Reaction rate taken from the Encyclopaedia Britannica
Collision theory and temperature effects on rates
Kinetic theory says that molecules are in constant motion. The kinetic energy and molecule velocity increase with temperature. KE = [1/2][mv2]
Reactions usually require collisions between reactant molecules or atoms. The formation of bonds requires atoms to come close to one another. New bonds can form only if the atoms are close enough together to share electrons. Some collisions are not successful. These are called ineffective collisions. The particles simply hit and then rebound. This animation illustrates what happens in an ineffective collision.
Collisions that lead to products are called effective collisions. An effective collision must happen with a great enough speed, energy and force to break bonds in the colliding molecules.
The animation illustrates an effective collision between two diatomic molecules. The two product molecules formed fly outwards.
Collisions between molecules will be more violent at higher temperatures. The higher temperatures mean higher velocities. This means there will be less time between collisions. The frequency of collisions will increase. The increased number of collisions and the greater violence of collisions results in more effective collisions. The rate for the reaction increases. Reaction rates are roughly doubled when the temperature increases by 10 degrees Kelvin. This means the rate can be quadrupled if the temperature is raised by 20 degrees Kelvin.
It should be clear that if you can increase reaction rates by increasing temperature you can decrease reaction rates by lowering the temperature. You do this every time you put something in the refrigerator. If you want to see the effect of elevated temperatures increased reaction rates you can leave some dairy product out of the refrigerator for a few days and compare its condition with the same age dairy product that was kept cold.
Temperature effects on rates and activation energy diagram
This illustration shows what happens to an exothermic reaction when the temperature is changed.
The dotted blue curve shows the energy for a reaction mixture that is heated. The reactants are "part way" up the energy barrier because they are "hot".
The dotted magenta curve shows what cooling does to the reactant energy. The energy goes down and the reaction happens with more difficulty.
NOTE: The energies of reactants and products have changed. They both have different energies because they were either heated or cooled. The heat of reaction is the slightly different. The relative amounts of reactants and products are slightly different because of the temperature changes.
Concentration
Reaction rates can be increased if the concentration of reactants is raised. An increase in concentration produces more collisions. The chances of an effective collision goes up with the increase in concentration. The exact relationship between reaction rate and concentration depends on the reaction "mechanism". This is the process involving elementary reaction steps. The slowest step controls the rate. The nature of the slow step is not obvious from the balanced equation. Only experimental observation reveals the link between concentration and reaction rates.
Collision Theory and Temperature effects on rates taken from http://scidiv.bbc.ctc.edc
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Safety
As with any investigations great care should be taken and the experiment should be undertaken in a sensible manner.
I will wear safety goggles throughout the investigation.
I will be careful so as not to spill any acid, but would ensure there was nothing near the experiment that could cause an accident or be damaged if an accident did occur.
In this experiment I will have to make sure I am careful with all glassware in case it breaks and if it does to make sure it is immediately cleaned up. As I will be using a conical flask its wide bottom gives a more secure base.
I will ensure that I do not use too much acid, which could escape and spill when the temperature is raised. Also, I must be careful especially with the stronger molarity acid because it will be corrosive so I must make sure I do not spill it on my hands or more especially in my eyes. One way to avoid this is to be extremely careful and wear safety goggles.
Fair Test
I will try to make sure that the marble chips will be of the same size, amount and mass – this is because I want to keep the chips as similar as possible throughout the experiment and hopefully be a fair test with the results in direct relationship to each other. If these varied then I would expect the result to be inaccurate and so ruin my experiment. I will take readings at the same time intervals throughout the experiment i.e. I will take a time reading every fifteen seconds in all my experiments.
Prediction
Having taken into account the Collision Theory I predict that the more concentrated the acid, it will contain more hydrogen ions causing more collisions so the quicker and more violent the reaction will be.
Also, the smaller the marble chips are, the surface area will be increased and so I would expect the reaction rate to be increased.
Apparatus List
? A conical flask – I chose this large enough to prevent spillage
? A rubber bung - To prevent accidents
? A gas syringe
? A delivery tube ( for connection to the gas syringe)
? A stand and clamp
? A stopwatch
? Calcium carbonate chips
? Hydrochloric acid – Different molarities
? Water
Method
When doing my experiments I will use the same procedure throughout. I will set out my equipment as shown in the diagram below:-
Throughout the experiment I was ready with the stopwatch, so that I started and stopped it immediately it was necessary to get accurate readings.
Once I have set up the apparatus I will measure out 5cm3 of acid in a measuring tube and then pour it into the conical flask. I will quickly place the chips into the acid, insert the bung in the top of the boiling tube and start the stopwatch. This must be done in as short a space of time as possible. From then on, I must take the reading of the volume in the gas syringe after a certain point is reached. I used eight different molarities of hydrochloric acid and then recorded the amount of carbon dioxide in the gas syringe. I will accurately record these roughly in a table and write them up neatly later.
I will take into consideration that the speed with which I put in the marble chips, place in the bung and press the stop watch may well affect how well the experiment works. Although these factors will affect the end result so long as they are kept constant throughout then it should not matter too much.
To get the best possible results I need to use the equipment in the best possible way, I could twist the gas syringe back and forth so as not to get the syringe stuck, which would affect my results. I will need three readings for each concentration and later if I need to repeat any experiments I can.
An important point is that I must use the right amount of acid and the best sized amount of marbles. If I used about 4 chips for 4ml acid then too much gas will be produced and the syringe will go over the 100 mm3 area in the syringe very quickly. Then I would not be able to take correct readings.
To overcome this problem I will test to see how many chips and what size will be the best to use. This is where my trial experiment that I did before I started came in handy because it allowed me to know what amount and size of chips to use in the proper experiment.
I also did an experiment beforehand to give an idea as to what to do in our investigation. It was the reaction between marble chips and hydrochloric acid. This was a very good thing to do because it showed me how to use the equipment properly and safely and gave an idea as to what was to come.
Evidence obtained
Molarity Time 1 Time 2 Average Time
0.50 2.51 2.34 2.425
1.00 1.2 1.17 1.185
1.50 0.32 0.45 0.385
1.75 0.44 0.39 0.415
2.00 0.29 0.39 0.34
2.25 0.28 0.37 0.325
2.50 0.26 0.35 0.305
3.00 0.25 0.35 0.3
Evaluating Evidence
I think that the experiment went very well. This is because I got all the results that I expected and they look very conclusive. I felt I have accurate results to the best of my ability and so I am able to make a good conclusion. From looking at my results I can see that there were no poor results which was a good thing and it proves that everything went very well.
The only possibility of error was if the temperature of the acid had changed from when I did the first reading to the end i.e. from when I did the first experiment the room temperature did not change and, hopefully the acid would have been the same temperature.
From looking at my results and graph I am able to conclude that the more concentrated the acid the quicker the reaction happened and the more gas was produced.
As we look at the graph we can clearly see that in all results that as the molarity got higher the reaction speeded up.
Before I did the experiments I predicted that the more concentrated the hydrochloric acid was the quicker the carbon dioxide would be produced. This was completely correct because what I thought would happen did.
At this point I am able to answer my main hypothesis which was how does changing the concentration of acid affect the rate of reaction between hydrochloric acid and calcium carbonate?
The answer to this would be that by changing the concentration of the hydrochloric acid it will either make the rate of reaction slower or quicker and more or less gas would be produced depending on whether the concentrations were more or less.
The effect of temperature on the reaction rate between marble chips and hydrochloric acid
Carbonates react with strong acids to give off carbon dioxide and water. Marble is calcium carbonate and thus behaves in the same way:
CaCO3 (s) + 2HCL (aq) CaCL2 (aq) + H2O (l) +CO2 (g)
In this experiment, I am going to see if temperature affects the reaction rate between marble chips and hydrochloric acid by timing the release of carbon dioxide in the reaction.
I predict the higher the temperature, the faster the reaction rate. This is due to the kinetic theory. The more heat that is given to matter, the faster the particles move. This happens in the acid, so the faster the particles move, the faster the reaction rate due to more collisions between the marble chips and the acid.
To help me plan out an adequate experiment and to find a suitable reading range, I will do a small preliminary experiment. Here is what apparatus I will use:
(Draw Apparatus)
I will wear goggles due to the fact that I am handling acid. I am going to take 3 readings in this preliminary experiment to see if the experiment works and takes place inside a suitable time range. I used 5g marble chips and 50ml hydrochloric acid. I am going to take readings from the top of my approximate range and from the bottom. I will also take one in the middle. This will help me to see if my range is appropriate.
I heated up the acid in the conical flask first until it reached the desired temperature. Next I placed the marble chips in and the delivery tube. I then timed how long it took to collect 50cm cubed of air in the burette. Here are the results of the preliminary.
(write or make up results)
I was careful to make this a fair teat by keeping all variables (apart from temperature) constant. To keep the marble chip surface area the same, I used only large chips. I used the same acid every time to keep the acid concentration the same and always used the same amount of materials.
The preliminary showed that the experiment worked. The timeframe is not too long or short either and can be easily measured. The range is OK, but I must be careful to not go over 70°C as I spotted some hydrochloric acid vapours, anything over 70°C would be dangerous. I will make one adjustment to the preliminary by heating the acid separately, and then, when it has reached the desired temperature to pour it in through the thistle funnel into the marble chips. This allows me to start timing the experiment straight away and there should be more accuracy involved. The results from the preliminary fit in with my prediction although more readings are needed.
Obtaining Evidence
(draw diagram)
I set up the apparatus as shown in the diagram above. I took precautions to wear goggles because I was handling acid. I followed the steps I had laid out in the plan with the changes that I made after the preliminary. I was careful not to start timing as I poured in the acid. This is because the gas released wouldn’t be produced by reaction but the displacement of air by the acid.
To keep this experiment a fair one, I had to keep all variables apart from temperature the same. The variables were:
Amounts of materials used
Surface area of marble chips
Concentration of acid
I made sure I used the same mass of marble chips each time and the same volume of hydrochloric acid too. I used the same concentration of acid every time so the concentration is the same. To keep the surface area of the marble chips the same, I made sure all the marble chips I used were large and therefore had similar surface area.
I repeated the experiment readings twice and took the average to gain a more accurate reading and recorded my results in a table.
Temperature Time taken to fill 50cm cubed with Carbon Dioxide (secs) Average time
(ºC) READING 1 READING 2 (secs)
20 35 36 35.5
30 15 17 16
40 12 10 11
50 8 8 8
60 7 8 7.5
70 7 7 7
Analysing Evidence
I used the results obtained to plot a graph of the time it took to fill 50 cm cubed with air against the temperature. The graph shows that as temperature rises so does the speed of the reaction. This is due to the kinetic theory. As temperature increases, particles speed up, this happens in the acid and so acid particles collide more with the marble particles and so the reaction happens faster. This fits in with my hypothesis too.
The graph shows a smooth curve which becomes more horizontal as it reaches the higher temperatures. This is because at what ever temperature, the reaction will never be instantaneous.
I conclude that as temperature increases, so does rate of reaction, but the reaction is never instantaneous.
(draw graph using the data given)
Evaluating Evidence
The experiment went quite well. There were no anomalous results and nothing went wrong. Although I have some ideas which may gain an even more accurate reading. Firstly, the surface area of the marble chips were not perfect. It could have been made better if I had used powdered marble chips because the surface area would have been a lot closer. (although if I did use marble chips, the concentration of the acid would have to be weaker because the reaction would be too fast.) Secondly, and inaccuracy may have been caused by the fact that the temperature may have dropped during the reaction. This could have been used if I used a water bath to heat up the acid instead. This way, I could also heat it during the reaction with relative safety.
There are many possible extensions to this experiment. I could try to use other carbonates and react them with hydrochloric acid. I could try different acids also and see what happens. All these could help me broaden my knowledge on reaction rates.
This essay was written atScarborough College (secondary school and college). The Teacher said "OK the marks were 8 - 8 - 7 - 6". The Student said " Have a look at the marks, even the tough evaluating of evidence was well tackled". This Essay was awarded a mark of A* 8 - 8 - 7 - 6