Purpose:
The purpose of this lab is to determine the mole ratio of the reactants
sodium hypochlorite (NaClO) and sodium thiosulfate (Na2S2O3) when reacted
in a chemical reaction.
Background:
A balanced chemical equation gives the mole ratios of the reactants
and products involved in the chemical reaction. However, when the formulas of
the products are not known, experimental measurements can be made to
determine those ratios. This experiment uses the method of continuous
variations to determine the mole ratio of the two reactants. First, solutions of
the reactants are prepared in which the concentrations are known. Second, the
solutions are mixed a number of times using different ratios of reactants. Third,
the change of temperature is the property to be measured. The reactions are
all exothermic, so the heat produced will be directly proportional to the amount
of reaction that occurs. Since the experiment is designed so that the volume of
solution is a constant for all measurements, the temperature change will also
be proportional to the quantity of reactants consumed. The total number of
moles of reactants is also kept a constant for the series of measurements.
Each measurement is made with a different mole ratio of reactants. The
optimum ratio (determined after the results of the trials are graphed) is the
stoichiometric ratio in the equation, which means that it should consume the
greatest amount of reactants, form the greatest amount of products, and
generate the most heat and maximum temperature change.
Materials:
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0.50M sodium hypochlorite (NaClO)
0.50M sodium thiosulfate (Na2S2O3) as “Solution B”
Styrofoam cup
Graduated cylinders, 10mL, 25mL, and 50mL
Thermometer
Wash bottle filled with distilled water
Procedure:
1) Measure the temperature of the NaCLO solution and of “Solution B,” and
record the data in a table. Use the same thermometer, rinsing with distilled
water when switching solutions. The solutions should be the same
temperature. If they are not, a correction must be made for the
temperature difference after every trial by adding the products of the
volume of each of the solutions by their respective initial temperatures and
dividing that product by the total volume of solution, 50mL.
2) Mix varying volumes of each solution (measured using appropriate
graduated cylinders) in the Styrofoam cup, covering the closed system
each time with a lid made by cutting off the top of another Styrofoam cup.
Stir the solution with the thermometer, and record the highest temperature
reached by the mixture.
3) Pour the solution down the sink, and repeat the process using a different
ratio of the two substances, always keeping the total volume at 50.0mL.
Continue testing various ratios until you have obtained at least 3
measurements on each side of the one that gave the maximum
temperature.
4) Plot the data. Draw two straight lines of best fit. The stoichiometric mole
ratio of the reactants is the point of intersection on the graph.
Lab Questions:
1) Explain how this method allows you to find the mole ratio of reactants.
Mixing different volumes of the two reactants resulted in different
temperature changes. The temperature change was directly
proportional to the quantity of reactants consumed because the
experiment was designed so that the volume of solution and the total
number of moles of reactants was a constant for the series of
experiments. Thus, the optimum ratio (determined after the results of
the trials are graphed) is the ratio of the volume of reactants that
consumed the greatest amount of reactants, formed the greatest
amount of products, and generated the most heat and maximum
temperature change.
2) Why must you keep a constant volume of reactants?
A constant volume of reactants must be kept so that the resulting
changes in temperature can be directly compared because they are
directly proportional to the quantity of reactants consumed in the
reaction.
3) Is it necessary that the concentrations of the two solutions be the
same?
Yes; in order to for the stoichiometric mole ratio of the reactants to be
accurate after the results of the trial are graphed, the concentrations of
the two solutions must be the same. This is because molar
concentration is moles of solute divided by liter of solution.
4) What is meant by the term limiting reagent?
A substance described as a limiting reagent is a reactant in a chemical
reaction that will be completely reacted before all other reactants in the
reaction are used up, causing the reaction to stop.
5) Does the measurement of temperature or the measurement of volume
limit the precision of your data? Explain.
The measurement of temperature limits the precision of the data to 3
significant figures, and the thermometer used was capable of
measuring temperature to the tenths of a degree. The measurement of
volume limits the precision of the data to 2 significant figures because
the graduated cylinders used were only capable of measuring volume
to the nearest mL.
6) Which reactant is the limited reagent along the upward and downward
sloping lines of your graph?
Na2S2O3 and NaCLO, respectively
7) What physical properties, other than temperature change, could use
the method of continuous variations?
Color intensity of a reactant or product, the mass of a precipitate that
forms, or the volume of a gas evolved
8) Why is it more accurate to use the point of intersection of the two lines
to find the mole ratio rather than the ratio associated with the greatest
temperature change?
Unless the optimum ratio was actually tested as a trial, the only way it
can be found is by using linear regression to accurately calculate the
proportion of volumes of reactants that would form the optimum ratio.
The ratio associated with the greatest temperature change in the
experiment cannot be assumed to be the optimum ratio because
minute variations to it will result in a higher temperature change.
9) If the two solutions used are not at the same initial temperature, a
correction must be made to find the correct change in temperature.
How should this be done?
Add the products of the volume used of each of the solutions by their
respective initial temperatures and dividing that product by the total
volume of solution, 50mL. This quotient is the initial temperature of each of
the varying solutions.
Conclusion:
The stoichiometric mole ratio of NaCLO to Na2S2O3 is 3:1.
Graphing the data obtained through the trials showed a optimum
volumetric ratio of the reactants to be approx.37mL of NaCLO to
13mL of Na2S2O3, which rounds to a 3:1 ratio. The stoichiometric
mole ratio of the reactants means that 3 moles of NaClO will react
with 1 mole of Na2S2O3 so that the reaction proceeds to completion, all
reagent is consumed, and no residues remain. Also, this ratio of reactants
will consume the greatest amount of reactants, form the greatest
amount of products, and generate the most heat and maximum
temperature change.