Friday, May 4, 2012

CONCENTRATION OF ACID

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CONCENTRATION OF ACID


Aim


The aim of this investigation is to find out how concentration of acid affects the rate of reaction between hydrochloric acid and magnesium ribbon. The rate of a chemical reaction is a measure of how fast the reaction takes place. It is important to remember that a rapid reaction is completed in a short period of time. Some reactions are very fast, e.g. (the formation of silver chloride precipitate when silver nitrate and hydrochloric acid solutions are mixed. In this investigation we will test different concentrations of acid reacting with magnesium.


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Prediction


I predict that as the concentration of the hydrochloric acid increases, the time taken for the magnesium to disappear decreases. I predict that when the concentration of the hydrochloric acid doubles, the rate of the reaction doubles.


Linking prediction to theory


Reaction rate and concentration.


The collision theory describes how the rate of reaction increase (the time taken for the magnesium ribbon to disappear when it is reacted with hydrochloric acid) when the concentration of HCL increases. The theory states that if, the more concentrated the reactants, the greater the number of collisions between particles increase. This also explains why the greatest rate of reaction is usually as soon as the reactants have been mixed, i.e. they are both at their highest concentrations. As the reaction continues, the concentration of the reacting substances decreases and so does the rate of reaction. We must consider what happens when a reaction takes place. First of all the particles of the reacting substances must collide with each other, and secondly a fixed amount of energy called activation energy (Ea) must be reached if the reaction is to take place. If the particles can produce the right amount of energy (i.e. if they collide fast enough and in the right direction) a reaction will take place. The reaction is speeded up if the number of collisions is increased. In this investigation we must consider the topic of variables.


Clearly, the time taken for the magnesium to disappear when it is placed in different concentrations of HCL is related in some way. The higher the concentration of HCL I use, the less time it takes for the magnesium to disappear and so the rate reaction increases. The concentration of HCL used is independant variable because it will vary, and the time taken for the rate of reaction to take place (i.e. the magnesium to disappear) is the dependant variable because it depends upon the concentration of


hydrochloric acid. Other variables throughout the investigation, which will vary are the volume of water used, and the volume of hydrochloric acid. The variables which will remain unchanged are the temperature (room temperature) will stay the same in order for it to be a fair test, because if the temperature changes it will effect the rate of reaction between the reactants, either by speeding it up if the temperature rises because the particles move faster and travel a greater distance in a given time and so will be involved in more collisions. Or the temperature may slow the reaction down due to particles moving slower. The amount of magnesium used will stay the same (cm long), so that it is a fair test. The time it takes for the magnesium to disappear will be measured accurately using a stop clock as soon as the HCL is poured into beaker with the magnesium in it. This way most results will be accurate. It is important to keep the reactants separate while setting up the apparatus so that the starting time of the reaction will be measured accurately.


Factors that may not be easy to control are, how well the solution is mixed when it is diluted, to get the correct concentration. A way to come around could be (stirring the test tube twice, or shaking the test tube twice). This factor is quite important because it determines exactly what the concentration of the solution is each time it is mixed, if the stirring factor is not carefully controlled it could lead to inaccurate results if the concentration of the acid solution is not what we calculated it to be. During a chemical reaction the particles have to collide with enough energy to first break the bonds and then to form the new bonds and the rearranged electrons, so its safe to say that some of the particles don’t have enough energy to react when they collide. The


minimum amount of energy that is required to break the bonds is called activation


energy (Ea). If the activation energy is high only a small amount of particles will have


enough energy to react so the reaction rate would be very small, however the activation energy is very low the number of particles with that amount of energy would be so high, so start, so the reaction rate would be higher. An example of low (Ea) would be in explosives when they only need a small input of energy to start their exceedly exothermic reactions. A change in concentration is a change in the number of particles in a given volume.


If I increase the volume


a) The particles are more crowded so they collide more often.


b) Even though the average amount of energy possessed by a particle does not change, there are more particles with each amount of energy - more particles with the activation energy.





Before starting the investigation, I decided to do some research about magnesium and hydrochloric acid.


Magnesium is a light, shiny grey metallic element; symbol Mg, atomic number 1, found in-group two in the periodic table. It is quite reactive giving vigorous reactions towards acids. It is one of the alkaline earth metals, and the lightest of the commonly used metals. It is used in alloys, flash photography, flares, fireworks and flash bulbs because it burns vigorously in air with a bright white light. Magnesium reacts with steam to release hydrogen and it also burns in carbon dioxide gas.


Hydrochloric acid, HCl, is a solution of hydrogen chloride (a colourless acidic gas) in water. The concentrated acid is about 5% hydrogen chloride and is corrosive. The acid is a typical strong, monobasic acid forming only one series of salts, the chlorides. Like most acids, it releases hydrogen ions when it is added to water and certain metals, and has a pH of less than 7. Hydrochloric acid is a common laboratory acid.


Extracts from a chemistry coursework from an Internet source and


The HUTCHINSON Dictionary of SCIENCE second edition.


Before looking at the factors that can alter the rate of reaction, we must consider what happens when a reaction take place.


First of all, the particles of the reacting substances must collide with each other and, secondly, they need a certain amount of energy to break down the bonds of the particles and form new ones. This energy is called the activation energy or Ea. If a collision between particles can produce sufficient energy (i.e. if they collide fast enough and in the right direction) a reaction will take place. Not all collisions will result in a reaction.


The investigation could be done using one variable and therefore have a set of results which were related in some way. The variables that could be used are


Concentration


Particle size/surface area


Pressure (for reactions involving gas)


Temperature


Light


Presence of a catalyst.


These variables can be used because


1. The more concentrated the reactants, the greater the rate of reaction will be. This is because increasing the concentration of the reactants increases the number of collisions between particles and, therefore, increases the rate of reaction.


. When one of the reactants is a solid, the reaction must take place on the surface area of the solid. By breaking up the solid into smaller pieces, the surface area is increased, giving a greater area of collisions to take place and so causing an increase in the rate of reaction.


. When one or more of the reactants are gases an increase in pressure can lead to an increased rate of reaction. The increase in pressure forces the particles closer together. This causes more collisions and increases the rate of reaction.


4. An increase in temperature produces an increase in the rate of reaction. A rise of 10ยบ C approximately doubles the rate of reaction. When a mixture of substances is heated, the particles move faster. This has two effects. Since the particles are moving faster they will travel greater distance in a given time and so will be involved in more collisions. Also, because the particles are moving faster a larger proportion of the collisions will exceed the activation energy and so the rate of reaction increases.


5. The rates of some reactions are increased by exposure to light. Light has a similar effect as temperature because it produces heat.


6. A catalyst is a substance, which can alter the rate of a reaction but remains chemically unchanged at the end of the reaction. Catalysts usually speed up a reaction. A catalyst, which slows down a reaction, is called a negative catalyst or inhibitor. Catalysts speed up reactions by providing an alternative pathway for the reaction, i.e. one that has much lower activation energy. More collisions will, therefore, have enough energy for this new pathway.


Extracts from Letts Study Guide, GCSE CHEMISTRY.





All this information is relevant to my investigation, as I now know what would happen to the molecules when using different variables. It also makes it easier to decide what variable I am going to use in this circumstance.


I decided to use the concentration of acid as my variable. I used 5 different strengths of hydrochloric acid. These strengths would determine the rates of reactions. I decided to measure the acid in molars. I predicted that the higher the concentration of the acid, the faster the reaction between magnesium ribbon and the hydrochloric acid. This would be because there were more acid molecules to react with the magnesium ribbon.





Before doing the actual experiment I decided to do some preliminary work. These were to tell me the details that I would need to know for my investigation to be successful. I saw from these preliminary investigations that the magnesium ribbon started to react with the hydrochloric acid the moment that I dropped it in. I decided that it would be a good idea to start timing the second that I dropped it in. When the ribbon had been eaten away by the acid, it stopped fizzing. I decided that I would stop timing the second that the fizzing stopped. I discovered from my preliminary experiments that when I used a low concentration of hydrochloric acid, for instance, 0.4 molar, it took a long time for the magnesium ribbon to be eaten away. I decided that it would be impractical to spend time on the following strengths of hydrochloric acid


1. molar


0.8 molar


0.4 molar


This was because they were the three slowest strengths of acids available to react with the magnesium ribbon over a period of time. I also learnt from my preliminary experiments that it was sometimes quite difficult to stop timing on the exact moment that the fizzing stopped. I decided therefore that I would carry out each of the experiments.


Apparatus/ methods


When the magnesium ribbon reacts with the hydrochloric acid, magnesium chloride is formed. I wrote down the equation to show this


Magnesium + Hydrochloric acid = Magnesium Chloride + Hydrogen


Mg + HCl = MgCl + H


The equipment I needed for the investigation was


Water (dilute the acid)


Magnesium ribbon- 15 pieces, cm long


Hydrochloric acid � 0 ml of molar


0 ml of 1.6 molar


0 ml of 1. molar


0 ml of 0.8 molar


0 ml of 0.4 molar


Measuring Cylinder


Stop clock (sensitivity 1/10s)


beaker


Pipette


Test tube rack


Test tubes


Safety goggles


I decided to repeat the experiments three times each, using all the information that I gained while I was doing my preliminary experiments.


To ensure a safe experiment and working environment I needed to have at least 1 meter squared of working space around me, wear safety goggles at all times when using acid, use a test tube rack instead of holding the test tubes, secure all equipment and make sure that all the equipment were fully functional and not damaged.


To make the experiment a fair test I used the same amount of acid for all experiments, only changing the concentrations. I used the same size of magnesium, I also started the stop clock when the magnesium touched the acid and stopped it when the magnesium stopped fizzing for each experiment. I always washed out the test tubes when an experiment had finished so the different concentration wouldn’t get mixed together causing strange results.


First I measured out the amount of hydrochloric acid using the measuring cylinder. I used a pipette to pour the acid into the measuring cylinder as to be accurate. I needed 5cm of acid in the cylinder and poured it into a test tube. I then put 0cm of water into the test tube to dilute the concentration of acid. I did this to see if the experiments with the same strength of acids affected the rate of reaction. I then got a piece of magnesium ribbon about cm long and dropped it into the acid and started timing the moment that the magnesium ribbon touched the acid solution. When the magnesium ribbon stopped fizzing, I stopped the clock and recorded the number of seconds (rounded up to nearest second) taken for the reaction from start to finish.


I made a table to record my results in. The table is shown below


Volume in cm Concentration of acid in molar Time taken to react


Acid Water


5 0 50


5 0 45


5 0 50


0 5 1.6 7


0 5 1.6 71


0 5 1.6 7


15 10 1. 156


15 10 1. 154


15 10 1. 146


10 15 0.8 8


10 15 0.8 6


10 15 0.8 40


5 0 0.4 88


5 0 0.4 65


5 0 0.4 6


To calculate the average time that it took for the magnesium to be eaten away by the acid, I did the following calculation


Test 1 + Test + Test = Average time /


The average result is shown below


Volume of acid cm Volume of water cm Concentration in M Average time


5 0 48.


0 5 1.6 7


15 10 1. 15


10 15 0.8 84.


5 0 0.4 41.7


As I already have mentioned, I used a measuring cylinder to make the measurements and used a pipette for further accuracy. I did each experiment three times so I would be able to calculate averages and thereby get more accurate results. I recorded the results in seconds instead of minutes in order to obtain more precise results. I used a stop clock instead of a 4-hour clock so I could look at the milliseconds and round it up to the nearest second, which made the results more exact.


It was noticeable, when looking at the results table, that the more concentrated acid had a faster rate of reaction than the less concentrated acid. This was probably because there are more particles in a concentrated acid and therefore more collisions will occur. For instance, 1. molars� average time, 15 seconds, is longer than molars� average time, which was 48. seconds.


I made a graph to show the results in a graph paper.


The graph above supports my original prediction of the more concentrated the acid the faster the rate of reaction because it shows the time difference between the different strengths of acids. In a higher concentration there are more acid particles to react with the magnesium ribbon and therefore it is eaten away faster.


I conclude that changing one factor does have a significant effect on the rate of reaction as we have seen.


Looking at the set of results obtained, you can clearly see that they all follow the expected pattern. This is pattern suggests that the reaction rate increase when the concentration of the acid increases because if you increase the concentration of the acid you are introducing more particles into the reaction which will in turn produce a faster reaction because there will be more collisions between the particles which is what increases the reaction rate.


The evidence I have been able to gather from this investigation seems to lead to a quite firm conclusion. I might not have been able to find the exact speed of the reactions but the pattern seems to be correct as I have repeated readings three times and as it agrees with the information I have researched.


I used the variable of concentration, which seemed to be of a good choice as it would show the results of how more acid molecules reacting with magnesium, would result in a faster reaction.


There will always be ways in which you can improve your investigations and the same thing goes to my investigation.


I found it very hard to measure out the exact number of millilitres for the acid even though I used a pipette and I was also in a hurry. If I were to redo this investigation I would put some more effort into measuring the acid. I could have used the wrong concentration of acid by accident and that would have affected the speed because there would have been fewer or more acid particles to react with the magnesium ribbon. Next time I do this experiment I would try to remember which acid I am using so it doesn’t get mixed up. Every time I washed a test tube or a measuring cylinder, I did not dry it before using it. This may have affected the rate of reaction, as water would dilute the acid. To improve my results, I could dry the test tubes and the measuring cylinder after they are washed to prevent diluted acids. The size and weight of the magnesium would have affected the rate of reaction. The experiment could be improved by measuring, adjusting and weighing the magnesium ribbons so they all are the same size and weight. I also found out from background information, that the magnesium ribbon is covered with a whitish deposit. This deposit was magnesium oxide where the magnesium had reacted with the air. I would imagine that some pieces had only a little of this oxide and some had a lot. The pieces of magnesium ribbon that did not have much oxide on them reacted faster than those with a lot. To improve my results, I could clean the magnesium oxide of all the magnesium pieces using some sandpaper, and this would mean that the acid would not have to eat through the magnesium oxide before reacting with the magnesium. In my investigation I also measured the temperature to see if there would be any change in the rate of reactions. From my results it was noticeable to look at each experiment and see how the temperature had affected the rates of reactions. To improve my investigation I could assure that the temperature was constant all the time. I could also do more readings to get nearer to a more accurate result.


In my investigation I used concentration as my variable. To improve my investigation further, I could use other variables such as, surface area, temperature, pressure for gas, and a presence of a catalyst.


These variables would hopefully prove that they all help speed up a chemical reaction.





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