Mole rations of products and reactants are obtained using a balanced chemical equation. With the formulas of all the reactants and products known, balancing chemical equations becomes easy. With knowledge of the chemical formulas known, experimental measurements are undertaken for the determination of the ratios. The determination of mole ratios is achieved through the use of a continuous variation method, which involves several steps. First, reactants’ solutions of known concentrations are prepared. Secondly, using different reactant ratios, the solutions are mixed several times. Thirdly, several reaction properties determined by the quantity of products or reactants formed, mass of precipitate formed, and gas volume evolved. In this experiment, changes of temperature was the property that was determined. Reactions were all exothermic and the quantity of heat produced is directly proportional to the amount of reaction that occurred.
Purpose
The purpose of this experiment is to use a continuous variations method in determining mole ratio of two reactants. The reactants’ stoichiometric ratios in the balanced chemical equation should produce most heat and hence generate maximum changes in temperature.
Materials
Chemicals
Sodium hypochlorite, NaClO, 0.50 M
Sodium thiosulfate, Na2S2O3, 0.50 M in NaOH, 0.20 M As “Solution B”
Equipment
Styrofoam cup
Thermometer
Graduated cylinders, 10-mL and 25- or 50-mL
Beakers
Safety Alert
Sodium hypochlorite (NaClO) is a bleaching agent. All solutions prepared were basic and thus harmful to the eyes and skin. In case of spillage accident, copious amounts of water were to be used. For fumes, a fume hood was to be used or experiment conducted under the funnel connected to an aspirator. It was a requirement to also wear googles if necessary and a lab coat was mandatory.
Procedure
1. The temperatures of NaClO solution and that of solution B chosen were measured using the same pair of calibrated thermometers (the two solutions should have the same temperature and if not, a correction of temperature difference was to be made). Data was recorded in a table.
2. 5.0mL of NaClO was poured into a Styrofoam cup and 45.0 mL of the second solution was added. The mixture was stirred using a thermometer with the highest temperature recorded. The solution was poured and the cup and thermometer rinsed.
3. The step was repeated using different ratios of the two solutions while ensuring the total volume remained at 50.0mL. Various ratios were continuously tested until at least three measurements were obtained.
4. Data obtained was recorded, two straight lines of best fit of the data were drawn and the point of intersection determined. During plotting, 0.50mL and 50.0mL ratios were included and if any point did not fall close to the lines, the measurements was repeated.
5. The stoichiometric mole ration of the reactants was determined from the point of intersection.
6. After the experiment, used solutions were flushed down the sink using excess water.
Observations/Data
At the lab sheet (excel sheet)
Calculations
4NaClO+ Na2S2O3 +2NaOH 4NaCL +2Na2SO4 +H2O
Four moles of NaCLO react with one mole of Na2S203
and since their molarity is the same, their volume can be compared.
To determine the number of moles of the reactants during the reaction that produced highest temperature, the above equation and the graph are used.
For NaCLO,
Molarity-0.5M,
Volume used- (5×0.5)/1000=0.0025 moles
Total moles that reacted=0.0025×4=0.01 moles
For NaS2O3,
Molarity-0.5
Hence, since four moles of NaCLO reacted with one mole of Na2S203
Then, moles for NaS2O3 will be (1×0.01)/4=0.0025 moles.
The ratio will thus be 4:1.
From the graph, plotting data at optimum volumetric ratios of the reactants at 37mL (NaCLO) and 13mL (Na2S203) gives a ratio of 3:1
Conclusion
The stoichiometric ratio of NaCLO to Na2S2O3 is 3:1. Plotting the data generated from the trials indicated optimum volumetric ratios of the reactants as about 37mL of NaCLO to 13mL of Na2s203 that rounds out to 3:1 ratio. The stoichiometric mole ratios of reactants hence mean that 3 moles of NaCLO reacted with 1 mole of Na2S2O3, making the reaction proceed to completion (all reagents are consumed while there is no residue that remained. Additionally, the reactants ratio would consumer greater reactant amounts, producing larger amount of products, generating most heat and maximum changes in temperature.
Discussion of Theory
The stoichiometry of chemical reactions is the relationship that exists between products and reactants and permits the calculation of the amounts of reactants and products. For indication of ideal mole ratios, coefficients are used in the balancing of chemical equations. A balanced chemical equation provides reactants and products mole ratios in a chemical reaction. Nonetheless, with known product formulas, experimental measurements are made to help determine these ratios. A continuous variation method is used in determining the mole ratio of the two reactants. This ration entails the combination of reactant volumes, which produce greater number of products while the product amounts are determined through the measurement of macroscopic properties that show the completion of a chemical reaction.
Sources of Error
First, there was systematic error which happened due to Ralph Hargrave’s confidence that there was the possibility things will be screwed. As such, since distilled water was used prior the experiment to rinse the cylinders and beakers used, it could have been left causing more dilution of the reagents. Secondly, there was the error in determining end point, which led to more volumes of a given solution being used. Thirdly, there was the possibility of improper calibrations of the graduated cylinders and beakers used, which led to wrong results.
Post-Lab Questions
1. Explain how this method allows you to find the mole ratio of reactants.
By mixing different volumes of the two reactants, temperature changes were observed. Changes in temperature was directly proportional to the amount of reactants used since solutions volume and total reactants’ mole number remained constant throughout the experiment. Hence, the maximum ratio was the ratio from the reactants’ volume consumed forming larger amount of products and generating larger heat and maximum temperatures.
2. Why must you keep a constant volume of reactants?
For easy comparison of temperature changes and quantity of reactants consumed during the experiment due to direct proportionality.
3. Is it necessary that the concentrations of the two solutions be kept the same?
Yes, concentrations of the two solutions must kept constant for the reactants’ stoichiometric mole ratios to be accurate after plotting graphs of the trial. Also, is due to the fact that molar concentrations is determined by division of solute moles with volume of the solution.
4. What is meant by the term limiting reagent?
It is a reactant in a stoichiometric chemical reaction that is completely consumed before the reaction is complete, making the reaction to stop.
5. Does the measurement of temperature or the measurement of volume limit the precision of your data? Explain.
Temperature measurement limits data precision to three significant figures whereas the thermometer had the capability to measure the temperature to tenth degrees. Volume measurement limited data precision to two significant figures since the graduated cylinders in use had the capability to measure volume to the nearest milliliter.
6. Which reactant is the limiting reagent along the upward sloping line of your graph? Which is the limiting reagent along the downward sloping line?
Na2S2O3 and NaCLO, respectively
7. What physical properties, other than temperature change, could use the method of continuous variations?
Color intensity changes for the product or reactant, formed precipitate mass and gas volume 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 ration associated with the greatest temperature change?
Unless maximum ratio was tested as trial, linear regression was the only way to ensure accurate calculation of volume proportions of reactants that could form maximum ratio. Ratio related to the greatest changes in temperature in the experiment cannot be taken to be optimum ratio since minute variations would cause higher changes in temperature.
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?
Adding volume of products from that of solutions using their respective initial temperatures and division of this product’s volume by total solution’s volume, 50mL. The quotient would be the initial temperature of each of these varying solutions.
