Yeast converts glucose to ethanol and
carbon dioxide by anaerobic fermentation, as represented by the equation glucose reacts to
form ethanol plus carbon dioxide. In a particular fermentation process,
200.0 grams of glucose is fully converted. What is the total mass of ethanol and
carbon dioxide at the end of the reaction.
Anaerobic means no oxygen. So glucose is our only reactant. And ethanol and carbon dioxide are the
only products. We can ignore the yeast because the
enzymes in the yeast are acting as a catalyst for this process. In this example, 200 grams of glucose is
being converted completely into ethanol and carbon dioxide. What we have to do is figure out the
total mass of the ethanol and carbon dioxide produced. To help us on our way, we can recall the
law of conservation of mass, which says that the mass of a closed system cannot change.
For a chemical reaction, the mass of the
reactants equals the mass of the products. The mass of the reactants refers to the
mass of glucose. And the mass of the products refers to
the mass of the ethanol and the carbon dioxide. This is what we’re looking for. We have 200.0 grams of glucose to begin
with that was fully converted into ethanol and carbon dioxide. So the total mass of ethanol and carbon
dioxide is 200.0 grams.
If the fermentation process were carried
out in an open container, would the mass of the container and contents increase, decrease,
or stay the same?
Let’s imagine an open container like a
beaker without a lid with yeast and glucose mixed together inside. The yeast will convert the glucose to
ethanol, a liquid, and carbon dioxide, a gas. The ethanol will stick around in the
container. And the carbon dioxide will escape. Since the carbon dioxide escapes the
system of the container and its contents, we can be sure that as the fermentation precedes
the mass of the contents and the container decreases. Since we’re dealing with an open system,
where mass is being lost, the law of conservation of mass does not apply because the law of
conservation of mass only applies to a closed system.
If the reaction produces 97.7 grams of
carbon dioxide, what mass of ethanol is produced?
What the question is saying is that when
200.0 grams of glucose is fully converted into ethanol and carbon dioxide, we have 97.7
grams of carbon dioxide. Our job is to work out the mass of the
ethanol. Going back to the law of conservation of
mass and part a), we know that the mass of glucose that is consumed in the reaction is equal
to the mass of ethanol plus the mass of carbon dioxide produced by the reaction. So 200.0 grams is equal to the mass of
ethanol plus 97.7 grams. The mass of ethanol is, therefore, 200.0
grams minus 97.7 grams which is equal to 102.3 grams. So the mass of ethanol produced when 200
grams of glucose is fully converted is 102.3 grams.