C Copy me Reaction of sodium thiosulphate and hydrochloric acid GCSE Investigation C Copy me The reaction… • Sodium thiosulphate, Na2S2O3 reacts with HCl to give a reaction mixture that gets more cloudy as yellow sulphur is formed. • Na2S2O3(aq) + 2HCl(aq)  2NaCl(aq) + S(s) + SO2(g) + H2O(l) C Copy me What will we measure? • The time for a cross beneath the reaction flask to disappear as the reaction gets cloudy. • We will do the reaction at several temperatures and try to work out the rate of reaction. C Copy me A The Investigation Answer Q’s • The investigation will involve looking at how concentration can affect the rate of this reaction. • We can control concentration by diluting solutions of known strenghts • We want to investigate 5 or 6 concentrations – what would be a suitable range for investigation of reactions in solution? • How many times would you attempt to repeat the reaction to improve reliability of results? A Answer Q’s Errors • There are several errors in the experiment. • The devices for measuring volume are at best accurate to 0.2 mL. • What would the result be of having too much or too little of each reagent? • If the total volume of the reaction is 50mL what is the percentage error from measuring volume? • Error % = error/range x 100 • Especially at lower temperature the end of the reaction is difficult to judge. • Why is it inappropriate to use reaction times to fractions of 1/100 seconds? D Do this Background • Using your background knowledge write a short account of how and why you would expect concentration to affect react rate. • Then use the reference materials to make a prediction of how rate will change with concentration – for higher (Level 8) you need to make a prediction that has numbers in it!. In your write up quote the source of the information (including web addresses if the internet is used). The Reaction Na2S2O3 + 2HCl  2NaCl + S + SO2 +H2O When hydrochloric acid is added to the sodium thiosulphate a reaction occurs which produces a cloudy suspension of solid sulphur. The time it takes for the cross to disappear can be used to investigate rate of reaction. The Plan (1) • Describe what you will be investigating and what you will be measuring. • Give a list of the apparatus you plan to use and a method which could be followed by a student who has not done the experiment. Your method should include quantities of chemicals. Plan (2) • Describe how you will make the experiment more reliable and what things you will have to look out for that could go wrong. • Include some details on safety of the chemicals and the lab. • Give background research into things that will affect reaction rate and try to make a quantitative prediction – how much faster will the experiment go when you change concentration? • Do a trial experiment that can be used to help decide how exactly to do the reaction. Changing Concentration • It is important that the total volume of the reaction stays the same. • If you add more acid you need to take away the same amount of water. For example… Amount of thiosulphate (cm3) Amount of acid (cm3) Amount of water (cm3) 40 40 40 40 10 8 6 4 0 2 4 6 Analysis of results (1) • Describe what the basic pattern of results shows – what is the effect of the changes you make? • Describe the pattern in more detail using the numbers you have obtained in your results. • Produce a graph of the results with a line of best fit. Make sure the axes are labelled with the correct units. Analysis of results (2) • Using your results calculate the reaction rate for each concentration using… • Rate = 1000/time to react (units = 1000/s) • Produce another graph with Rate on the y-axis and concentration on the x-axis. • Does the rate double if you double the concentration? If it does not then how much faster does it get? How does the increase in rate relate to collision theory? Evaluation • Did the experiment work effectively to answer the aim of the investigation? • Did any of the results you obtained not fit the expected pattern? If they did say which ones and think of why this result might be off. • What could you do to improve the experiment or make it more accurate? • Describe in detail an experiment that you could do to test more ideas about collision theory. I’ve checked results from the experiments and while some were not too bad most were pants!! So I have put together the best results so we can use them as a class 1000/av g 1 2 3 4 5 6 7 8 9 avg 10 34 36 36 37 34 35 37 33 35 35.1 28 8 37 38 38 39 38 38 39 38 39 38.2 26 6 43 41 44 44 45 44 45 42 44 43.6 23 4 50 49 52 51 50 48 50 50 51 50.2 20 2 61 63 66 62 65 66 62 65 65 63.8 16 Choose 3 sets of results from sets 1-9 – do not use the same set as the person sitting next to you. Then work out values for your average and then 1000/average. Remember to put the correct units into the table for volume and time… Rewrite your own table using the results you have Making sure you have the correct units in the table…. thiosulphate acid water 40 10 0 40 8 2 40 6 4 40 4 6 40 2 8 1 2 3 average 1000/aver age