Reactions only happen if the particles collide with sufficient energy. At a higher temperature there will be more particles colliding with sufficient energy to make the reaction happen at a much more rapid rate. This initial energy is known as activation energy and it is needed to break the initial bonds that exist between some of the reactant particles. In other words, the activation energy is the minimum amount of energy required for a reaction to take place.
- CONCENTRATION (or PRESSURE)
An increase in the concentration of the reactant particles will cause an increase in the rate of reaction because it means that there are more reactant particles present, which means there will be more collisions and more collisions mean a faster rate of reaction. So, in a way, increasing the concentration has a similar effect to increasing the temperature because it encourages more collisions between particles. In a gas, increasing the pressure will make the gas molecules more congested and tightly packed and there will therefore be more collisions and a faster rate of reaction as a result. However, only increasing the temperature will cause faster collisions:
In this investigation, the concentration of the nitric acid will be the only variable that will be changed because I am investigating its effect on the rate of reaction; all other variables will remain constant. The highest concentration of nitric acid available will be 2M (two molars). I will be testing five different concentrations of acid because this will give me a sufficient and manageable range of results to make a more valid analysis. All other concentrations will be below this. I will achieve more concentrations of acid by diluting the nitric acid with water to weaken the strength of the acid.
- SIZE OF SOLID PARTICLES (or SURFACE AREA)
Surface area also plays a big part in the rate of reactions. When the surface area of the reactant particles is greater, there will be a greater rate of reaction because there is bigger area for particles to collide with (useful collisions). For example, marble chips (calcium carbonate) will dissolve more slowly in hydrochloric acid than marble chip granules (ground marble chips) will. This is because when the marbles are ground or granulated, there is a greater surface area exposed for collision with the hydrochloric acid particles than there is when the marble chips are still in their chip form:
- CATALYSTS
Catalysts are substances which speed up a reaction but remain unchanged after the reaction has ended. Catalysts are often in the form of enzymes which are biological proteins. In a reaction, the catalyst would work by giving the enzymes a surface to attach themselves to where they can collide. This surface increases the number of useful collisions between the reactant particles and, in turn, increases the rate of the reaction:
BRIEF OUTLINE OF MY PLAN:
- I am going to investigate the effect of the concentration variable on the rate of the reaction between calcium carbonate (marble chips) and nitric acids.
- I am going to the determine the rate of reaction by calculating volume of carbon dioxide gas produced in a certain time length (1 minute), using a gas syringe
- I am going to do a preliminary investigation so that I can determine whether the apparatus is correct and sufficient, what steps I need to take in order to make my main investigation as fair as possible, and also, to make sure I avoid as many mistakes and problems as possible when I conduct my main investigation. The preliminary investigation will also help me to make a more accurate prediction and analysis for the relationship between the concentration of nitric acid and the rate of reaction for my main investigation. However, as I am not going to rely solely on the results of my preliminary investigation to conclude it, I am not going to pay so much attention to things like fair testing, although I will try and conduct the investigation as fairly as I can. I will, however, I will insist on fair testing in my main investigation. Therefore, I am aware that my preliminary results will not be good enough to base my entire investigation around. The results will just be used as mere guidelines to help with the planning and execution of my main investigation.
PRELIMINARY INVESTIGATION:
AIM:
To investigate the effect of concentration on the rate of the reaction between nitric acid and magnesium.
APPARATUS:
- Magnesium Granules (2.5 grams)
- Nitric Acid (50cm³) [see method for more details]
- Conical Flask
- Measuring Cylinder
- Retort stand and clamp
- Gas Syringe
- Stopwatch / timer
- Top-pan balance / scales
METHOD:
- Set up the apparatus as shown in the diagram above.
- Measure 2.5 grams of magnesium granules using a top-pan balance.
- Measure out 50cm³ of acid solution using a measuring cylinder. The acid should be diluted with water to form a final 50cm³ solution at the correct concentration. (See conversion table)
- Place the magnesium granules into the conical flask.
- Immediately afterwards, pour the acid into the conical flask and quickly seal it with the rubber cork.
- At the same time, start the timer and begin to observe the amount of gas produced using the gas cylinder. Record the amount of gas produced at 10 second intervals.
- Repeat all of the abovementioned steps once more for more accurate results and take an average of the two results. If there is great variance in the two results, repeat a third time and take an average.
- Repeat steps 1-8 for five the different concentrations of nitric acid (2 molar, 1.6 molar, 1.2 molar, 0.8 molar and 0.4 molar). (See conversion table)
N.B.: At least people should be involved in conducting the experiment to make the process a little easier resulting in better, more accurate and more reliable results (which is what I did).
CONVERSION TABLE
Use this acid to water ratios to achieve the correct concentrations:
For the 50cm³ acid solution:
SAFETY:
- Goggles should be worn at all times during the experiment to reduce the risk of acid entering the eye.
- Acid should be handled very carefully to avoid contact with skin. If acid is spilt on hands, run hand under a tap with cold running water immediately, and then seek medical aid or advice.
- Keep breakable objects away from the edge of the work surface.
PREDICTION:
I predict that the relationship between the concentration of the nitric acid and the rate of the reaction between the nitric acid and the magnesium will be a directly proportional relationship. In other words, if the concentration of the acid (in molars) is increased, the rate of reaction (i.e. the amount of gas produced) will also increase, and if the acid concentration is decreased then the rate of reaction will also increase. It will be fastest at 2.0 molar and slowest at 1.4 molar. This is because as the concentration of a reactant (in this case the acid) is increased, there will be more reactant particles available and, therefore, more likelihood of effective collisions (i.e. collisions which produce a significant reaction) between the reactant particles.
I would therefore expect the shape of the graph to look something like this:
RESULTS FROM PRELIMINARY INVESTIGATION:
N/A – see observations and conclusions.
OBSERVATIONS AND CONCLUSIONS FROM PRELIMINARY EXPERIMENT:
I could not conclude that the experiment showed that an increase in the concentration of the nitric acid increases the rate of the following reaction:
Magnesium + Nitric acid Magnesium Nitrate + Hydrogen Gas
The reason I was unable to draw this, or any other, conclusion was because the reaction was too quick and too ‘violent’ for me to record any results. I decided that this was a result of excess amounts of magnesium granules being used. The significance of this was that it made the results unfair and unreliable as the speed of the reaction was such that a lot of gas was escaping before the rubber cork had been put in place to seal the conical flask.
Another thing I noted was that the reaction had also generated a lot of heat energy. This also caused disruption and unreliability to any potential results because the temperature is also a variable which affects the rate of reaction and needs to be kept constant because it is not being investigated.
I decided that I would need to reduce the amount of magnesium used considerably in order for the reaction to occur at a pace which was suitable enough for me to obtain and record reliable results and be able to draw conclusions from them. I therefore changed the method for my main investigation. If I had had more time, I would have conducted a second preliminary investigation to make sure that the changes would be adequate before then conducting my main investigation
This concluded my preliminary investigation.
I decided that I had enough knowledge to pursue my main investigation. I opted to keep the same method, the only alteration being to broaden the scale of results by recording results at 5 second intervals rather than 10 second intervals.
MAIN INVESTIGATION
AIM:
To investigate the effect of concentration on the rate of the reaction in the following reaction:
Magnesium + Nitric acid Magnesium Nitrate + Hydrogen Gas
APPARATUS:
- Magnesium Granules (0.3 grams)
- Nitric Acid (50cm³) [see method for more details]
- Conical Flask
- Measuring Cylinder
- Retort stand and clamp
- Gas Syringe
- Stopwatch / timer
- Top-pan balance / scales
METHOD:
- Set up the apparatus as shown in the diagram.
- Carefully measure 0.3 grams of magnesium granules using a top-pan balance.
- Carefully measure out 50cm³ of acid solution using a measuring cylinder. The acid should be diluted with water to form a final 50cm³ solution at the correct concentration. (See conversion table)
- Place the magnesium granules into the conical flask, making sure each time that the granules go to the bottom of the conical flask without any becoming stuck to the sides so that the surface area remains constant.
- Immediately afterwards, pour the nitric acid solution into the conical flask and quickly seal it with the rubber cork.
-
At the same time, start the timer and begin to observe the amount of gas produced using the gas cylinder. Record the amount of gas produced at 5 second intervals.
- Repeat all of the abovementioned steps once more for more accurate results and take an average of the two sets of results. If there is great variance in the two sets of results, repeat a third time and take an average of al three sets of results.
- Repeat steps 1-8 for the five different concentrations of nitric acid (2 molar, 1.6 molar, 1.2 molar, 0.8 molar and 0.4 molar). (See conversion table)
N.B.: At least people should be involved in conducting the experiment to make the process a little easier resulting in better, more accurate and more reliable results (which is what I did).
CONVERSION TABLE
Use this acid to water ratios to achieve the correct concentrations:
For the 50cm³ acid solution:
SAFETY:
- Goggles should be worn at all times during the experiment to reduce the risk of acid entering the eye.
- Acid should be handled very carefully to avoid contact with skin. If acid is spilt on hands, run hand under a tap with cold running water immediately, and then seek medical aid or advice.
- Keep breakable objects away from the edge of the work surface.
PREDICTION:
Based partly on the results of my preliminary investigation combined with my own background knowledge of the factors involved in the investigation, I predict that the relationship between the concentration of the nitric acid and the rate of the reaction between the nitric acid and the magnesium will be a directly proportional relationship. In other words, if the concentration of the acid (in molars) is increased, the rate of reaction (i.e. the amount of gas produced) will also increase, and if the acid concentration is decreased then the rate of reaction will also increase. It will be fastest at 2.0 molar and slowest at 1.4 molar over the one minute time period. This is because as the concentration of a reactant (in this case the acid) is increased, there will be more reactant particles available and, therefore, more effective collisions (i.e. collisions which produce a significant reaction) will be encouraged between the reactant particles over the time period. However, the particles only collide more rather than collide faster when you increase the concentration. Only an increase in temperature can encourage faster collisions because the reactant particles use the heat energy and turn it into kinetic energy to collide with other reactant particles.
FAIR TESTING:
I have to carry out certain procedures to make sure that the test is fair. If the investigation is carried out as fairly as possible then the results I collect will be more reliable and valid and therefore so will my analysis and evaluation of the results. In the ideal investigation, these are some of the procedures I would take to ensure that the investigation was carried out as fairly as possible under the circumstances and conditions available:
-
The only variable I will change is the concentration of the nitric acid using the conversion table to dilute it appropriately. All other variables (i.e. temperature and surface area) must be kept constant through out the investigation as they are not being tested. Otherwise, the results obtained will be unreliable as the other variables will also have an effect on them.
- I will rinse and dry the conical flask with distilled water and a clean towel after each individual test to ensure that the remnants of the reactants and any products are removed and will have no effect on any further tests. I will also expel any air from the gas syringe after each test by pushing it back into its starting position.
- As the investigation will probably require more than one participant, the same people will perform the same designated tasks throughout the investigation.
- The same type of acid (nitric acid) will be used throughout the investigation.
RESULTS:
Here are the results of my main investigation:
*assuming that at 0 seconds, 0cm³ of gas were produced.
ANALYSIS OF RESULTS FROM THE MAIN INVESTIGATION:
Firstly, I plotted a graph of the average volume of gas produced for each concentration of nitric acid every 5 seconds. I found a few results that slightly did not fit the patterns and have highlighted these in the table. However, these were so slight that I did not interpret them as anomalies as they had barely any effect on the other results at all. I decided to exclude them when drawing my conclusions.
I then drew tangents on the graph for each concentration of nitric acid to enable me to calculate the rate of reaction (volume of gas produced/time). I drew the tangents at the beginning of each concentration graph (i.e. from 0-5 seconds) because this is the where the rate of reaction was at its fastest and would give a better representation for analysis. I calculated the rate of reaction for each concentration of nitric acid as shown in the table below:
From the table, graphs and results I could see that, as predicted, the rate of the reaction:
does indeed increase as the concentration increases. My results show that there is a proportional relationship between the concentration of nitric acid and the rate of the reaction. This is because when you increase the concentration of nitric acid, you increase the number or nitric acid particles available for collision with the other reactant particles (magnesium particles). By so doing, the probability of effective collisions (i.e. collisions which form the products magnesium nitrate and hydrogen gas) that occur is higher and the rate at which the reaction takes place is increased. The reverse effect occurs if you decrease the concentration of the nitric acid.
I then decided to work out what type of relationship my results showed. I therefore plotted a second graph of rate of reaction against concentration of nitric acid. The graph produced by my results this time showed the characteristic curve of a 2nd order relationship. This meant that when the concentration of the nitric acid was doubled, the rate of the reaction was approximately four times as fast. There was one anomalous result: the result for a 1.2 molar concentration of nitric acid. However, I once again decided that its effect on the other results was too small to reconsider. I therefore disregarded the result but have circled it on the graph to identify it as a result which falls slightly out-of-pattern.
EVALUATION OF INVESTIGATION:
Overall, I think I could call the investigation a success because my results supported my prediction and there were only a few anomalous results, which I have acknowledge and dealt with appropriately. There are a number of reasons why these anomalies could have occurred, however. Firstly, because we had no means of regulating the temperature of the environment in which the investigations were conducted, the temperature would have affected the results in some way because temperature is one of the variables which affects the rate of a reaction as an increase in temperature gives reactant particles more energy to move around and collide with each other, thus increasing the likelihood of productive collisions and increasing the rate of reaction. The significance of not controlling the temperature in the investigations can therefore not be calculated and the results can not be deemed entirely reliable. Also, as temperature was not the variable which was being investigated, it should, ideally, have remained constant throughout the investigation. A second explanation for the occurrence of certain anomalies could be the fact that, when we added the nitric acid to the magnesium in the conical flask, the reaction took place almost instantly. This meant that there was a strong chance that some hydrogen gas had been formed and had escaped before the rubber cork was placed over the top of the conical flask. The results would, therefore, not have been absolutely reliable. A third and final explanation for some of the anomalous results encountered during the investigation could be that the reaction between the nitric acid and the magnesium granules is an exothermic reaction in which heat energy is also emitted. This, again, relates to the role of temperature in a reaction, i.e. the increase of energy leading to more rapid particle movement, more effective collisions and therefore a faster rate of reaction. Therefore, the results are not completely reliable when reaching conclusions about the relationship between concentration and the rate of a reaction.
Nonetheless, I believe the investigation was carried out as fairly as possible and the results obtained were reliable, taking into consideration the conditions and equipment that were available when conducting the investigation. Also, even with certain slightly more advanced equipment, there is still always margin for error: for example, human error when reading the volume of gas from the gas syringe, therefore, obtaining entirely reliable results without any anomalies is unlikely.
Here is a short table explaining, summarising and suggesting improvements for the investigation which could be introduced for more reliable results, if the investigation was to be repeated at any point in the future:
There are other improvements which I could make to my method to make my results more reliable in a future investigation. For similar future investigations, I would adapt the suggested improvements into my method where possible, to ensure that I have more reliable results and that I can reach fairer and more valid conclusions. However, I believe that, with the apparatus and conditions which I had available to me the investigation was conducted as fairly and sincerely as possible. With more advanced apparatus, the results of the experiment could have been even more reliable and I will take note of this for future reference.
DEVELOPING THE INVESTIGATION:
This investigation is fairly flexible in the sense that there are a few other variables which could be investigated and the experiment can be easily modified to accommodate this. For example, the apparatus could be modified to investigate:
- The effect of temperature – by using a thermostatically water bath and changing the temperature at which the reaction takes place to investigate the effect of temperature on the rate of reaction.
- The effect of a catalyst – bye introducing a catalyst to the reaction to see what effect this has on the rate of reaction. A catalyst gives particles a surface to stick to, thus promoting more effective collisions.
- The effect of surface area – by changing the surface area of the magnesium e.g. using magnesium strips as an alternative to granules and seeing the effect that this has on the rate of reaction. A larger surface area will give reactant particles a bigger area to collide with, therefore promoting more effective collisions and increasing the rate of reaction.