lesson #4| Stoichiometry and yield
Stoichiometry and yield
By Amir Syahir
Introduction
Stoichiometry is the study of mass measurement to reveal the relationship
between chemical / biochemical formulas and equations, and the actual molecular
mass. Although mass usually measured in laboratory in units such as grams,
milligrams, or micrograms, it is also a numerical coefficient in equations to balance
off the biochemical reactions, yielding a balanced reaction. Subscripts in formulas of
number of atoms, ions, and molecules, gas molar volumes and other measured
quantities have common basis in mass, whether it is mole, grams or number of
particles.
In balancing equation of chemical reactions, inspection method is used. This
method allows you to balance the equation not by trial-and-error, but with effective
and systematic way. In other words, by using this method, you ‘will not require a
liquid paper’. Thus, you must learn this special technique. It consist of two simple
steps
Methods
Step 1 : Select a starting point. You must choose a starting compound to put the
first molar number. Once you put it, that is it and do not change it. The starting point
can be selected based on;
i)
Look (inspect) for element that present only once in both sides, reactants
side and products side
ii)
If you have more than one choices, prefer compound rather than a single
element
iii)
If you still have multiple choices, select the compound that consist most
of the elements.
*Thus, the can be simplified as complex à simple chemicals
Step 2 : Start your balancing in a chain or ping-pong process. In other words, do not
break the chain, until you finish writing the last molar number.
Note:
1. One (1) is also a molar number that is required to be written
2. Fraction is okay in balancing chemical / biochemical reactions
3. Sometimes, you need to use algebric equation in order to get the molar
number
lesson #4| Stoichiometry and yield
Try this! (use pen, and do not use liquid paper)
NH3
+
O2
NO
à
HCl
+
KMnO4
+
S
+
HNO3
à
SO2
S
+
NaOH
à
Na2S
P 2I 4
+
P4
CH4
+
NH3
As2S3
+
+
Zn
NO3−
+
H 2O
+
HNO3
Pb(N3)2
NO
KCl
à
+
O2
+
+
H+
Cl2
+
NO
+
+
+
PH4I
à
HCN
à
+
H3AsO4
Cr2O3
à
Zn2+
+
+
+
H 2O
H3PO4
+
à
H 2O
H 2O
Na2S2O3
à
H 2O
Cr(MnO4)2
+
H 2O
H 2O
+
NO
MnO4
NH4+
+
+
+
S
Pb3O4
H 2O
+
lesson #4| Stoichiometry and yield
Experiment: Fermentation
In this section, we will deal with the reaction of sugar fermentation and find
the relationship between the biochemical formula and actual measured mass unit.
When organisms are exposed to anaerobic conditions, enough oxygen is not
available as a terminal electron acceptor. Without oxygen to accept electrons, the
electron transport chain does not function, and NADH has nowhere to donate its
electrons. Under this condition, fermentation of glucose occurs primarily. The cells
attain a maximum specific growth rate of about 0.45 hr-1 with a low biomass yield of
0.15 g dry mass per gram glucose consumed and a high respiratory quotient (the
ratio of CO2 production rate to the O2 consumption rate) and a low energy yield of
only about 2 ATP per mole of glucose metabolized. The stoichiometry of this
reaction is
C6H12O6
C2H5OH
à
+
CO2
+
(Not Efficient)
Note that the oxidation of glucose predominates under aerobic condition,
results in the cells attain a maximum specific growth rate of only about 0.25 hr-1 with
a biomass yield of about 0.5 g dry mass per gram glucose consumed, a respiratory
quotient of about 1, and a high energy yield of < 38 ATP per mole of glucose
metabolized. The stoichiometry of this reaction is:
C6H12O6
+
O2
CO2
à
à
+
H 2O
+
(Efficient)
In animals or human body, fermentation will produce lactate, but in bacteria
or yeast such as Saccharomyces cerevisiae, fermentation will resulted the
production of ethanol. Ethanol is a renewable and environmentally benign substitute
for the current fossil-fuel-based transportation fuels. Fermentation of sugars, which
is, in general, a slow process, is an essential step in the production of ethanol from
renewable sources.
Materials
1. Yeast (Saccharomyces cerevisiae)
2. Sugar : Glucose, Fructose, Sucrose
3. 5 Test tubes (with rack) and 5 balloons
4. Sonicator (equipped with temperature controller)
5. Water bath and thermometer
6. Graduated cylinder (25 ml)
7. 6 beakers (25 ml)
8. Glass rod
9. Permanent marker for labelling
10. Gloves
lesson #4| Stoichiometry and yield
Methods
1. Dissolve 5 g of sugar (glucose × 2, sucrose, and fructose) with 15 ml distilled
water in 4 different beakers.
2. Then, dissolve 0.5 g of yeast (Saccharomyces cerevisiae) with 10 ml water in
5 different beakers.
3. Mix each one of the sugar solution into the yeast solution.
4. Then, fill in 4 different test tubes (including control experiment by using
water) until almost full to create unsufficient oxygen condition. Make sure
every test tube has the same level of solution.
5. Shake the tube gently before sealing with balloons (use rubber glove).
6. Then, incubated the all the test tubes in 30°C water bath. CAREFUL! If the
water is too hot you will kill the yeast. The process will take 30-60 min.
7. Observe the balloon of which tube will be filled with gases in sequence (#1
#2 #3 and #4).
8. Measure the gas volume using water displacement method.
9. The experiment will be made parallel with another test tube fill with glucose
and yeast, incubated with ultrasonic wave (mild sonication) at 30°C.
Take note of the time, and take a picture of the result. Discuss your result.
Questions
1. Calculate the theoretical and actual yeild of your sugar fermentation reaction.
Discuss.
2. Match the chemical reaction formula with the actual measured mass unit. And
discuss your results.
3. How much ethanol can you generate from 1000 kg of glucose?
4. Discuss/search for a method, in which we can detect and quantify ethanol that
being produce from the fermentation process.
5. What are the current problems of bio-ethanol production?