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It has come to our attention that some reference materials used to support teaching of the OCR AS/A Level Chemistry A (H032/H432) specification do not give appropriate descriptions of some of the new topics covered. We are pleased to clarify what we consider to be the correct approaches to these topics.
The descriptions used in the specification make clear that the category of aliphatic compounds includes the alicyclic compounds. That is, aliphatic compounds can be straight-chain, branched-chain or cyclic (non-aromatic). Alicyclic compounds are therefore also aliphatic.
E/Z nomenclature, applied using the Cahn–Ingold–Prelog priority rules, can be used to name any organic compound containing a C=C double bond.
Cis–trans nomenclature can be applied to any such compounds in which two of the substituent groups (one on each carbon atom of the C=C group) are the same. This nomenclature is therefore not confined to compounds where each carbon atom of the C=C group is bound to a hydrogen atom.
This means that E is not consistently equivalent to trans and Z to cis. For example, 1-bromo-1,2-dichloro-2-fluoroethene can be classified as cis or trans based on the position of the substituent chlorine atoms. Due to the priority rules, the cis isomer is also the E isomer.
E/Z nomenclature should be used by preference for systematic names.
The pre-exponential factor A in the Arrhenius equation can be derived from the y-axis intercept of the best-fit line on an Arrhenius plot; that is, a plot of ln k against 1/T. The y-axis intercept is equal to ln A.
Care must be taken when a graph has been plotted using experimental data. If a sensible scale is used for the x-axis, it is not likely to begin at 0. In this situation, it would be incorrect to determine A simply by extrapolating the line of best fit to the vertical axis. The line would need to be extrapolated back to the y axis at the point where x = 0.
To avoid this difficulty in determining A graphically, it is possible to determine A mathematically. Once the gradient of the line of best fit has been determined, a value from the line can be substituted into the linear Arrhenius equation and the equation rearranged to solve for the unknown ln A.
More on this approach to the Arrhenius equation can be found in the Mathematical Skills Handbook.
The complex [Cr(H2O)6]3+ is violet. However, solutions often contain other mixed complexes of chromium(III) ions, which are green. It is therefore common for Cr(III) to appear green in solution. Green is the expected observation when chromium(III) ions are produced in the oxidation of alcohols and aldehydes using acidified dichromate.
Addition of aqueous ammonia to [Cr(H2O)6]3+ initially results in formation of the grey-green precipitate Cr(OH)3. On adding further ammonia, the precipitate dissolves to form the complex ion [Cr(NH3)6]3+, which is purple.