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:
1. Concentration
2. Particle size/surface area
3. Pressure (for reactions involving gas)
4. Temperature
5. Light
6. 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.
2. 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.
3. 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 10º 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 millilitres. 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. I decided that I would do 5 experiments and the different concentrations of hydrochloric acids were:
v 1.0 molar
v 1.25 molar
v 1.50 molar
v 1.75 molar
v 2.0 molar
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, 0.25 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:
v 0.25 molar
v 0.50 molar
v 0.75 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 5 experiments three times and find the average time as this would result in a more accurate figure.
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 + 2HCl = MgCl + H
The equipment I needed for the investigation were:
v Magnesium ribbon- 15 pieces, 1cm long and weighed 0.01g
v Hydrochloric acid – 30 ml of 1.0 molar
30 ml of 1.25 molar
30 ml of 1.50 molar
30 ml of 1.75 molar
30 ml of 2.0 molar
v Test tubes – 5
v Test tube rack
v Stop clock
v Pipette
v Measuring Cylinder
v Thermometer
v Safety goggles
I decided to do 5 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 ribbon and weight (approximately 0.01g). 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 10 ml of acid in the cylinder and poured it into a test tube. I then put a thermometer into the test tube for 1 minute to check the temperature. I did this to see if the experiments with the same strength of acids affected the rate of reaction if there was a change in temperature. I then got a piece of magnesium ribbon about 1 cm long weighing 0.01g 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.
Experiment Strength of hydrochloric acids in 10 ml No. of Mg ribbon pieces (0.01g) Test 1 secs. Temp Test 2 secs. Temp Test 3 secs. Temp AverageSecs.
1 1.0 molar 1 151.0 18 ºc 141.0 18 ºc 117.0 18.4ºc 136.3
2 1.25 molar 1 81.0 17 ºc 76.0 17.5ºc 74.0 18 ºc 77.0
3 1.50 molar 1 58.0 17.5ºc 56.0 18 ºc 56.0 18 ºc 56.3
4 1.75 molar 1 50.0 18 ºc 41.0 18º c 31.0 18 ºc 40.7
5 2.0 molar 1 20.0 19 ºc 22.0 19 ºc 16.0 21 ºc 19.3
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 2 + Test 3 = Average time
3
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 24-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, 1.0 molars´ average time, 136.3 seconds, is longer than 2.0 molars´ average time, which was 19.3 seconds.
I made a graph to show the results.
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 was 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. For instance, if we refer back to the table on page 5 and look at experiment 5, test three is slightly faster than test 1 or test 2 because it has a temperature of 21º, where as the other two tests both have a temperature of 19º. 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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Hypothesis
What are the factors Affecting the rate of reaction between magnesium and Hydrochloric acid?I
predict that as the temperature increases, the speed of the reaction will increase therefore the gas will be produced faster. I believe this because most chemical reactions happen faster when the temperature is higher. At higher temperatures molecules mover around faster, which makes it easier for them to react together. Usually, rises of 100C will double the rate of reaction.
Chemical reactions take place by chance. Particles need to collide with enough velocity so that they react. As the temperature is increased the particles move faster since they have more energy. This means that they are colliding more often and more of the collisions have enough velocity to cause a reaction. Since there are more collisions the chemical reaction takes place faster.
Pilot Experiment
To decide on the best volume and concentration of hydrochloric acid and best mass of magnesium a number of calculations were done and a pilot experiment conducted.
The equation for the reaction is:
Magnesium(s) + Hydrochloric Acid(l) Magnesium Chloride(l) + Hydrogen(g)
Mg(s) + 2HCL(l) MgCl2(l) + H2(g)
We were advised to use 0.1g of magnesium ribbon (found to be 10.9 cm long). The Relative Molecular Mass (RMM) of magnesium is 24, therefore the moles of magnesium to be used was:
Moles= 0.1
24
Moles= 0.00416
In the reaction above, 1 mole of magnesium reacts with 2 moles of hydrochloric acid. The concentration of acid was 1mol/dm3. Therefore the volume of hydrochloric acid used was:
Volume = 0.00416 * 2
1
Volume = 0.0083dm3
Volume = 8.3cm3
It was decided to use an excess of hydrochloric acid to ensure all the magnesium reacted, therefore 10cm3 of acid was used in the pilot experiment. At room temperature 10cm3 of hydrochloric acid was added to 0.1g of magnesium and the gas was collected (see fig 1). The volume of gas produced was measured every 15 seconds. It was found that the reaction was too rapid to be effectively measured, therefore 10cm3 of water was added to halve the concentration of the acid.
Investigation Experiment
I am going to investigate how temperature affects the rate of reaction between magnesium and hydrochloric acid. The procedure for the experiment is as follows;
* Using a measuring cylinder, measure 10cm3 of water and pour it into the side arm tube.
* Measure 10cm3 of hydrochloric acid (1 mol/ dm3) and add it to the water.
* Place the side arm tube in a water bath at 20OC, set up the apparatus below.
* Measure 10.9 cm of magnesium ribbon and check on the balance that it weighs 0.1g.
* Coil the ribbon around a pencil and then drop it into the side arm tube and quickly put a bung on the side arm tube (this must be done quickly to prevent gas escaping).
* Every 15 seconds measure the volume of gas produced until less than 1cm3 of gas is produced every 15 seconds.
* Repeat experiment two more times (for accuracy) and record all results in a table.
* Repeat the experiment for temperatures of 0.5M, 1.0M, 1.5M and 2.0M (1mole/dm)
It is important that only the temperature is changed since this is what is being investigated.
Method
* Apparatus
I have chosen to use a 10cm3 measuring cylinder to measure the volumes of substances used since it is more accurate than a pipette. I will use an electronic water bath for maintaining the mixture at a temperature since the temperature is more accurate than a water bath above a Bunsen burner.
A 100cm3 gas syringe should be appropriately accurate for measuring the gas produced since it is accurate to 1cm3 of gas. I will use a three figure balance to measure the mass of magnesium to be used since it is vital that as close to 0.1g of magnesium is used as possible.
* Variables
After deciding how to approach all of the variables in the experiment I decided how to carry out my experiment. I decided to use different concentrations of HCl in 0.5M, 1.0M, 1.5M and 2.0M (1mole/dm ). To obtain these different concentrations I used a solution of 1.0M which was already prepared and the same with a 2.0M solution. However in order to create a 0.5M and 1.5M solution I had to mix different solutions. The 0.5M solution was made using 10cm of deionised water and 10cm of 1.0M solution, this was then stirred with a glass rod. To make 1.5M solution the same concept was used, I mixed 10cm of 2.0M solution with 10cm of 1.0M solution.
I then measured and recorded how much volume of gas (cm ) was given off each 10sec using a stopwatch. Finally the gas syringe is an excellent piece of apparatus as it is very accurate, however I will have to make sure that the clamp does not hold on to it too tightly as this could effect the results.
To help obtain the best possible results I will repeat each experiment twice and then find an average set of data.
I predict that the high molarity concentration solution will have a much faster rate of reaction than the weaker solution. I have predicted this from my knowledge of a previous experiment and scientific knowledge. I think that this happened because the more HCl in the solution the greater the concentration and there will be more particles colliding and more energy and therefore a faster rate of reaction. The reason I think that the rate of reaction will increase is that the experiment is exothermic, this means that it will give itself more energy and therefore more collisions and faster collisions. Also in the 2.0M solution compared with the 0.5M solution there are much more HCl particles in the same volume so the are going to collide with each other more often
Therefore I predict that the rate of reaction will increase with the concentration of the HCl solutions.
* Rates of Reaction
Increasing the temperature increases the speed of the particles. The faster the particles move, the greater the number of collisions, and therefore the rate of the reaction increases. A 10OC rise in temperature almost doubles the rate of most reactions.
Chemical reactions take place by chance. Particles need to collide with enough velocity so that they react. As the temperature is increased the particles move faster since they have more energy. This means that they are colliding more often and more of the collisions have enough velocity to cause a reaction. Since there are more collisions the chemical reaction takes place faster.
The temperature of the reaction - When the temperature is low, the particles in the reaction do not have much energy so collision is unlikely to occur. However on heating, particles take in energy causing them to move faster and collide more often. Because the collisions have more energy they are more likely to be successful in breaking and reforming bonds. Therefore the rate of the reaction will increase. When the temperature increases by 10C, the rate of the reaction roughly doubles.
Prediction:
I can predict that there will be a positive correlation between the rate of the reaction and the temperature so that as the temperature is increased, the rate of reaction will increase
Scientific explanation:
For a reaction to occur, not only must particles collide with one and other but they must have enough energy so that original bonds are broken and new bonds formed. The energy needed to break a mole of bonds is known as bond energy. If the collision has insufficient energy no reaction occurs. The more successful collisions, the faster the reaction. Successful collisions depend on particles moving faster to increase the chance of colliding. Increasing the temperature means particles take in more energy causing faster movement and therefore more collisions. Heated particles both collide more often and also these collisions will have more energy so will be more likely to be successful. Conversely, reducing the temperature will decrease the amount of energy, slow down movement and decrease the number of collisions. All reactions need energy to start them off. The activation energy is the minimum energy needed to break enough bonds to start a reaction.