In this investigation we must consider the topic of variables that affect the rate of reaction (reaction velocity). 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 you use, the less time it takes for the magnesium to disappear and so the rate reaction increases. The concentration of HCL you use is the independent 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 dependent variable because it depends upon the concentration of hydrochloric acid. The only other variable throughout the investigation, which will vary are the volume of water used. This is because, it has to dilute the acid, in order to have the same amount of liquid each time.
The variables which will remain unchanged are the temperature (room temperature) 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. If the starting temperature of the acid is different each time the speed at which the acid particles collide with the magnesium ribbon will increase more the higher the temperature goes. This means the acid particles move with more energy, which means they will collide with the magnesium with more energy, which will give more successful collisions per second.
The amount of magnesium will also be a controlled variable. If the magnesium had a bigger surface area each time the experiment was done, then the acid particles will have a bigger area to collide with, so more collisions will occur every second and the more collisions per second than the more successful collisions per second. The surface area has an effect on the rate of reaction. If the solid has a large surface area then there are more opportunities for collisions to occur between the solid and liquid. If the surface area is small, collision can only occur with the outer atoms and is therefore limited.
Factors that may not be easy to control are; how well the solution is mixed when it is diluted, to get the correct concentration. 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.
A way of altering the rate of reaction without being chemically changed after it is by using a catalyst. Catalysts are usually transition metals. In the presence of a catalyst, less energy is needed by a collision in order to be successful. They increase the rate by providing an alternative reaction path with lower activation energy. Therefore there are relatively more successful collisions and so the reaction rate increases. Increasing the amount and surface area of the catalyst used can make reactions faster.
In order to ensure a fair test with reliable results, I will need to keep all variables constant apart from the one I am intending testing.
I have chosen to use the concentration of the acid as my factor that I will change. I chose this because several different concentrations can be made up before the experiment by the lab technicians and they will be able to make them accurately.
There will be several different concentrations of acid, which will give me a wide range of results, which will be reliable and reproducible.
Equipment needed:
- Hydrochloric acid – A reactant
- Magnesium ribbons – A reactant
- Conical flask – To house the reaction (with the magnesium and acid dilute in)
- Trough – Containing an amount of water to act as a water bath
- Rubber tube – for the reaction to travel down
- Stop watch – To time the reaction
- Acid – One of the main parts to the investigation (to vary the concentration). The concentrations I will be using are; 50 ml of HCl, 40 ml of HCl with 10 ml of H2O, 30 ml of HCl with 20 ml of H2O and finally, 10 ml of HCl with 40 ml of H2O. I have chosen not to use 50 ml of H2O, as this would take too long, as the reaction would be very slow.
- Water (to dilute the acid)
- Magnesium ribbons – the second most important part of the experiment (to react with the acid). Each ribbon will be 1.5 centimetres long.
- A burette – With water up to ml (to show how fast the acid dissolves the magnesium ribbon)
- Bung – to seal the top of the conical flask to ensure gas doesn’t escape.
Method:
The experiment will be done using the equipment shown below. To begin with, set up all the apparatus in a safe position. Then I will make up the different concentrations of acid that I mentioned in the ‘equipment’ section.
The acid will be measured in a measuring cylinder and then put into a conical flask. The water (if necessary) will be measured in the same way and added to the same conical flask as the acid. When I have set up all of the apparatus as required, I will cut a piece of magnesium ribbon to 1.5 centimetres long. I will then ask some one else to start the stopwatch as soon as the magnesium ribbon is in and the bung to the conical flask placed in the top. Next, I will take a reading of the level of the water in the burette at certain intervals. For 50 ml of HCl, 40 ml of HCl with 10 ml of H2O, 30 ml of HCl with 20 ml of H2O and 20 ml of HCl with 30 ml of H2O I will take a reading every five seconds, however, I expect that it will take a very long time for the magnesium ribbon to dissolve in an acid concentration of 10 ml of HCl with 40 ml of H2O, therefore, I will take a reading every ten seconds. When the magnesium is no longer visible, I am going to stop the stop clock and then repeat this method for all of the acid concentrations.
To find out the volume of gas produced, I will take the amount of water lost from the burette away from the starting amount of water in there.
Fair Test:
In order to keep our investigation fair, we must make sure that we use the same size, amount and roughly the same mass of magnesium throughout the investigation, so that it will be a fair test and the results we be in relation to each other, otherwise the results will be completely different to what we will expect and it will ruin the experiment. As well as this, I will have to make sure that the surrounding temperature remains the same (room temperature), as a higher temperature can quicken up the reaction, and a lower temperature can slow the reaction down. The following factors that I will change are, the concentration of acid and the amount of water used, as these are my controlled variables.
Safety:
The safety of this experiment is very important, as I will be using a dangerous, reactive chemical. The things that I will need to do to keep my experiment a safe one for myself and other students around me are as follows:
· Wear safety goggles as I am using concentrated hydrochloric acid
· Care to eyes and the skin besides all the other people is always vital and necessary
· Care in using glassware since it is sharp when broken and can cut skin
· Safe disposal of reagents and laboratory chemicals
· Care when returning all used glassware and equipment at the end of the experiment
Results:
Below are the results to my experiment. The tables show the volume of gas produced after certain amounts of time:
Analysis:
Evaluating and Interoperating Evidence:
I believe that the experiment was successful but some of the results were unexpected/unreliable.
I believe that the experiment was designed well but there were a few problems. Although the initial rate of reaction (which is what I am concerned with in this investigation) seemed to fit a trend, the rate of reaction curves of some temperatures on the graphs crossed. This could have been because some of the magnesium had corroded forming a magnesium oxide layer, which would have affected the rate of reaction.
Another factor which could have given me unreliable results could have been that the gas syringes were wet causing them to jam and so not giving correct results or that the bung was not placed on the top of the side arm tube fast enough which allowed gas to escape. I conducted all three experiments for each temperature at the same time to save time. An error in my graphs (plotting, drawing curves or calculating gradients) could have also affected the calculated rates of reaction.
To improve the experiment I would find a way of attaching and releasing the magnesium inside the side arm tube above the acid (with a bung at the top of the side arm tube) so that the magnesium could be dropped into the acid without any gas being lost.
Additional work, which could be carried out include, to repeat the experiment using, a wider range of acid concentrations. The investigation could also be extended to investigate other factors affecting the rate of reaction such as catalysts, temperature or particle size of the magnesium.
There are many reasons why our results for the 1/time graph did not prove the point that I predicted. There are:
1) When the reaction takes place bubbles of H2 are given off, which might stay around the magnesium, which therefore reduces the surface area of the magnesium, and so the acid cannot react properly so this affects the results.
2) We could have controlled factors in the investigation better (e.g. the stirring of the solution because if this isn’t done properly it can lead to incorrect results).
3) Using larger concentrations of acid would give a bigger more accurate conclusion instead of using small test tubes; we could have used large test tubes. This way graphs would be more spaced out and give an accurate form or curve
In this experiment I found out that many different factors including; Concentration, Temperature and Surface Area affect the rate of reaction. For the experiment I found out that the higher the concentration of the acid the quicker it takes to dissolve the magnesium. I know this because the more acid molecules that is in the beaker there is more chance of a successful collision to take place. Another factor that I learned is that the rate is constantly changing during the reaction because as time goes on there are fewer acid molecules. There are fewer acid molecules because when the acid reacts the same particle cannot react again, so there is less to end with than there are to start with. As time goes on the number of acid particle decrease so there is less chance of a successful reaction to take place.
Before I started the experiment I made up a prediction. I predicted that the more concentrated the acid solution the faster the magnesium and hydrochloric acid will react. I thought that this would happen because when there are more acid particles in the test tube the more chance there is of a successful collision. I was right in my prediction but I did not predict that the rate of reaction slows down as time goes on which did happen in my experiment.
Conclusion:
I can conclude that if you double the concentration of the acid the reaction rate would also double, this is because the ions are closer together in a concentrated solution. The closer together they are, the more often the ions collide. The more often they collide, the higher the chance of a reaction between the magnesium and the hydrochloric acid. Also because there are more particles in the solution which would increase the likelihood that they would hit the magnesium so the reaction rate would increase. The graph gives us a good device to prove that if you double the concentration the rate of reaction doubles. If you increase the number of particles in the solution it is more likely that they will collide more often. In the reaction, when the magnesium hit the acid, it fizzed and produced many bubbles it was silver in colour (which is one magnesium’s physical properties silvery white metallic element), the activation energy of a particle gets higher with heat, the particles which have to have the activation energy are those particles which are moving, in the case of magnesium and hydrochloric acid, it is the hydrochloric acid particles which have to have the activation energy because they are the ones that are moving and bombarding the magnesium particles to produce magnesium chloride. The graph for 1/time had the form of an s curve and it did have some relationship with the other graph.
Help on Analysis
First three graphs were drawn (one for each time the whole experiment was carried out). So that the graphs did not become over crowded the curves were drawn on separate pieces of see through OHP film and overlaid each graph. Note: They have been attached all overlaid their appropriate graph but should be viewed one at a time overlaid the appropriate graph. A tangent was drawn at the beginning of each curve and its gradient calculated, the gradients are shown in the table below.
The average gradient was calculated and then the gradients plotted on a graph. As the temperature increases so does the rate 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 increase.
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.
In my design I predicted that as the temperature increased the rate of reaction would increase. I predicted that for every 10OC increase in temperature the rate of reaction would double. I was correct that as the temperature increases so does the rate of reaction however the rate of reaction does not double for every 10 OC increase in temperature although it is a linear relationship.
