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Introduction Sulphate ions and iodine can be formed from reactions between peroxodisulphate ions and iodine ions. S2O82-aq + 2I-aq -----------> 2S042-aq + I2aq The reactants and the sulphate ions are colourless so the achievements of the experiment must be determined by the iodine produced. If starch is added to the reaction iodine is more clearly visible as a strong complex of blue-black colour is formed. For this experiment I will be measuring the time taken for the iodine to appear in the solution. To make this more clear and accurate thiosulphate ions are added to the mixture which will emphasize the changes. This chemical also returns iodine to iodine ions. 2S2O32-aq + I2aq -----------> S4062-aq + 2I-aq As the reactants of this reactions are colourless it is difficult to tell when the colour change will happen, and when it does it is quite sudden. This happens because the thiosulphate is being used up in the reactions, and when it is all used up the starch-iodine complex is formed resulting in the colour change. Equipment and apparatus needed: "¢ Test tubes "¢ Boiling tubes "¢ Thermometer 0-110°C "¢ Stopwatch "¢ Burettes "¢ 15cm3 Potassium iodide solution,KI, 1.00 mol dm-3 "¢ 10cm3 Potassium peroxodisulphate solution,K2S2O8, 0.0400 mol dm-3 "¢ 10cm3 Sodium thiosulphate solution, Na2S2O3, 0.0100 mol dm-3 "¢ 5cm3 fresh Starch solution "¢ Test tube rack Method "¢ Read and understand carefully the table below illustrating the different mixtures of concentrations for individual experiments. mixture Volume of KLaq/ cm3 Volume of water / cm3 Volume of Na2S2O3aq / cm3 Volume of starch solution/cm3 Volume of K2S2O8aq / cm3 1 5 0 2 1 2 2 4 1 2 1 2 3 3 2 2 1 2 4 2 3 2 1 2 5 1 4 2 1 2 "¢ When making the mixtures, make sure that all chemicals are added to a boiling tube, except the potassium peroxodisulphate which must be added separately as this is what will trigger the reaction. "¢ Make up mixture 1 and place a thermometer in the boiling tube. Add the potassium peroxodisulphate at begin the timing with the clock "¢ Stop timing when the solution turns blue-black "¢ Constantly stir the solution. "¢ Take a temperature reading "¢ Repeat procedure for each mixture of different concentrations. Results A Table mixture Concentration of I-aq / mol dm-3 Clock time / seconds Rate / mol dm-3 s-1 Temperature / °C 1 0.5 28.8 3.45 X10-7 23 2 0.4 40.9 2.44 X10-7 23 3 0.3 58.0 1.72 X10-7 26 4 0.2 136.7 7.32 X10-8 26 5 0.1 455.5 2.20 X10-8 24 Questions B Rate = concentration of I2 Time = [ I2 ] = mol dm-3 s-1 S [ I2 ] = 1.0 X10-5 Mixture: 1. = 1.0 X10-5 = 3.45 X10-7 28.8 2. = 1.0 X10-5 = 2.44 X10-7 40.9 3. = 1.0 X10-5 = 1.72 X10-7 58.0 4. = 1.0 X10-5 = 7.32 X10-8 136.7 5. = 1.0 X10-5 = 2.20 X10-8 455.5 C The Iodine ions are in excess in the reaction mixtures. The reactant that is not in excess in the reaction will be used up and it is this which will determine the total amount of iodine produced. D Moles = mol dm-3 x dm3 = 0.04 x 2/1000 = 8x10-5 mols of S2O8 2- E i 2cm3 of Na2S2O3 0.0100 mol dm-3 Moles of Na2S2O3 = 0.01 x 2/1000 = 2x10-5 moles of S2O3 ii Moles of = 1.0 X10-5 I2 iii Percentage % of reaction = 1.0 X10-5 x100 8.0 X10-5 = 12.5 %
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Introduction Sulphate ions and iodine can be formed from reactions between peroxodisulphate ions and iodine ions. S2O82-aq + 2I-aq -----------> 2S042-aq + I2aq The reactants and the sulphate ions are colourless so the achievements of the experiment must be determined by the iodine produced. If starch is added to the reaction iodine is more clearly visible as a strong complex of blue-black colour is formed. For this experiment I will be measuring the time taken for the iodine to appear in the solution. To make this more clear and accurate thiosulphate ions are added...
mixtures. The reactant that is not in excess in the reaction will be used up and it is this which will determine the total amount of iodine produced.

D Moles = mol dm-3 x dm3

= 0.04 x 2/1000

= 8x10-5 mols of S2O8 2-

E i 2cm3 of Na2S2O3 0.0100 mol dm-3

Moles of Na2S2O3 = 0.01 x 2/1000

= 2x10-5 moles of S2O3

ii Moles of = 1.0 X10-5

I2

iii Percentage % of reaction = 1.0 X10-5 x100

8.0 X10-5

= 12.5 %

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Experiment: Determining pH of identical strong...Experiment: Determining pH of identical strong and weak solutions Date: 31/10/03 Aim: Plan and perform a first-hand investigation to measure the pH of identical concentrations of strong and weak acids. Equipment: · Deionised water H2O · Tartaric Acid C4H6O6 · Citric Acid C6H8O7 · Sulfuric Acid H2SO4 · Acetic Acid CH3COOH · Hydrochloric Acid HCl · Ammonium Chloride NH4Cl · Tap Water H2O · Nitric Acid HNO3 · Probe · Ferric chloride FeCl3 · Wash bottle · Sodium Chloride in tap water NaCl · Beakers · Sodium Chloride in deionised water NaCl · Data logger Method: 1 Make sure there is an equal molarity of each substance. In this case 0.1 moles/litre. Therefore there are 0.1 moles of each substance in solution. 2 Place each of the substances in a beaker. 3 Calibrate the data logger by using buffer zones of pH 4 and 10. 4 Place the probe into one beaker and click the start button on the data logger. Record the pH level of the substance. 5 Remove the probe and wash thoroughly using the wash bottle. 6 Repeat for all the other substances and record their pH's. 7 Determine which solutions are strong and which are weak. Results: Substance pH Acidic, Basic or Neutral Strong or Weak Citric Acid C6H8O7 2.4 Acidic Weak Acetic Acid CH3COOH 3.0 Acidic Weak Tartaric Acid C4H6O6 2.3 Acidic Weak Nitric Acid HNO3 1.5 Acidic Strong Hydrochloric Acid HCl 1.5 Acidic Strong Sulfuric Acid H2SO4 1.4 Acidic Strong Ferric chloride FeCl3 1.5 Acidic Strong Ammonium Chloride NH4Cl 7.0 Neutral - Sodium Chloride in deionised water NaCl 5.6 Acidic Very Weak Sodium Chloride in tap water NaCl 7.2 Neutral - Deionised water H2O 6.0 Neutral - Tap Water H2O 7.0 Neutral - Discussion: By observing our results it can be seen that even though all solutions have the same concentration they can still be strong or week. It can be seen that all organic acids eg. Acetic acid, Citric acid and Tartaric acid are all weak acids whereas all inorganic acids such as Sulfuric acid, Nitric acid and Hydrochloric acids were strong. It was also discovered that ionic molecules could be acidic eg. Ferric Chloride. In the past it has been said that all ionic substances tend to be more neutral than acidic or basic. This has all been changed now and we have the Brønsted Lowry concept. With the Brønsted-Lowry concept we usually refer to a hydrogen ion as a proton. That is because a proton is all that is left when a hydrogen atom loses an electron to become an ion. Brønsted and Lowry independently came up with the idea that an acid is an acid because it provides or donates a proton to something else. When an acid reacts, the proton is transferred from one chemical to another. Note that in order for an acid to act like an acid, there needs to be something for it to react with. There needs to be something to take the proton. There needs to be a base. A base is a proton acceptor. Compare this to the definition that an acid is a proton donor. Bases are the opposite of acids. Bases are basic because they take or accept protons. Hydroxide ion, for example can accept a proton to form water. Brønsted and Lowry realized that not all bases had to have a hydroxide ion. As long as something can accept a proton it is a base. So anything, hydroxide or not, that can accept a proton is a base under the Brønsted-Lowry definition. Example: HCl aq + H2O l à H3O+ aq + Cl - aq [strong acid] And CH3COOH aq + H2O l à H3O+ aq+ CH3COO "“ aq [weak acid] Citric Acid: A water-soluble weak organic tribasic acid found in many fruits, esp. citrus fruits, and used in pharmaceuticals and as a flavouring E330. It is extracted from citrus fruits or made by fermenting molasses and is an intermediate in carbohydrate metabolism. Tartaric Acid: Dihydroxysuccinic acid; made from crude tartar; a laxative and refrigerant; used in the manufacture of various effervescing powders, tablets, and granules. The main disadvantage of this experiment was that the probe had to be washed thoroughly after each substance was tested or else some residue might be left behind altering the pH of the next substance. Also it was necessary to keep the electrical items used in the experiment away from the solutions so nothing is spilt onto the electrical items. Risk Assessment: Care must be taken when handling the substances, as some can be harmful. Especially those substances with a low pH level, because if spilt on the skin, irritation may occur or burning. Conclusion: The pH of identical strong and weak substances was determined by performing a first-hand investigation.   

Experiment: Determining pH of identical strong and weak solutions Date: 31/10/03 Aim: Plan and perform a first-hand investigation to measure the pH of identical concentrations of strong and weak acids. Equipment: · Deionised water H2O · Tartaric Acid C4H6O6 · Citric Acid C6H8O7 · Sulfuric Acid H2SO4 · Acetic Acid...

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