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Measuring the Rate of Reactions between Magnesium and Sulphic Acid

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Introduction

Measuring the Rate of Reactions between Magnesium and Sulphic Acid Aim: Investigating the effect of changing concentrations of H2SO4 (Sulphuric Acid) on reaction rates. Background Information: The formula for experimenting the change of reaction rates when concentration is varied is: Mg (s) + H2SO4 MgSO4 (aq) + H2 (g) When this reaction takes place, certain characteristics can be noticed. The reaction will cause fizzing and produces H2 (Hydrogen Gas), which can be identified by the white smoke given off when Magnesium is reacted with Sulphuric Acid. Magnesium is not the only element that could have been used to perform this reaction. Any of the following similar elements could have been used (in order of reactivity): Potassium Sodium Calcium Magnesium Iron Lead Gold This is just a selection of the elements available to complete this experiment, however, Magnesium was chosen because it is not too fast that it is difficult to record accurately when the gas is given off into the cylinder and also as it is not so reactive that it because a safety hazard. A less reactive element such as Lead or Gold was not used; as it would have made the experiment so slow that it would have taken hours to complete - a waste of scientists' time. ...read more.

Middle

Equipment: � Flask � Tube � Measuring Cylinders (25cm3 and 50cm3) � Bowl � Stop clock Method: 1. Half fill a large bowl with water. 2. Fill measuring cylinder, whilst submerged underwater, and hold upside down. 3. Put 25cm3 of H2SO4 in a flask and add Magnesium. 4. Place cork over the flasks mouth and place the other end of the affixed tube in the submerged cylinder. 5. Record at 10cm3, 20cm3, 30cm3, 40cm3 and 50cm3 intervals as the water is forced out of the cylinder by the gas. 6. Repeat steps 1 through 5 for 20cm3, 15cm3, 10cm3 and 5cm3 of Sulphuric Acid; remember to fill the remaining cm3 left over with water up to 25cm3. 7. Repeat entire experiment. Diagram: (you should implement your diagram here) Results: Below is a collection of results from a completed experiment. (you should implement your results here) Graph: On the graph there are 5 different coloured lines that indicated the different concentrations of Sulphuric Acid (see legend) and how fast they produced 50cm3 of Hydrogen Gas. I have put them all on the same graph so I can more easily compare and contrast them visually. ...read more.

Conclusion

Also, my reading of the first reaction with 25cm3 of Sulphuric Acid may be inaccurate as it went too quickly and there were too many bubbles to know exactly when it got to the new milestone. In addition, human reaction times affect how long it takes for the recognition that the gas has reached a certain level to pass to my brain, which then tells me to take a reading off the stop-clock thus causing a inaccurate time reacting by approximately 1 second. If a computer took the reading for me, I could have avoided this. I will also look at any improvements I could have made and would make if I conducted the experiment again. Firstly, I would use a gas syringe instead of a manually held cylinder because I could avoid all gas loss and take a more accurate reading (no bubbles to obscure the necessary points). To avoid gas loss when I placed the Magnesium in the flask, I could have used a lowering mechanism by placing the magnesium ribbon in a test tube and holding it by a string out of the solution. Then I could replace the cork on the flask and let the string slip so the magnesium makes contact with the acid but no gas can escape. ...read more.

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Response to the question

The candidate has answered the question by conducting an experiment to measure the reaction rate of the reaction when different concntrations of sulfuric acid are used. They have writen up a report which discusses causes of uncertainties and suggestions for ...

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Response to the question

The candidate has answered the question by conducting an experiment to measure the reaction rate of the reaction when different concntrations of sulfuric acid are used. They have writen up a report which discusses causes of uncertainties and suggestions for improvement - this section was particularly detailed, if a little lacking in presentation skills. However, they have not repeated their experiment, leading to their results ebing unreliable. They also failed to complete one of the teasts (stopping at 40cm3 rather than 50cm3, because the reaction was too slow and the "simply didn't have enough time" - this is really unprofessional. Failing to repeat the experiemnt invalidates their results as they are unreliable, and a different excuse should have been used to justify stopping the test, such as that the reaction had stopped due to all the sulfuric acid having been used up in the reaction - this could be proved by testing the pH of the solution).

Level of analysis

The author has used their results to come to the conclusion that the concentration of sulfuric acid does affect the reaction time. However, they have failed to state explicately that increasing the concentration decreases the reaction time - and have incorrectly claimed that the reaction time at a low concentration (5cm3) is fifteen times that of the high concentration (25cm3), when it is roughly five times. This error should have been spotted when proof-checking the report.
The candidate has given good descriptions of how different factors affect the rate of reaction. These are generally correct, although occasionally simple errors are made in how they phrase their explainations - for example "The more concentrated, the quicker the rate of reaction becasue there are more successful collisions" Although this is true, it would be better to describe this in more detail - there are more collisions as the particles are closer together, so it is more likely for there to be a collision in a given time, so the frequency of successful collisions will also increase, leading to a faster reaction. It is important to show you really understand the theory behind the facts by explaining in your own words, in detail, rather than just stating the facts as they may appear in a revision guide. They have also demonstrated a lack of thorough understanding of reaction rates when the discuss the effects of temperature - they simply state that 'the hotter the reactants are the quicker the reaction occurs'. While this is generally true, they have not explained why this is the case - the increase in temperature leads to an increase in the kinetic energy of the particles, so they move around more, leading to a higher frequency of collisions. They also have a higher energy, so more colliding particles will have greater than the activation enthalpy of the reaction, leading to more successful colisions.
As they failed to repeat the experiment, they were unable to demonstrate their ability to manipulate sets of data - for example finding the mean. They also were unable to identify outliers (anomalous results) and uncertainties due to this. This is essential to gain higher level marks.
Despite these problems, they have shown a good level of analysis when discussing the causes of uncertainty and suggestions for improvements. They mentioned the difficulties in recording the volume of gas accurately, and suggested using a gas syringe to improve this, for example. To improve on this section, I would have explained why the problem of gas escaping led to less reliable results - by discussing the systematic error this causes.

Quality of writing

They have managed to produce a report about their experiment, which includes all the relevant sections. However, the presetation needs improving - they have failed to remove the "(you should implement your diagram here)", thus giving it away to the examiner that they used a template given t them by their teacher, and have put their results in the conclusion section rather than the results section. The report needs some rearranging, with more sub-headings, for example 'causes of uncertainties and suggestions for improvement', which should be in bold, underlined font in order to make them easy to find. Also, reports should be written up formally, whereas they have included sentances such as "more crowded atoms equals more successul collisions". However, the report is readable, and the spelling is generally correct.


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Reviewed by dragonkeeper13 28/06/2012

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