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Copyright reserved Faculty of Natural and Agricultural Sciences Fakulteit Natuur- en Landbouwetenskappe Chemistry Department/ Departement Chemie

CHM171

Practical 1: Stoichiometry

Remember to bring your LAB COAT, MICROKIT & SAFETY GLASSES Introduction Stoichiometry is the study of quantitative relationships between the reagents and products of a chemical reaction (Greek: stoicheia – simplest part or quantity; metrein – to measure.) During stoichiometric calculations, balanced chemical reaction equations are used to determine the relationships between the molar quantities of reactants and products. Knowledge of (a) the mol concept and (b) the relationships between concentration, moles and atomic mass units are essential. For redox reactions (c) oxidation numbers and (d) the use of standard reduction potentials from a given table are also applicable.

Experiment 1: The reaction of iodine with sulphite ions The (redox) reaction between iodine (I2) and sulphite ions (SO3

2- ) can be represented as follows: a I2 + b SO3

2- c (I-product) + d (S-product) + e (unknown)

with the stoichiometric coefficients (a, b, c, d and e) and the three products unknown. In this experiment we wish to determine the ratio in which iodine and sulphite ions react i.e. a / b in the above equation and the identities of the unknown products. Two standard solutions (i.e. solutions whose concentrations are accurately known) are provided, an iodine solution and a sulphite solution. A known volume of the iodine solution of known concentration is reacted (titrated) with the sulphite solution until all the iodine has reacted. We can calculate the mole ratio of iodine to sulphite ions from the volumetric data using the equation [VA.CA]/[VB.CB] = a/b where VA is the volume of iodine, cA is the concentration of iodine, VB is the total volume of sulphite solution added and cB is the concentration of sulphite solution. In addition, qualitative chemical analyses can be used to identify the unknown reaction products. Since the reaction between iodine and sulphite ions is a redox reaction, a redox indicator would normally be required to indicate the equivalence point of the reaction. However, a starch indicator will be used in this practical as iodine reacts with beta-amylose starch to form a blue-black complex. The disappearance of this blue-black colour is thus used to indicate the equivalence point in this titration i.e. when all the I2 has reacted. However, please note that since the iodine-starch complex is insoluble in water it is very difficult to extract high concentrations of iodine from it. For this reason the starch indicator solution is added to the titration mixture during the final stages of the titration, when the iodine concentration is low. Note: Attention should be given to precision, accuracy and significant figures in this experiment

Overview of the experiment

During the experiment a drop technique is used to determine the stoichiometric ratio in which the iodine reacts with sulphite ions. A known volume (measured in drops, e.g. 30 drops) of a standard iodine solution is measured. The number of drops of a standard sulphite solution required to react with all of the iodine is then determined. Furthermore, qualitative tests are performed on the reaction mixture in order to identify the reaction products. Experimental Procedure Experiment a: Determination of the stoichiometry of the redox reaction between iodine and sulphite ions 1.1 First it is necessary to measure the volume of one drop of the standard iodine solution as accurately

as possible. This is done as follows:

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Remove the plunger from the syringe and seal the open end of the syringe with the prestic provided in the micro kit.

Using a propette and the drop controller, add the iodine solution to the syringe until the volume of the solution just reaches one of the measuring marks on the syringe. Let this mark be the “zero mark”

Count the number of drops that need to be added to reach another measurement mark 0.5 ml above the “zero mark”.

Record the results.

1.2 Repeat step 1.1 with the standard sulphite solution. 1.3 Place exactly thirty drops of the iodine solution in one of the large wells of the comboplate®. 1.4 Add the sulphite solution drop wise until the brown colour of the iodine has almost disappeared (the

solution should be pale yellow). Count the number of drops added carefully, and mix the solution continuously with a clean glass rod or micro-spatula.

1.5 When the reaction mixture has a pale yellow colour, add 2 - 3 drops of the beta-amylose starch solution. The reaction mixture will turn blue-black as a result of the iodine-starch complex.

1.6 Continue dropping sulphite solution into the mixture (with continuous mixing) until the blue-black

colour of the iodine-starch complex just disappears. Record the total number of drops of sulphite solution added. (i.e. drops added before the starch indicator + drops added after the starch indicator).

1.7 Repeat steps 1.3 to 1.6 above until at least three similar results have been obtained. 1.8 Retain all test mixtures for the qualitative tests that follow. Experiment b: Identification of the reaction products (Qualitative analysis) Part 1: Identify the reaction medium (unknown product)

1.1 Tear one strip of yellow indicator paper (from your micro-kits) into three squares of roughly equal

size.

1.2 Place the three squares of paper on the underside of an upturned micro-kit container (lunchbox) and moisten each square with a drop of distilled water.

1.3 Treat the squares of paper as follows:

1.3.1 Place one drop of the test mixture (step 1.7 above) on the first square of pH paper.

Compare the colour of the paper with the colour chart in the micro-kit to determine whether the solution is acidic, basic or neutral.

1.3.2 Place one drop of the sulphite solution on the second square of paper, and determine whether it is acidic, basic or neutral.

1.3.3 The iodine solution is intensely coloured. The intense brown colour must first be removed before the acidity of the solution can be tested with the pH paper. To remove the colour place about 0.5 mL of the iodine solution plus about 0.5 mL water

in a glass test tube and add the same volume of xylene. Shake the mixture gently and then allow the two layers to separate Do not use the comboplate® for this part of the experiment, as the xylene may attack the

polymer of which the comboplate® is manufactured. Discard the coloured organic layer. Repeat if necessary until the aqueous layer is clear. Place one drop of the clear aqueous layer on the third square of pH paper to determine

the acidity of the iodine solution.

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Part 2: Identify the sulphur product. The sulphite ions may possibly be reduced during the reaction. The two most obvious reduction products would then be either sulphur (oxidation number (ON) = 0) or sulphide ions (ON = 2). 6H+ + 6ē + SO3

2- → S2- + 3H2O reduction half-reaction 6H+ + 4ē + SO3

2- → S + 3H2O reduction half-reaction Sulfur can be recognized as a yellowish-white milky suspension that will precipitate gradually. Use the following procedure to test for the presence of sulphide ions: 2.1 Transfer 10 drops of the reaction mixture (step 1.7 above) to one of the small wells of the

comboplate®. 2.2 Add two drops of dilute hydrochloric acid. 2.3 Carefully smell the vapours above the solution: the characteristic rotten egg odour of hydrogen

sulphide will confirm the presence of sulphide ions in the reaction mixture. (NB: H2S is poisonous) If, on the other hand, the sulphite ions were oxidised during the reaction, the most probable reaction product would be sulphate ions (ON = +6).

SO32- + H2O → 2H+ + 2e- + SO4

2- oxidation half-reaction

Test the reaction mixture for the presence of sulphate ions with the classical barium sulphate test that is described below: 2.4 Add 2 drops of dilute hydrochloric acid to the remaining reaction mixture of step 1.7.

This will ensure that any remaining sulphite ions in the reaction mixture are converted to hydrogen sulphite, and this will decompose spontaneously to sulphur dioxide.

2.5 Add approximately 10 drops of barium chloride solution to the acidified reaction mixture, and watch

carefully for the appearance of the characteristic white barium sulphate precipitate that will be observed only if sulphate ions are present in the reaction mixture.

Part 3: Identify the iodine product The iodine may have been reduced during the reaction. The obvious product would then be iodide ions (ON = 1). Iodide ions form a cream coloured precipitate of silver iodide with silver nitrate. This precipitate is insoluble in excess ammonia. If the iodine is oxidised during the reaction, the hypoiodite ion (ON = +1), the iodite ion (ON = +3) and the iodate ion (ON = +5) would be the most probable reaction products. The iodate ion also forms a whitish precipitate with silver nitrate, but this precipitate dissolves readily in excess ammonia. Possible half-reactions are: I2 + 2ē → 2I−

I2 + 2H2O → 4H+ + 2e- + 2IO− I2 + 4H2O → 8H+ + 6e- + 2IO2

− I2 + 6H2O → 12H+ + 10e- + 2IO3

− Test for iodide ions: 3.1 Place approximately 0.5 cm3 of the reaction mixture in one of the large wells of the comboplate®. 3.2 Add to this 3 drops of silver nitrate solution. Mix thoroughly and allow it to stand for a few minutes. 3.3 Add approximately 0.5 cm3 of the ammonia solution to the precipitate. 3.4 Add a further 0.5 cm3 ammonia solution to determine if the precipitate will dissolve in the excess ammonia.

Disposal of chemical waste The chemical waste of this practical can safely be disposed of in the basins, with the exception of: Silver residues: Silver is a relatively expensive metal, and can be recycled from the silver residues.

Place the silver residues in the waste container that has been provided for this purpose.

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Organic residues: Dispose of the xylene residues in the waste container that has been provided for this purpose.

Experiment 2: The Stoichiometry of Precipitation Reactions. Reactions that result in the formation of an insoluble product are known as precipitation reactions. Precipitation reactions are exchange reactions, which occur when certain pairs of oppositely charged ions attract each other so strongly that they form an insoluble ionic solid. The relationship between the quantities of chemical reactants and products is called stoichiometry, and the coefficients in a balanced equation are the stoichiometric coefficients. These coefficients in front of each formula in a balanced chemical equation reflect the principle of the conservation of matter. Focus question: What is the mole ratio in which lead nitrate, Pb(NO3)2 (aq), and sodium iodide,

NaI(aq), react? Procedure 2.1 Place 5 clean, dry test tubes in a rack. Label the test tubes 1 to 5. 2.2 Using a clean pipette, dispense 0,5 mL of the lead nitrate solution into test tube 1. Similarly, dispense the

volume of lead nitrate solution, in mL, into the other test tubes as indicated in Table 1 below. Table 1:

Test Tube 1 2 3 4 5 Volume (mL) of Pb(NO3)2 [0.50M] to add 0.5 1.0 1.5 2.0 2.5

2.3 Using a clean pipette, dispense 2,5 mL of the sodium iodide solution into test tube 1, to make a total

volume of 3.0 mL. Similarly, dispense the volume of sodium iodide solution, in mL, into the other test tubes as indicated in Table 2 below.

Table 2:

Test Tube 1 2 3 4 5 Volume (mL) of NaI [0,50M] to add 2.5 2.0 1.5 1.0 0.5

2.4 Pour boiling water into a beaker to a depth of about 2 cm. Place the test tubes in the water for five

minutes. 2.5 Remove the test tubes and allow them to stand for about another five minutes. 2.6 Use a ruler to measure the approximate height of the precipitate that has formed in each test tube. 2.7 Prepare a graph with the height of the precipitate (mm) on the Y axis. On the X axis put the volume of

lead nitrate solution (from 0 mL to 3.0 mL), as well as the volume of sodium iodide solution (from 3.0 mL to 0 mL)

2.8 Draw a best fit line through the set of points between 0 and the volume of lead nitrate that gave the most precipitate. Now draw a best fit straight line through the set of points between the most precipitate and 3.0 mL of lead nitrate (i.e. 0 mL of sodium iodide and no reaction). The two lines will intersect at the true maximum point on the curve. Drop a perpendicular from this point to the X axis and record the volume of Pb(NO3)2(aq) and NaI(aq) where the perpendicular touches the axis.

Disposal of chemical waste Lead salts and lead solutions are poisonous. Please dispose of them in the waste bottle provided, and do not wash them down the sink. References 1. RADMASTE Microchem by S Durbach, B Bell, M Liwanga and J Bradley (Ed); 1997 2. Microscale General Chemistry Laboratory by Z Szafran, RM Pike and JC Foster; second edition Wiley

Publishing Company, 2003. Chapter 4. 3. Chemistry- the Central Science by TL Brown, HF LeMay and BE Bursten. Prentice Hall, 2006. Chapters 4, 13,

16. 4. Chemistry and Chemical Reactivity by JC Kotz en P Treichel, fourth edition. Saunders College Publishers,

1999