[Dolphinsiu]

I have read many reaction like the following:

CH2 = CH2 + H⁺ --> +H2O --> - H⁺ ---> CH3CH2OH

5% conversion
95% yield

What is the difference between conversion and yield?

[burnt]

A conversion is the pourcentage of sarting material (SM) converted (that has reacted) into the excpected product or byproducts, or even into products of degradation. While a yield is the pourcentage of SM converted into one product only. In fact, conversion is related to the SM and yield to the product.
For instance consider a reaction between a SM and a reagent that leads to 3 products. If you don't observe any SM left at the end of the reaction, then the conversion is said total (100%) and each of the 3 product will receive a yield, the sum of which cannot exceed 100%. Thus, the conversion is always superior or equal (in the case of a selective reaction) to the yield.
So in your example, the yield cannot exceed 5% as you mentioned it, but 95% is most likely the selectivity of the reaction, in other words what would be the yield of the expected product if the conversion was total.

Hope this helps
burnt

[Custos]

Hmm. That's a complicated, but correct, explanation. I think of it like this...

Say you had a reaction of A going to B. And say at the end of the reaction you had 10% yield of B but are able to recover 80% of your unreacted starting material A.

In this case your yield is 10%. But because you have recovered 80% of your starting material only 20% has been consumed - so your conversion is 10/20 or 50%.

A low yielding reaction that has a high conversion rate can sometimes still be useful because, theoretically, you can recycle the unused starting material through the reaction again to increase your yield.

[movies]

I don't think that is correct!

Conversion is the amount of starting material that has reacted, regardless of what product it goes to.  So in that example, you would have 20% conversion.

Yield is the percentage of theoretical maximum for each product, so if you isolate 10% of your material as your desired product, it's 10% yield.  If you account for the rest of the converted material as another product in 10% yield, then you got 10% yield of each of those.

What you described as yield of desired product/% conversion is sometimes called "yield based on recovered starting material."  I think that this term is overused, however, and I think it's a bit disingenuous to report a reaction like you described as "50% yield based on recovered starting material" unless you also state the actual chemical yield of that step (10%).  It's just misleading to say that you could get 50% yield when you would have to run that reaction more than 5 times to actually get 50% isolated yield.

[kristo]

Custos, I thought your explanation was far more confusing. Burnt your explanation made great sense to me.

[Custos]

What you described as yield of desired product/% conversion is sometimes called "yield based on recovered starting material."  I think that this term is overused, however, and I think it's a bit disingenuous to report a reaction like you described as "50% yield based on recovered starting material" unless you also state the actual chemical yield of that step (10%).  It's just misleading to say that you could get 50% yield when you would have to run that reaction more than 5 times to actually get 50% isolated yield.

Yes, you're quite right. In production we call it "process yield", and it can be used disingenuously to make a reaction yield look better. It's used properly in process chemistry on those specific occasions when the recovered starting material can actually be reprocessed, and when the starting material is valuable enough to warrant collection and reprocessing.

[movies]

Cool.  That makes sense.  I have to admit I don't know much at all about what goes on in industry!