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# the determination of a rate equation

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Introduction

The determination of a rate equation: Skill P This experiment will involve the determination of a rate equation derived from experimental data. The experiment will involve the reaction between hydrochloric acid and sodium thiosulphate. 2HCl(aq) + Na2S2O3(aq) � 2NaCl(aq) + SO2(aq) + S(s) + H2O(l) Background theory (1, 2, 3, 5): The rate expression tells us how the rate of reaction depends on the concentration of the species involved. The generalised equation for this experiment: Rate= k [HCl(aq)]a [Na2S2O3(aq)]b The orders of reaction, a, b and c, have to be determined by experiment. a and b are constants whose values are usually 0, 1 or 2, and k is the rate constant. The rate constant is the proportionality constant k in a rate equation. a is the order of reaction with respect to reactant HCl, and b is the order of reaction with respect to sodium thiosulphate. The overall order of reaction is the sum of the powers of the concentrations of the two individual reactants, hydrochloric acid and sodium thiosulphate, in the rate equation, that is (a+ b). By rate of reaction we mean the change in concentration of a reactant (or a product) in a given period of time. Order of a reaction is the power to which we have to raise the concentration to fit the rate equation. ...read more.

Middle

sodium thiosulphate has a low hazard > sulphur has a low hazard > sulphur dioxide is toxic and inhalation of sulphur dioxide can cause breathing difficulties such as asthma > Wear goggles all the time Apparatus list (for one investigation only excludes repeats): Item Quantity Concentration and volume Measuring cylinder 2 100ml, 25ml Thermometer 1 - stop clock 1 - Goggles 1 - Tap water 220cm3 pH 7 Hydrochloric acid 200cm3 2.0 moldm-3 Sodium thiosulphate 900cm3 0.4 moldm-3 Conical flask 2 100ml, 200ml Tile with black cross in the middle 1 - Lab coat 1 - Table to show reasons for choice of apparatus: Item What it is used for Reason for choice Measuring cylinder To measure the volume of sodium thiosulphate, water and hydrochloric acid used Easy to handle and accurate Thermometer To measure the room temperature Accurate Stop clock To time the start of the reaction and the end Accurate and easy to handle Goggles For eye protection The only way to protect eyes Tap water To dilute the concentration of hydrochloric acid and sodium thiosulphate It is harmless Hydrochloric acid One of the reactants Low hazard Sodium thiosulphate One of the reactants Low hazard Conical flask Chemicals will be mixed together and the place where the reaction takes place Always in the school laboratory Tile with black cross in the middle To see when the mixture in the conical ...read more.

Conclusion

15. Rate= k [HCl(aq)]a When varying the concentration of sodium thiosulphate: Use the same method as when varying the concentration of sodium thiosulphate. Use a 200ml conical flask instead and keep the hydrochloric acid concentration constant using 45cm3 only. Deduce the order of reaction from the resulting graph and put your result into the rate equation: Rate= k[Na2S2O3(aq)]b Dilution tables: Serial dilution table for sodium thiosulphate: Sodium thiosulphate (cm3) Water (cm3) Concentration(moldm-3) 100 0 0.40 80 20 0.32 60 40 0.24 50 50 0.20 40 60 0.16 Serial dilution table for hydrochloric acid: Hydrochloric acid (cm3) Water (cm3) Concentration(moldm-3) 25 0 2.0 20 5 1.6 15 10 1.2 10 15 0.8 5 20 0.4 When varying the concentration of Hydrochloric acid: 1. Concentration (moldm-3) = (moles/volume (dm3)) 2= (moles/0.025) Moles=0.05 it is a 2:1 ratio, so moles for sodium thiosulphate: 0.025 2. 0.4=(0.025/volume) Volume=0.0625 dm3 Volume=62.5cm3 use 65cm3 of sodium thiosulphate in excess when varying the concentration of hydrochloric acid. When varying the concentration of Sodium thiosulphate: 0.4=(moles/0.1) Moles=0.04 as it is a 2:1 ratio for hydrochloric acid, so moles for hydrochloric acid: 0.08 2=(0.08/volume) Volume=0.04dm3 Volume=40cm3 Use 45cm3 of hydrochloric acid when varying the concentration of sodium thiosulphate. Bibliography: I. Essential A2 Chemistry for OCR by Ted Lister and Janet Renshaw, page 98-105 II. Chemistry A2 endorsed by OCR, page 108-109 III. AS Chemistry by Andrew Hunt,page 114-115 IV. ...read more.

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