Fermentation of glucose mass changes (1 Viewer)

thebigticket

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In a practical exam we had a question on the fermentation of glucose to ethanol , why the mass change stopped at a given time

would it be fair to say that due to the limited amount of active sites on yeast only a limited number of glucose molecules will react to the yeast?

Also would ethanol really affect the active site on yeast as its pH is neutral and so the pH will probably slightly change
 
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Aykay

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Wouldn't a reason be because CO2 is released (you connect the apparatus to limewater to prove this), and once all of the CO2 is released the mass stops decreasing.

C6H1206 ---> 2CO2 + 2C2H5OH
 

Kimyia

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Couldn't you also say that after a given time, the ethanol content would have been high enough to kill the yeast hence stopping the reaction and so no further mass changes would occur? Or all of the sugar was metabolised?
 

nightweaver066

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That's a weird way of thinking of it lol. "Active sites" on yeast.. They're microorganisms performing anaerobic respiration!

Couldn't you also say that after a given time, the ethanol content would have been high enough to kill the yeast hence stopping the reaction and so no further mass changes would occur? Or all of the sugar was metabolised?
+1
 

golgo13

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Kimyia's answer was what we were taught, the mass change was due to the loss of CO2, and it stopped because the ethanol content was high enough essentially stopped the metabolism of the glucose, generally you won't reach the point where the glucose is used up irl, but you could probably write it down as a factor,

From memory it the ethanol content reaches about 7-8% fell free to scrutinise this number (distiling after will help determine your mixtures content)
 
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xXnukerrrXx

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Fermentation by the yeast upon the glucose ceases at a certain concentration of ethanol. (i forgot the concentration level). Any concentration higher, they yeast could not survive as a result dies. Hence fermentation ceases and mass changes stops.
 

jet

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In a practical exam we had a question on the fermentation of glucose to ethanol , why the mass change stopped at a given time

would it be fair to say that due to the limited amount of active sites on yeast only a limited number of glucose molecules will react to the yeast?

Also would ethanol really affect the active site on yeast as its pH is neutral and so the pH will probably slightly change
As night weaver said, yeast organisms don't have active sites. An active site is the area of an enzyme which catalyses a chemical reaction.

In a traditional liquid reaction, mass is conserved so you wouldn't expect a mass change to occur, but we know in fermentation, carbon dioxide is produced and since this is a gas, the removal of carbon dioxide the cause of the mass change.

Further, just like any catalyst, enzymes are re-usable. They don't just catalyse one reaction and stop working whatsoever. So if the number of enzymes inside the organism was limited, this wouldn't stop the reaction from going. So carbon dioxide will still be produced and the reaction will occur. In fact, the amount of enzyme controls the rate of the reaction.

Assuming this is the only reaction going on in the solution, then only a few things can change - the amount of glucose and the amount of ethanol. The carbon dioxide is gaseous so this shouldn't change the pH of the solution itself.

All you need to do is google yeast and ethanol see that yeast stops functioning above a certain ethanol concentration. It's likely that this causes the change in mass to stop.

However, the amount of glucose could be the limiting factor here. Say (for example) you run the reaction in 100 mL of water. Say (just for example, as I'm not sure of the exact value) the limiting concentration of ethanol was 10% (v/v). Thus 10 mL (~7.89 g) of ethanol should kill the yeast, i.e. 7.89/46.07 = 0.171 mol. Assuming the only reaction which occurs is the fermentation process, then this amount will be produced by 0.0856 mol of glucose. This is 15.4 g of glucose. So say you added 10 g of glucose to 100 mL and ran a fermentation reaction, you would probably run out of glucose before the yeast stopped functioning, so the reaction would stop.

A really nice way to check this would be to wait until the mass change stops and add extra glucose. If you get more carbon dioxide/or see another mass change then you know the yeast are still alive.
 

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