Yield is a term used in chemistry and chemical engineering to describe the performance of a reactor or a plant. More specifically, it is a measure for the amount of a desired product formed in a chemical reaction. This factor largely determines the cost of a chemical process and the desired product. There are several definitions of yield, and it is important to always state the basis of any yield figures. Unfortunately, this isn't always explicitly done.
The first definition of yield, the reaction yield or chemical yield is only dependent on chemical losses to undesired products, or side products. The choice of what is a desired resp. undesired in this case depends on the objective of the chemical process. However, it is important to understand how undesired products are formed in a chemical process. This can occur by either parallel reactions (competing reactions), or series reactions (consecutive reactions).
Parallel reactions are of the type:
R → D R → U
O / \ CH2=CH2 + O2 → CH2=CH2 CH2=CH2 + O2 → 2 CO2 + 2 H2O
R → D → U
CH2=CH2 + H2O → C2H5OH 2 C2H5OH → (C2H5O)2O + H2O
YD = ND / (NR0 - NR)
Where YD is the reaction yield of product D, NR0 is the initial number of moles of reactant R, and NR is the number of moles of reactant R at the end of the reaction. This is the definition for batch reactors. The definition for flow reactors is similar: in this case the ratio of flow rates for the reactor species is taken instead of the number of moles. The second definition of reaction yield is based on the ratio of reaction rates. In this case the yield is equal to the rate of formation of the desired product divided by the rate of disappearance of reactant R:
YD = rD / -rR
The reaction yield does not take into account physical losses of product, but only chemical losses to side products. A second definition, the reactor yield also include physical losses. And third, there is a plant yield, which is applied to a complete chemical plant, or a stage of a chemical plant:
Plant Yield = (mols D produced)×(stoichiometric factor)/(mols R fed to process)
In python, a function can yield a value, rather than return one (e.g. yield x). This makes it a generator, which is a rather nifty (though frequently unnecessary) kind of iterator (actually, an iterable object). When the "function" is called, rather than executing, it returns a generator-iterator. Whenever the iterator is nexted, function execution resumes where it was left off, with local variables intact, as they were. When a yield statement is executed, the iterator returns the yielded value, and execution is suspended until the next next.
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Professor Pi's writeup was very good (I have never seen the stuff on reaction rate yields before and I'm midway through my 2nd year in a Masters in Chemistry) but I would like to add more on the percentage yield of batch products.
Usually, the most useful yield calculation for batch reactions is the percentage yield. The reasons for this should be obvious, assume one person is running a crappy reaction, say an aldol reaction on the kilogram scale of an aldehyde with a hindered ketone and looking for the aldehyde-ketone product. The aldehyde will self-condense and there will be minimal aldehyde-ketone formation, maybe 3g. Another chemist, who has thought about what they are doing, runs an aldol reaction on a milligram scale which can only go in one direction and isolates 0.3g. On the pure yields, the crappy aldol reaction is more succesful. However, the percentage yield cancels out the inequalities between the amounts of starting materials.
It can be defined crudely as:
Amount of product obtainedx 100
Theoretical maximum product
While it is usually worked out in moles (chemical measure of number of molecules) the logic remains the same. If the reaction you are doing should double the size of your starting material, starting with a kilo should give 2 kilos of product. Obtaining 1.8 kilos would be a 90% yield etc.
It should be noted that yields of >100% are well known and yields of up to 250% have been seen in undergraduate laboratories. This is because the sample is contaminated, either with solvent or reaction byproducts. It is impossible to get >100%yield of a pure compound.
Typically, unless a very simple reaction is being used batch processes will be utilised. In organic synthesis this is very important as the final yield is found by multiplying all the steps together eg. a 5 step synthesis, each step having a %yield of 80% (incidentally,a reasonably good yield for an organic reaction)
Total yield = 0.8*0.8*0.8*0.8*0.8*0.8= 0.327% or for every 100g you put in the first reaction, 33 come out as your finished product
Incidentally, a low %yield does not necessarily mean a process will be expensive. The Haber process (the major source of ammonia for fertilisers) fuses nitrogen and hydrogen to make ammonia. It is ridiculously inefficient, most plants operating at around 10-20% efficiency, mostly due to nitrogen's and hydrogen's unreactivity. However, the reactants can be easily recycled and so the low efficiency is a moot point.
(before anyone downvotes this because I said hydrogen was unreactive, it is. the stability of water drives it's aerobic combustion, it is still bloody difficult to get it to react. for example, alkenes (carbon-carbon double bonds) add to bromine in water. To get the same reaction with hydrogen, several atmospheres pressure and a palladium catalyst are required. For a metaphorical version, its the difference between asking politely to open the door and smashing it open with a sledgehammer)
Yield (?), v. t. [imp. & p. p. Yielded; obs. p. p. Yold (?); p. pr. & vb. n. Yielding.] [OE. yelden, [yogh]elden, [yogh]ilden, AS. gieldan, gildan, to pay, give, restore, make an offering; akin to OFries. jelda, OS. geldan, D. gelden to cost, to be worth, G. gelten, OHG. geltan to pay, restore, make an offering, be worth, Icel. gjalda to pay, give up, Dan. gielde to be worth, Sw. galla to be worth, galda to pay, Goth. gildan in fragildan, usgildan. Cf. 1st Geld, Guild.]
1.
To give in return for labor expended; to produce, as payment or interest on what is expended or invested; to pay; as, money at interest yields six or seven per cent.
To yelde Jesu Christ his proper rent. Chaucer.
When thou tillest the ground, it shall not henceforth yield unto thee her strength. Gen. iv. 12.
2.
To furnish; to afford; to render; to give forth.
Milton.
[He] makes milch kine yield blood. Shak.
The wilderness yieldeth food for them and for their children. Job xxiv. 5.
3.
To give up, as something that is claimed or demanded; to make over to one who has a claim or right; to resign; to surrender; to relinquish; as a city, an opinion, etc.
And, force perforce, I'll make him yield the crown. Shak.
Shall yield up all their virtue, all their fame. Milton.
4.
To admit to be true; to concede; to allow.
I yield it just, said Adam, and submit. Milton.
5.
To permit; to grant; as, to yield passage.
6.
To give a reward to; to bless.
Chaucer.
Tend me to-night two hours, I ask no more, And the gods yield you for 't. Shak.
God yield thee, and God thank ye. Beau. & Fl.
To yield the breath, the ghost, ∨ the life, to die; to expire; -- often followed by up.
One calmly yields his willing breath. Keble.
© Webster 1913.
Yield, v. i.
To give up the contest; to submit; to surrender; to succumb.
He saw the fainting Grecians yield. Dryden.
To comply with; to assent; as, I yielded to his request.
To give way; to cease opposition; to be no longer a hindrance or an obstacle; as, men readily yield to the current of opinion, or to customs; the door yielded.
Will ye relent, And yield to mercy while 't is offered you? Shak.
To give place, as inferior in rank or excellence; as, they will yield to us in nothing.
Nay tell me first, in what more happy fields The thistle springs, to which the lily yields? Pope.
Yield (?), n.
Amount yielded; product; -- applied especially to products resulting from growth or cultivation.
Bacon.
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