GCSE Science/Rates of reaction coursework
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In order to get maximum marks in this coursework it is vital that you discuss all factors which affect the rate of a reaction. These include:
- Temperature. The higher the temperature the more energy there is within the reaction. When there is energy in the reaction more reactants hit each other and then there are more successful reactions.
- Surface area. Smaller substances react better then large substances because the small substance has a larger surface area and if there are more reactants exposed there is more successful collision.
- Concentration. The more molecules in a chemical the more concentrated it is. When a chemical has a greater concentration of molecules the chances of successful collisions increases and therefore speeds up the reaction.
- Catalyst. Adding a catalyst will speed up a reaction by decreasing the energy needed to break the bonds. A catalyst is not chemically consumed by the reaction so does not have an effect on the final products.
- Pressure. Pressure results in there being more particles of each reagent in a smaller area, resulting in more reactions.
Skill P: Planning - the theory and your experiment design!
First, you can start by describing the reaction situation you are intending to investigate. For example, with the word and symbol equation, short description about the reaction, and so on. This sets the scene. If you are confident and chosen the VARIABLE you want to investigate you should try to make a prediction and maybe justify it with some theory if you can. You can continue in a broader context by introducing some background theory and descriptions of the factors or VARIABLES which may have an effect on the rate of the reaction you are studying (include briefly factors which might not apply). In your 'method' description use the correct units or descriptors. The factors to discuss might be ... amount of limestone?, temperature of reactants?, acid concentration?, volume of acid?, size of limestone pieces? (relate to surface area?), stirring of the reacting mixture, size of reaction vessel, volume of thiosulphate, any added water to dilute etc. Example of the theory is the factors will increase the molecules inside the chemical and it will eventually increase the rate of reaction.
Is there any other factor for the reaction you are studying?, will any of the reactants or products be affected by change in temperature or pressure? e.g. there are several reasons why the same acid should be used if its a reactant in the investigation, e.g. (1) its the hydrogen ion, H+(aq), is the active ingredient that actually 'attacks' the metal or carbonate, and acids can ionise to different extents, (2) 1 molar or 1 mol dm-3 (1M) H2SO4 is twice as acid as 1M HCl because each H2SO4 provides 2 H+'s whereas each HCl just 1. If you have decided, for example, to investigate the effect of acid concentration on the speed of a reaction, then everything else should be kept constant for a fair test, and this should be obvious in your plan for the reasons discussed above! If you haven't already chosen the VARIABLE, do so now, and make a prediction and justify it with some theory which you may have previously described and should refer to. Next you should describe initially, but briefly, some methods for following the reaction = measuring the speed of the reaction. If a gas is formed, there are at least two ways of collecting a gas e.g. initially empty gas syringe or a measuring cylinder/burette full of water inverted over water with appropriate tube connections and there is one other very different method available for 'following' the reaction using a balance to record the mass loss. The hydrochloric acid - sodium thiosulphate reaction depends on the time for a certain amount of sulphur precipitate to form and obscuring a marked black X on white paper. Briefly explain how the method can be used to measure the speed - the results of the first few minutes is usually the most crucial - you can discuss (briefly) other methods, but perhaps better in evaluation as a means of further evidence. When you have decided on the method, give a detailed description of how you might carry it out. Include details of the amounts of chemicals you might use mass, volumes, dilutions + UNITS etc. etc. Clearly indicate why the method would be expected to produce precise and reliable evidence - the results! Include 'health and safety' points. If you are looking at changing the reaction temperature, its not easy to accurately vary and control the temperature of the reactants without a thermostated water bath to hold the reaction flask in. Even with a thermostated water bath (normally only available to advanced level students), all the reactant solutions should be pre-warmed in the bath before mixing and start the timing and recoding of results. If you are varying temperature, you need to heat up the reactant solutions separately and take their temperatures, mix, start stopwatch. However, they will cool a little standing out in the laboratory, so not completely satisfactory solution to the problem. In the case of the sodium thiosulphate - acid reaction, you can leave the thermometer in the flask and take the temperature at the end, then use an average for the temperature of the reaction. If temperature isn't a variable, it must be kept constant. The simplest solution here, is to make sure all the chemicals have been standing in the laboratory prior to the lesson. Then, they will all be at the same temperature, which should be recorded. If more experiments are conducted at another the time, the temperature must again be checked and recorded. Refer to any previous laboratory experience with 'rate of reaction' experiments which may have helped you decide and design the experimental method. A clearly labelled diagram of the method with a brief outline of how you intend to carry out the experiments - this cuts down on the writing and makes the scene clear! You must give details of how long you might time the experiment as well as the time interval between experimental readings REMEMBER you can change your 'recipe' or way of doing the experiment. If you have to change anything, describe and explain the changes you have made to the procedure (some of this might count as valuable marks for the EVALUATION skill) AND DON'T FORGET AT ALL TIMES QUOTE THE CORRECT UNITS in P, O, A or E.
Skill O: Obtaining evidence
Observations, measurements, in other words the results! (Possibly some data you might have been given)
These must be clearly recorded in neat tables showing all the units e.g.
Run 3: acid concentration? temperature ???? Time? Gas volume???? Gas volume??? (Repeat) corrected gas volume??? 0??? ??? ??? 1 5 3??? 2 11 9???
You can produce a summary table with the average/corrected (if necessary) gas volumes v time for all the different acid concentrations or whatever variable For the hydrochloric acid - sodium thiosulphate reaction you are recording just the reaction time for different thiosulphate or acid concentrations or temperatures, so the data gathering and subsequent processing is 'simpler'. All experiments should be repeated where time allows checking for accuracy and consistency; this may become more necessary after you have done a preliminary analysis The 'bung effect'! - look up about dead volumes and its correction when dealing with gas volumes! Your recorded results should indicate the accuracy of the measuring equipment e.g. 0- 2 decimal places. Some of the work done here in presenting the results, e.g. working out averages etc. actually counts towards the mark for analysing (described below). Have you got enough results, do they seem ok? Starting the analysis as soon as possible will help you decide whether further, wider ranging or repeat experiments - best decided after examining the graphs of results (see below) - difficult to decide just looking at tables of data.
Skill A: Analysing and considering the evidence
What do the results mean in terms of your prediction and theory!
The results are initially processed into graphical form ('graphing') for several reasons for both the analysis and evaluating the experimental .... they can clearly show the general trend of the effect of changing that factor or variable, highlighting experimental 'runs' that don't seem to fit the pattern of the other sets of results for the other runs, individual points that don't seem the pattern of a particular sets of results - BUT ITS UP TO YOU Ideally you should plot the average(*) corrected gas volumes on the y axis and time on the x axis - what should the origin be? (* May depend on the consistency of your results). For the hydrochloric acid - sodium thiosulphate reaction you can plot either (i) reaction time, or (ii) 1/time versus a concentration or temperature (1/time = relative rate of reaction). It is best, if possible, to have all the average results points plotted on the same graph for easy comparison - take care because this may involve 4 or 5 lines for 4 or 5 different acid concentrations Make sure you use a clear KEY for the different line points and a clear title for the graph AND clearly label the axis including the units or whatever .. Use smooth 'best curves' for as many of the points as possible, though some parts of the graph might be linear, watch out for the 'scatter' - the experiment is not that easy to get good results. See Rates of Reaction Notes. From the graph you can then describe in words what the results mean, always refer to the graph lines and gradients directly - don't make vague comments. So what we are after is the main 'trend(s)' or 'pattern(s)' describing with reference to the graphs. Does the 'trend' of all the graph lines support you're your prediction, are all the results consistent with your prediction AND theory? For different the acid concentrations you can do a 2nd and more advanced graphical analysis of the limestone results. This involves measuring from the graph, the speed of the reaction at the start. Explain why best data at the start? (i.e. first 3-5 mins?). What graph could you then plot?... where does the graph line start?, origin?, what is the 'shape' of the graph? is it a better way of showing consistency (or inconsistency!) in your results? We are basically talking about plotting the initial rate versus e.g. acid concentration. If you are doing something like the hydrochloric acid - sodium thiosulphate reaction, your reaction time measures the formation of a fixed amount of sulphur per 'time'. So the rate is 'x amount of sulphur per time', which means the speed or rate is proportional to 1/time, then plot this 1/time against the concentration of the acid. From this graph re-discuss your findings in a more mathematical way and relate this to the particle collision theory of reactions! It's all about chance! and explain why/why not the results support your prediction.
Skill E: Evaluating
How good are your results then? error sources? can we improve the existing method? are there other experimental methods?
Do your results seem consistent and accurate - always refer directly to the graph or graphs in your analysis ... do any of the sets of results not fit in with the others?, do most/all sets of results fit a pattern?, are there any particular points that don't fit the pattern? (anomalies), can some results be ignored in drawing your conclusion(s)? if so, which results and why? QUOTE DIRECTLY - WITH REFERENCE TO YOUR GRAPH(s) Discuss possible sources of error which might lead to inconsistent results i.e. points or sets of results that don't fit the pattern AND how could the method be improved to minimise these sources of error ... e.g. chip size? ,temperature or pressure checks for each experiment? dead volume?, ? gas syringe operation? draughts? where these or any other factor OK? in other words how suitable was the method overall? Do think the results are reliable bearing in mind any anomalies? For the hydrochloric acid - sodium thiosulphate reaction think about the precipitate, observing it etc. What further experiments, using the same method or another method, could be done to support your prediction or conclusion? In other words give some detailed ideas on further work that would provide additional relevant evidence. e.g. in the case of the sodium thiosulphate - hydrochloric acid experiment , you can use a light gate to detect the precipitate formation. The system consists of a light beam emitter and sensor connected to computer and the reaction vessel is placed between the emitter and sensor. The light reading falls as the sulphur precipitate forms. Keeping the temperature constant is really important for a 'fair test' if you are investigating speed of reaction/rate of reaction factors such as concentration of a soluble reactant or the particle size/surface area of a solid reactant. On the advanced gas calculations page, temperature sources of error and their correction are discussed in calculation example Q4b.3, although the calculation is above GCSE level, the ideas on sources of errors are legitimate for GCSE level. Note that if the temperature of a rates experiment was too low compared to all the other experiments, the 'double error' would occur again, but this time the measured gas volume and the calculated speed/rate of reaction would be lower than expected.