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  • Level: GCSE
  • Subject: Science
  • Essay length: 3586 words

An Experiment to Investigate the Effect of Changing the Concentration of Hydrochloric Acid on the Rate of the Reaction between Hydrochloric Acid and Sodium Thiosulphate

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An Experiment to Investigate the Effect of Changing the Concentration of Hydrochloric Acid on the Rate of the Reaction between Hydrochloric Acid and Sodium Thiosulphate Aim When hydrochloric acid reacts with sodium thiosulphate the solution becomes cloudy and eventually opaque due to formation of sulphur. The time taken for the solution to become opaque i.e. a certain amount of sulphur to form can indicate the rate of reaction. In this experiment the effect of changing the concentration of hydrochloric acid on the rate of reaction below is investigated. The products of reaction are: * Sodium chloride, which will be in the aqueous solution and will appear colourless. * The solid sulphur, which forms in the solution, causes the water to look cloudy. It covers the black cross under the flask * Sulphur dioxide gas which smells like purgent. Prediction I predict that the rate of reaction above will increase as the concentration of hydrochloric acid increases. This will be indicated by the increased rate at which the solid sulphur is produced and the solution becomes opaque. I also predict that as the concentration of hydrochloric acid increases the average time taken for the laminated, black cross to disappear decreases. I can explain my prediction with the collision theory. Collision Theory The "collision theory" states that if there are more particles of a fixed amount of substance i.e. a higher concentration the particles are more likely to collide with each other then they would be in a less concentrated solution. The diagram below illustrates this. So as I increase the concentration of hydrochloric acid, the number of collisions with the sodium thiosulphate particles increase and hence the rate of reaction increases and less time is taken for cross to be covered. Definition of a molarity solution Another way of expressing concentration, the way that we will use, is called molarity. Molarity is the number of moles of solute dissolved in one litre.


* Some paper towels (to wipe spillages) * Laminated paper with a black cross in the centre Apparatus * A 150cm³ conical flask * A 50cm³ measuring cylinder * A 25cm³ measuring cylinder * A 5cm³ measuring cylinder * Stopwatch * 2 Pipettes * Thermometer Method 1. Prepare my attire by putting on a laboratory coat and safety goggles and tie back loose hair safely. Make sure that you prepare a table to write down your results and have near a pen to do so. 2. Gather all the apparatus and the materials and keep them so that they can be used in a systematic way during the experiments. The main containers of sodium thiosulphate a hydrochloric acid were labelled to avoid mix up. The distilled water was made available in a blue container (for identification) with a dispenser. 3. Place the laminated cross (pre-prepared using the computer) on the table. The same cross will be used for all the experiments ensure a fair test. 4. Place a 150cm³ conical flask on the laminated paper with the cross. Pour 50 cm³ of sodium thiosulphate into the 50 cm³ measuring cylinder. To make sure the reading is accurate bend your knees to the ground so that your eyes are in line with the measuring cylinder and make sure the measuring cylinder read 50 cm³. It is important that no hydrochloric acid gets into this cylinder or else the reaction will start in the cylinder. If there is too much or too less sodium thiosulphate use a pipette to add or take away some sodium thiosulphate. Then pour into the conical flask this 50 cm³ of sodium thiosulphate. 5. In 20-cm³ cylinder pour 20 cm³ of concentrated hydrochloric acid (look at the table below for volumes to change HCl concentration). Bend my knees so that my eyes are in line with the measuring cylinder and makes sure the reading is correct.


Error bars show the reliability of data. The error bars on Graph 1 show that my data is quite reliable. The most unreliable data on the graph is the error bar drawn for the concentration of hydrochloric acid of 0.25. The error bar drawn for concentration of hydrochloric acid of 0.75 shows that it was the most reliable because all the readings gave the same results. Overall my error bars show that my results are very reliable because all four data pass through my curve of best fit and there are no percentage errors. Although my results were quite reliable it would have helped to have more readings so I could draw more conclusions however, the time limit was a problem and only allowed me to collect data for five different hydrochloric acid concentrations. If I repeated the experiment I would have a wider range of concentrations so that my results were even more reliable and so I could conclude more from my results. The range would be 0.0, 0.1, 0.2, 0.3...0.8, 0.9, 1.0M. On graph three, I discussed earlier that there was an anomaly as highlighted on the graph. It did not fit in with the line of best fit I drew on the graph. This could be due to several reasons, however; I think it was most likely down to my reaction time when stopping the stopwatch was a little fast. There is no practical way of controlling this, as it is how fast my reaction takes place when seeing the cross disappear. It could be due to inaccuracy when reading volumes. Perhaps I misread this meniscus line and there was too little volume of the solution measured. It would be better if I made sure that my eye was level with the meniscus line and making sure that using modern technology such as electronic pipettes/ syringes to excrete the exact amount of volume of the solutions. Overall I think that my investigation has been a fair success!

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