through space through waves, which have an electric field and magnetic field components.

Energy and frequency decrease with increasing wavelength. Visible light is between 400 nm and 700 nm.

A.2.2: The emissions spectrum is formed when substances release light of quantized wavelengths, after being provided sufficient energy. The absorption spectrum is formed by the wavelengths of light which are not absorbed by a substance when light is passed through it.

A.2.3: When UV and visible light is absorbed by a compound, the extra energy and excite electrons the substance to jump to higher energy levels. This is used determining the identity of metal complex ions. Infrared radiation is absorbed by molecules when they undergo a change in the dipole moment through vibration. The weak radio waves are absorbed by molecules during rotational transitions.

A.3.1: Below is seen a schematic diagram of a simple double-beam spectrometer.

A.3.2: Most organic molecules have dozens of different bond stretching and bending motions, resulting in dozens of absorption and on an infrared spectrum. Because functional groups have characteristic infrared absorptions that do not change from one compound to another, it is possible to identify compounds in terms of which functional groups are or are not present.

A.3.3: In the absorption of infrared radiation by H2O, if the frequency of radiation applied to the molecule matches one of the discrete energy levels of bond vibration of the molecule, the applied radiation can change the bond polarity, causing the bonds to stretch and bend.

A.3.4: Most organic molecules have dozens of different bond stretching and bending motions, resulting in dozens of absorption and on an infrared spectrum. Because functional groups have characteristic infrared absorptions that do not change from one compound to another, it is possible to identify compounds in terms of which functional groups are or are not present. However, is important to realize that it is not the functional group which produces the absorption peak, but the bonds that make up that group.

A.4.1: When a sample is bombarded with electrons in a mass spectrometer, it forms positive ions. The biggest molecular ion is called M+. The detector in the mass spectrometer detects only cations. The array of all the readings is called the fragmentation pattern. In a fragmentation pattern, the peak of highest abundance is assigned the name of base peak and the arbitrary abundance of 100. All the other fragments are then represented as peaks with heights that give their relative abundance. The molecular mass is the relative average of all isotopes of the compound present in the sample.

A.4.2: As the sample is bombarded by electrons, groups of atoms can be knocked off in the process, resulting in a other large peaks on the fragmentation pattern. Based on the difference between the mass of the fragment and the mass of the parent, it can be determined what the fragment is. For a parent mass M, (M-15)=lost CH3

(M-29)=lost C2H5 or CHO(M-31)=lost CH3O(M-45)=lost COOH

A.5.1: 1H NMR spectroscopy is performed using four basic procedures. First, the number of different absorption peaks are counted. Each peak corresponds to a different approach on environment in the compound. By integrating the area under each peak, the lowest ratio of hydrogen atoms in each electrical environment is obtained. By looking to see where the peak occurs on the horizontal axis of chemicals shifts, relative to the reference standard of zero chemical shift of Si(CH3)4, and comparing to a list of standard chemicals shifts for each functional

group, it is possible to determine what functional group this proton environment is part of. By analyzing how many individual smaller peaks make up each a large peak, it can be determined how many hydrogens are attached to the adjacent carbons.

A.5.2: Since protons in water, lipids, carbohydrates and proteins give different signals, they are used to make a map of each plane of the section of the body that was scanned. It is used to determine the abnormalities of the tissues, for example to detect tumors. Since there are no known side effects or damages to the body, the technique is used regularly.

A.6.1: Atomic absorption spectroscopy is used to determine the concentration of metals in water, blood, soils, oils, and food.

A.6.2: Atomic absorption is the reverse process of atomic emission spectra. In AA, the energy absorbed by electrons as they are excited to higher energy levels is measured. By measuring the intensity of the light after passing through an aerosolized sample burning in a flame cell relative to the intensity of the light before passing through the sample, it is possible to determine the concentration of an element in a particular sample. The light source is a sample of the metal being measured, and this method produces extremely sensitive results.

A.6.3: In the spectrophotometer, the fuel is a liquid solution of the compound containing the element in question, burning in a flame. It is from this that the concentration of the element being measured can be measured. The atomizer breaks the compounds making up the sample are broken into free atoms. The monochromatic light source is a sample of the metal being measured in an individual trial. Since the emissions spectrum and the absorption spectrum of an element are the same, this allows the absorption of just the sample to be accurately measured. The monochromatic detector detects the amount of light absorbed by the atoms and converts it into an electrical signal using a photomultiplier.

A.6.4: Since the path length can be fixed and the molar absorpitivity constant is constant, the absorbance is directly proportional to the concentration. The absorbance of the unknown sample at a given point is read and the concentration can be directly interpolated from the calibration curve.

A.7.1: Chromatography is a separating technique. Substances separated by chromatography are analyzed and identified by mass spectroscopy. It is extensively used in drug and food testing and can also be used as a technique to determine the purity of substances.

A.7.2: Components in a mixture have a different tendency to absorb onto a surface or dissolve in a solvent. This provides a means of separating the components, therefore all chromatographic techniques require a mobile phase (the solvent in the case of paper chromatography) and a stationary phase (the paper in the case of paper chromatography). The mobile phase in a chromatographic technique passes the sample over a stationary phase, which causes different species of molecules in a sample to separate.

A.7.3: Paper chromatography: Stationary phase is paper, the mobile phase is a solvent. The sample is placed on paper, the solvent then separates the solvent by capillary action and absorption of the sample.

Thin-layer chromatography: Stationary phase is a thin layer of gel on a hard surface, the mobile phase is a solvent. Similar functioning to paper chromatography.

Column chromatography: Stationary phase is an inert gel soaked with a solvent, mobile phase is a second solvent. The solvent is poured on the solid, and left to soak. The second solvent with sample is poured over and left to separate. Different components form different bands.

A.8.1: For d-block elements to behave as transition metals and form complex ions, the d-orbital has to be partly filled. It is now believed that the d-orbital is divided into five sub-orbitals. Three of them are less energetic and two of them are more energetic. Ligands, including NH3, H2O, and Cl-, act as Lewis bases, and donate a pair of non bonding electrons to form a coordinate bond. As the ligands approach the metal along the axes, they repel the two orbital oriented along the axes, causing the five d orbitals to split, three to lower energy and two to higher energy.

A.8.2: The electronic configuration of the transition element, its oxidation state, the identity of the ligand, and the molecular geometry of the complex all affect the color of the transition metal complex ion.

A.8.3: Organic molecules containing a double bond absorb ultraviolet radiation.

A.8.4: Unsaturated compounds containing conjugation ( alternating double single carbon-carbon bonds) require high energy to absorb. As a result, many organic compounds containing conjugation absorb in the ultraviolet range and thus appear colorless.

A.8.5: A particular molecule must absorb either ultraviolet or visible radiation, as all matter has color, and color is the result of absorption.

A.8.6: Since the path length can be fixed and the molar absorpitivity constant is constant, the absorbance is directly proportional to the concentration. The absorbance of the unknown sample at a given point is read and the concentration can be directly interpolated from the calibration curve.

A.9.1: TMS is used as a reference standard, due to the fact that all the protons are in the same environment, it is not toxic, it is very unreactive, it absorbs well away from most other protons, and has a low boiling point, making it easily removable from the sample.

A.9.2: 1H NMR spectroscopy is performed using four basic procedures. First, the number of different absorption peaks are counted. Each peak corresponds to a different approach on environment in the compound. By integrating the area under each peak, the lowest ratio of hydrogen atoms in each electrical environment is obtained. By looking to see where the peak occurs on the horizontal axis of chemicals shifts, relative to the reference standard of zero chemical shift of Si(CH3)4, and comparing to a list of standard chemicals shifts for each functional group, it is possible to determine what functional group this proton environment is part of. By analyzing how many individual smaller peaks make up each a large peak, it can be determined how many hydrogens are attached to the adjacent carbons.

A.10.1: Gas-liquid chromatography: Stationary phase is a liquid, mobile phase is a gas carrying the samThe ple. The gas is bubbled through the liquid, different substance separate, are detected and condensed.

High-performance liquid chromatography: this method is similar to column chromatography, but under high pressure. Stationary phase is a grating, the mobile phase is a solvent, it is poured over the grid, and then high pressures are applied to speed up the process.

A.10.2: If food or drugs are involved, Gas-Liquid chromatography should be used. In the case of pigments, paper chromatography can be applied. In a mixture of ions, ion exchange chromatography should be

used. In the case of molecules of different sizes present, high performance liquid chromatography should be applied. In the case of separation of Amino acids, thin layer chromatography should be used.