IB Chemistry home > Syllabus 2016 > Data Processing > Structural identification of unknown compounds

Syllabus ref: 11.3 Syllabus ref: 21.1

Chemical analysis means the separation and identification of substances, both qualitatively and quantitatively.

Purity and identification

Chemistry is the study of pure substances. Chemical analysis has two separate strands:

However, this leads directly into a problem, as most substances occur as part of a mixture. We have to be able to separate substances into their individual components and then determine just how successful we have been to ensure that we really are dealing with purity. This means that we can also add a third endeavour of the chemical analyst:

We will deal with each of these areas in turn.


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Qualitative analysis

This involves identification of unknown substances. It is rather like a puzzle where you gather the clues from a series of tests and arrive at logical conclusions as to cause of the evidence.

In the past most, of this kind of analysis was conducted using chemical reaction in test tubes, so-called 'wet chemistry', however, nowadays such crude methods are largely superceded by methods involving instrumentation. Most instrumental methods involve directing energy in the form of electromagnetic radiation into the substance and recording the effect.

Although these are the most important, they are just a few of the many techniques available. The actual choice of technique very much depends on the nature of the material under investigation.


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Determination of structure

Determination of structure (discovering the structure of a compound) is a very important part of chemistry. The problem is that the molecules, or crystal lattices, cannot be 'seen' as such and indirect evidence for the structure must be gleaned by a variety of methods. This is particularly important in organic chemistry where the structure may be complicated.

A structural determination may be derived from evidence gathered from a variety of techniques, each one providing evidence leading towards the final conclusion. It is not likely that one technique alone provides sufficient evidence.

The final 'proof' is often the synthesis of the compound from simpler units, producing a substance with identical characteristics to the unknown.


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Quantum theory

Particles in the microscopic world behave in a different way to the big wide world in which we live. Any form of motion that particles may have which gives rise to a fluctuating electromagnetic field can interact with electromagnetic radiation causing a change in energy level. In the microscopic world of particles the energy levels are not continuous, they are said to be 'quantised'. This means that only certain values are allowed.

This concept of quantisation seems counter-intuitive. It is obvious that a car can have any velocity between its slowest state (i.e. zero) and its maximum speed. However, this is not the case in the quantum world. It is as if a car is only able to travel at 10 km/hr and 20 km/hr, but not at any speed in between. In order for the car to change velocity from 10 to 20 km/hr it must gain exactly the required amount of energy. Too little energy and it cannot attain 20 km/hr and so must stay at 10 km/hr. Too much energy and it would go beyond 20km/hr, which is also forbidden, so it can't happen.

The energy levels are usually represented by horizontal lines with the lowest energy level at the bottom - the ground state. The difference in energy between two levels is the energy required for transition to a higher state, i.e. the quantum that must be absorbed to change from one energy level to another. Most systems show convergence as the energy level gets higher, i.e. the levels get closer together.


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Mechanism of absorption

Electromagnetic radiation is absorbed by molecules and atoms in different ways depending on the wavelength, i.e. energy of the waves. In all cases, however, the energy is absorbed and used to change the energetic state of some characteristic property within the atom or molecule.

Atoms can change energy levels:

Molecules can also change energy levels:

In all of the above processes the higher energy states attained are unstable and the atoms or molecules involved return back to the lowest state again soon after. This is a process known as relaxation.

In all cases the fundamental mechanism follows the same steps:


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Electron transitions

Electrons can only have certain allowed energy levels, called quantum states. Any one electron within an atom can be defined by four quantum numbers:

Hundt's rule states that no two electrons within an atom can have the same four quantum numbers.

Although this may seem confusing at first sight, in fact any student who has studied the Aufbau Principle is familiar with the idea that electrons can only occupy distinct positions within an atom.

The principle quantum number corresponds to the main energy level. The subsiduary quantum number corresponds to the sub-level. The magnetic quantum number corresponds to the specific orbital and the spin quantum number corresponds to the spin of the electron with the orbiital.

It should already be known that two electrons cannot occupy the same orbital with the same spin, hence Hundt's rule above.

Electrons can absorb electromagnetic radiation of appropriate wavelength to move between different energy levels. These are called electron transitions. Absorption of energy moves the electron to a higher level and on relaxation the electons emits the same amount of energy and returns to a lower level. Animation

Visible and ultra-violet light causes transitions between outer shell electrons and higher levels, whereas X-rays cause transitions from the inner shells of atoms to higher levels. In this book we are only concerned with transitions caused by visible and UV light.


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Bond vibration

Polar molecules have an associated electromagnetic field that can change when the molecule vibrates. If the overall polarity of the molecule does not change when a vibration occurs then this type of vibration cannot interact with, and hence absorb, electromagnetic radiation. Electromagnetic radiation in the Infra-Red region of the spectrum is able to change the vibrational states of polar molecules.

There are many types of vibration available to molecules, which get ever more complex as the complexity of the molecule increases. We can appreciate this by looking at the simple cases of di- and tri-atomic molecules.

A di-atomic molecule can only vibrate in one way, i.e. the atoms can move apart and back together again. This is called a stretching vibration. This animation shows the stretching vibration of the hydrogen chloride molecule, HCl.

Tri-atomic molecules, such as water, are more complex and can vibrate in several ways, broadly described as stretching and bending. This is shown in the next animation.


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Molecular rotation

Molecules are also free to rotate and, once again, if they are polar this rotation constitutes a changing electromagnetic field that can interact with electromagnetic radiation to change the molecules' rotational energy levels.

Rotational energy levels are very close together and are of limited value in analysis, although they can be used to get information about the bond length in simple molecules.

To understand rotational energy levels the simplest case is that of a diatomic polar molecule such as HCl that behaves as if it were a tiny dumbell.


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Nuclear transitions

Nuclei that have an odd number of nucleons produce an associated electromagnetic field when they spin. This field interacts with electromagnetic radiation of appropriate energy to change the spin state. The number of possible spin states that are available to the nucleus depends on the number of nucleons. Hydrogen for instance has only 1 nucleon, a proton, and has only 2 available spin states, designated +1/2 and -1/2.

Nuclear magnetic resonance makes use of these spin states to investigate the environment of hydrogen atoms in compounds. This is dealt with in greater detail in the section on NMR.


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Summary

Movement of electrical charge creates electromagnetic fields that can interact with electromagnetic radiation. This effect is made use of in the investigation and analysis of compounds and elements using different techniques.

The following tabe shows the relationship between the energy of electromagnetic radiation and the transitions possible.

Radiation Wavelength Interaction and effect
X-rays 1 x 10-12 Inner shell electron transitions
UV radiation 1 x 10-9 Molecular orbital electron transitions
Visible light 6 x 10-7 'd' orbital transitions
IR radiation 1 x 10-7 Molecular vibrations
Microwave radiation 1 x 10-6 Molecular rotations
Radiowaves >1 x 10-3 Nuclear spin transitions

REM Micro = 10-6, as in micrometre meaning one millionth of a metre.


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