Mechanism common mistakes

All areas of the cooling water system where a specific form of damage is likely to be found are described. The corrosion or failure causes and mechanisms are also described. Especially important factors influencing the corrosion process are listed. Detailed descriptions of each failure mode are given, along with many common, and some not-so-common, case histories. Descriptions of closely related and similarly appearing damage mechanisms allow discrimination between failure modes and avoidance of common mistakes and misconceptions. 

Experience tells us that whilst many students find mechanisms easy and logical, others despair and are completely bewildered. We cannot guarantee success for all, but we hope that by showing a few of the common mistakes we may help some of the latter group join the former. In order to make the examples chosen as real as possible, these have all been selected from students examination answers. The mechanisms relate to reactions we have yet to meet, but this is not important. At this stage, it is the manipulation of curly arrows that is under consideration. You may wish to return to this section later. 

Note also that we illustrated some common mistakes in drawing mechanisms, all taken from students examination answers, in Box 5.1. 

Before we illustrate this procedure, let s consider a few common mistakes. Avoiding these mistakes will help you to draw correct mechanisms throughout this course. 

Chemat et al. [14] found the ]oint use of US and microwaves for the treatment of edible oils for the determination of copper to shorten the time taken by this step to about a half that was required in the classical procedure (heating in a Buchi digester) or with microwave assistance, nitric acid and hydrogen peroxide. However, they did not state the specific medium where the microwave-US-assisted method was implemented and assumed US to have mechanical effects only, even though they mentioned a cavitational effect. This is a very common mistake in working with US that is clarified in an extensive discussion by Chanon and Luche [15] of the division of sonochemistry applications into reactions which were the result of true and false effects. Essentially, these terms refer to real chemical effects induced by cavitation and those effects that can be ascribed to the mechanical impact of bubble collapse. The presence of one of these phenomena only has not been demonstrated in the work of Chemat et al. [14] — despite the illustrative figure in their article — so their ascribing the results to purely mechanical effects of US was unwarranted. 

Reversing the direction of one or more arrows during a chemical step is the most common mistake made by students when writing organic reaction mechanisms. Backward arrow pushing usually derives from a student thinking about the movement of atoms, not the movement of electrons. Hence, to avoid this mistake it is important to remember that arrows depict how electrons move, not where atoms move, within or between chemical structures. Further, one can avoid this mistake by remembering that every arrow must start at an electron source (a bond or lone pair) and terminate at an electron sink (an atom that can accept a new bond or lone pair). 

Concerning the luminescence of the lanthanides, the f f transitions are actually described by the spectroscopic levels of the ion, either down to the groimd state or down to an intermediate level. Usually, the visible emissions of the lanthanide ions have transitions that change the total spin number of the ion, i.e., the (2 5 + 1) multiplicity, whereas the NIR emissions do not change the spin. Since several lanthanides exhibit both mechanisms, the term litminescence is preferred over fluorescence or phosphorescence for the lanthanide ions. In this way, the common mistake of calling fluorescence all kind of emissions is avoided. Becatrse of their forbidden character, f-f transitions are slow and the lanthanide luminescence may take up to a few miUiseconds (jns = 10 s). 

In summary, the mechanism for a nucleophilic acyl substitution reaction involves two core steps— nucleophilic attack and loss of a leaving group. Notice that these are the same two steps involved in an 5 2 process. However, there is one important difference. In an Si,i2 process, the two steps occur in a concerted fashion (simultaneously), but in a nucleophihc acyl substitution reaction, the two steps must occur separately. It is a common mistake to draw these two steps as occurring together. 

The use of curved arrows to depict electron-pair movement is a nseful technique that will prevent us from making the common mistake of changing the total nnmber of electrons when we draw resonance forms. It is also advantageons in keeping track of electrons when formulating mechanisms (Sections 2-2 and 6-3). 

A few dozen grammar, spelling, punctuation, and capitalization mistakes account for the majority of common writing errors. Once you become acquainted with these common errors and learn how to avoid or correct them, your writing will greatly improve. Therefore, this section on mechanics will focus on the errors that occur most frequently. 

Before you submit your essay, there s one more important step proofreading. Good proofreading invoives far more than a simple run of spell and grammar check on your computer. In fact, those programs are not foolproof, and therefore, a reliance on them alone to find your errors is a mistake. However, they are not a bad place to start. This lesson explains how to use these tools to your advantage, as well as how to find and correct the most common grammar and mechanics errors. 

The case report form (CRF) should be unambiguous and simple to use. Its completion should minimise the need for text. CRFs should consist of three modules. One module is common for all trials (laboratory data, etc.), one is common for all trials in the clinical programme for a given compound and one is specific to the study in question. In this way, data handlers become familiar with the forms and can therefore manage a larger number with fewer mistakes. A mechanism should be in existence to ensure that the clinician completes the CRF adequately. 

The explosive valves used in the liquid poison injection system in BWRs have the characteristic of not being subject to leaks as their closure is ensured by a membrane which is destroyed by the explosive charge. They, moreover, have a high reliability because of the absence of mobile mechanical parts. Operating experience, however, indicates a certain number of cases where the electric connections for their actuation were erroneously made, making the valve inoperable. If this mistake is due to erroneous installation instructions, then the latter comprise a dangerous common cause failure. 

Each of the following mechanisms contains one or more errors—that is, the curved arrows may or may not be correct. In each case, identify the errors and then describe what modification would be necessary in order to make the curved arrows correct. Explain your suggested modification in each case (the following examples represent common student errors, so it is in your best interests to identify these errors, recognize them, and then avoid these mistakes) ... 

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