3 replies to this topic

I am a starting chemical engineering student. During my lab section last week the teacher talked about actual and theoretical yield, consequently also about percentage error and percentage yield. My question is the following:

What are factors that contribute to the likelihood of percentage yield calculations in differing from 100%?

I would appreciate technical answers. I know human and instrumental error can lessen the possibilities of a substance yielding exactly 100%

Thank you for your help!

[Art Montemayor]

Kylkova

This is a good way to start to learn how Chemical Engineering works and how it is applied. However, you first have to learn how to communicate – or relate exactly what it is that you are talking about or concerned about. As a means of self-improvement, I would ask you to please return to your original post and read it carefully. Note that you:

• Don’t state WHAT lab your refer to;
• Don’t identify WHAT kind of yield you refer to – is it the “yield” from a chemical reaction? Or is it the yield from a Unit Operation?

You say you want technical answers but we can’t get technical without being SPECIFIC and detailed.

I think you mean a chemical reaction – but being a Chemical Engineer, I refuse to try to read your mind and prefer that you be specific so that I can respond as an engineer and not a crystal ball gazer.

Can you kindly elaborate?

Of course. The lab I am referring to is:

• Separation of the components of a ternary mixture.

The lab procedure requires the (1) heating of a mixture of Naphthalene, NaCl and sea sand. As some of the Naphthalene is sublimed the rest turns into a residue of NaCl and sea sand. Then we (2)extract the sea sand using a filter, this step separates the mixture by draining out the NaCl(aq) solution and leaving the sea sand as a residue. Lastly we (3)evaporate the remaining water from NaCl and the sea sand and calculate the percentage yield.

My question is more conceptual though by "technical" I meant something, any factor during the procedure that would make for example : make the sand weigh more exceeding the theoretical amount of it. A factor that would not be instrumental or human error.

I know I explained the lab, but I am not referring to it particularly. why do I not get the 100% yield or why do I get 101% yield? is it because maybe a mixture has other liquids affecting its weight? I know for the lab mentioned above, we evaporated(3) the water from sand, I set the timer for 5 minutes, but my lab partners decided to take it out before the 5 minutes ended (sand was not completely dry). I assume that this is what affected the weight of the sand.

Thank you for your reply and please excuse my vagueness.

Edited by Art Montemayor, 17 February 2011 - 07:38 PM.

[Art Montemayor]

Kylkova:

Your lab example is difficult to understand. You say you mix 3 solids (Naphthalene + NaCl +sea sand) and then you filter out the sand. There is no way you can filter out a solid out a mixture of solids.

You further state that the filter step “separates the mixture by draining out the NaCl(aq) solution” and follow that by adding that you “evaporate the remaining water from NaCl and the sea sand and calculate the percentage yield”. Where did the water come from? You never mentioned that you were adding water (or had it present) in the solids mixture. Additionally, what percentage yield do you calculate? The salt (NaCL) recovered? Or the sand? Very confusing.

If you got 101% recovery of something, it obviously points to a measurement error – either at the beginning or at the end. But you have an error. Only God can create matter in the practical sense, so there must be a mistake in measurement(s). But that is something that happens from time-to-time. If you are weighing your ingredients to calculate the mass, you are prone to measure humid sand, for example, and identify more sand at the end due to the moisture it still contains.

In real life engineering, I have never come across a chemical reaction or a Unit Operation that yields 100% efficiency. This is just not reality. This fact is related to the “tried-and-true” engineering TRUTH: “There are no free rides or free lunches.” Everything in life has a price or a tradeoff – and usually it comes in the form of inefficiency. Other times, as in Thermodynamics, it comes in the form of the Second Law of Thermodynamics – the existance of Entropy. In almost all engineering applications you cannot practically expect to get back everything you put into a process. Almost all processes are irreversible – for many reasons. Friction is one great enemy and virtually destroys any concept of perpetual motion. The topic of yields is related to efficiency in a process.

As a future Chemical Engineer, you will spend much of your life fighting and working around inefficiencies – either process or human related – but inefficiencies nevertheless. Your job will never end.