How the Concentration of a Solution Affects Rates of Reaction.
An Investigation to Find:
How the Concentration of a Solution Affects Rates of Reaction
In this investigation I am going to be investigating the rate of reaction between Hydrochloric Acid (HCl) and Calcium Carbonate (CaCO3).
There are several factors that affect rates of reaction, and I could investigate any of these. These factors are:
* Surface Area
* Concentration
* Temperature
* Affects from Catalysts
Surface Area:
The rate of reaction is affected by the surface area of what is being reacted; in this case the surface area of the calcium carbonate. If the particles of calcium carbonate have a greater surface area (i.e. using many small particles rather a large chunk) the reaction time will decrease; and larger particles would take longer to react.
This can be explained using the collision theory. For the reaction between hydrochloric acid and calcium carbonate to occur, the particles in the chemicals must collide with each other. The more the collisions occur, the faster the reaction.
An object with greater surface area gives a greater number of exposed particles to collide with others.
This diagram (taken from the 'Chemistry For You' textbook) shows this:
You can also further demonstrate this by putting numbers to it. Imagine having a 1000cm3 block. Each side of the cube will have an area of 100cm2. The total surface area of the cube would be 6*100 = 600cm2.
Now let's imagine that I cut this 1000 cm3 into lots of smaller 1 cm3 blocks. I would need exactly 1000/1 = 1000 of these smaller blocks to give me the same volume as the larger block. All of the 1000 smaller blocks have the same volume as the single larger block; however their collective surface area is far greater.
Each side of the smaller blocks would have a surface area of 1cm2, and therefore the total surface area of a single small cube would be 6*1 = 6cm2. There are 1000 of these smaller blocks in one single large one, so 6*1000 = 6000cm2 is the total surface area of all 100 small blocks.
The surface area of the small blocks is 100 times greater than the surface area of the single large block.
Thus, we can see that there is a much larger amount of particles exposed for collisions to happen, and therefore more chance for a reaction to occur.
[Source: Chemistry For You - Lawrie Ryan]
Concentration:
The concentration of the hydrochloric acid will also affect the rate of reaction in this investigation. An acid of a higher concentration will have a faster rate of reaction than a lower concentration.
This is once again can be explained using the collision theory.
A higher concentration of acid would have more particles of acid in a specific volume of water than a lower concentration would.
Here is a small diagram to show this:
(A) (B)
Beaker (A) (imagine as being 1 molar) has half as many acid particles in the same volume of water as beaker (B) (imagine as being 2 molar), meaning it is a lower concentration. There is also half as much chance of the acid particles hitting the marble chip in beaker (A) as in beaker (B), therefore the rate of reaction should be half as much in a 1 molar (lower) concentration as in a 2 molar (higher) concentration.
[Source: Chemistry For You - Lawrie Ryan]
Temperature:
Temperature can have a large affect on rates of reaction. As the temperature raises, reaction rates increase, and as it drops, the rates decrease.
This can partly be explained again by collision theory. As the temperature increases, the particles gain more energy than they had before. As this energy increases, the particles move faster than at lower temperatures. When the particles are travelling faster, there are a greater number of collisions, and the collisions start the reactions.
There is also another factor, however. Some particles do not have enough energy to start a reaction, even if they are colliding with other particles, they will just bounce off. The greater energy from the temperature increase will increase the chances of a reaction happening when they collide.
[Source: Chemistry For You - Lawrie Ryan]
Catalysts:
Catalysts are substances which can speed up reactions.
So if you add a catalyst, then the rate of the reaction rises, the time taken to react drops.
All reactions have an 'activation energy', which is the energy ...
This is a preview of the whole essay
There is also another factor, however. Some particles do not have enough energy to start a reaction, even if they are colliding with other particles, they will just bounce off. The greater energy from the temperature increase will increase the chances of a reaction happening when they collide.
[Source: Chemistry For You - Lawrie Ryan]
Catalysts:
Catalysts are substances which can speed up reactions.
So if you add a catalyst, then the rate of the reaction rises, the time taken to react drops.
All reactions have an 'activation energy', which is the energy needed to start a reaction. When the amount of energy is lower than the activation energy, no reaction takes place when particles collide.
Catalysts lower the activation energy of chemicals, making the energy needed to start a reaction that much lower, therefore it is easier for a reaction to occur.
In this investigation, I will explore the ways in which concentration affects the rate of reaction between chemicals. The two chemicals I will be investigating are hydrochloric acid (HCl) and calcium carbonate (CaCO3).
'Microsoft Encarta Encyclopaedia 2003' says that "increasing the concentration can increase the reaction rate by increasing the rate of molecular collisions". This is saying that as the concentration is increased, the number of collisions between particles will also increase.
From looking at the information from other sources and my own knowledge, I predict that as the concentration of the hydrochloric acid increases, the rate of reaction will drop, and vice versa. I have formed this prediction as in a higher concentration, there is going to be a larger amount of acid particles.
This will mean there are more chances for the particles to collide with the calcium carbonate particles, and a grater chance of a reaction occurring.
After recording down all the results, I predict that my graph will appear like this:
The line/curve does not touch the bottom (x axis) as it is not possible to have a reaction occur in an inexistent amount of time. That is also the reason that I predict it must be a curve, and not a straight line.
[Source: 'Chemistry For You' - Lawrie Ryan;
'Microsoft Encarta Encyclopaedia 2003' - Microsoft Corporation]
In order to come to a correct conclusion, I need to make this investigation as fair as possible.
I will be actively changing a single variable, and that is the concentration of the solution. I will change this by adding specified proportions of water and hydrochloric acid to each other, to give me the desired molar of concentration, so I should be able to get this fairly accurate.
I will be measuring the time taken for the reaction to occur, however this may prove to be difficult, as a reaction still may be occurring even though there are no physical signs.
I will have to specify a certain point at which to continually stop timing the reaction throughout the experiment. The point I will choose is when there is no more sign of any calcium carbonate left in the base of the boiling tube, and there are no signs or bubbles (one of the products of this reaction is carbon dioxide, so there will be bubbles of this gas given off).
I will attempt to keep the other variables that affect rate of reaction the same (these include temperature, surface area of the calcium carbonate, and added catalysts).
Keeping the amount of catalysts will be easy to do, as I simply need to make sure I do not add any. However keeping the temperature and surface area of the calcium carbonate particles will be difficult.
I cannot control the temperature in the laboratory easily with what resources I have, so this may prove to cause some inaccurate results if the temperature varies a lot. The particle size will also be difficult to monitor, as the particles are very small.
I will grind up the calcium carbonate into small pieces using a pestle and mortar, and then check the particle size through a microscope to try and make them as similar size as possible.
Other variables that I will try to control are the amount of calcium carbonate and solution I am using. I will use some accurate scales to measure out 0.1 grams of calcium carbonate, and I will use a measuring cylinder to keep the amount of hydrochloric acid and water totalled at 30 ml.
Before I started this investigation, I did some preliminary work to try and find out what concentrations to time the reaction at, what particle sizes were the best to use, and what amounts of calcium carbonate would be the best to use.
Here are the results I got:
Concentration of Acid
Volume of Solution
Size of CaCO3 Particles
Amount of CaCO3
Time Taken for Reaction
(Molar)
(ml)
(Large Chunk / Fine Powder)
(g)
(min)
5
Chunk
.5
Unfinished in Time
5
Powder
Full Spatula
7
0.5
5
Powder
Full Spatula
Unfinished in Time
0.05
5
Powder
Full Spatula
Unfinished in Time
I first tried using a 1 molar solution (15 ml) with various particle sizes, to see which particle size would be best suited, as I was planning to include 1 molar as one of the values of measuring the rate of reaction at.
My results showed me that it would be best for me to use a powder rather than a chunk or calcium carbonate, as the chunk took too long to react, and was unsuitable for my investigation.
I then tried the powder at other concentrations, to see the types of times I would get for the reaction. Each of the reactions at other concentrations took too long, so I can therefore conclude that if I am going to use a calcium carbonate power, I need to either use a lower amount of calcium carbonate, or a larger volume of solution, or both together to increase the speed at which the reactions occur.
I have decided to double the volume of the solution to 30ml in the investigation, and to reduce the amount of calcium carbonate to 0.1 grams.
In this investigation, I will be using the following apparatus:
*
* Accurate Digital Scales
* 12 Boiling Tubes
* Calcium Carbonate
* 1 molar solution of Hydrochloric Acid
* 50ml Measuring Cylinder
* Mortar
* Pestle
* 2 Spatulas
* Stopwatch
* Test/Boiling Tube Rack
* Water.
Here is my Apparatus Diagram:
Here is my method for what I plan to do in this investigation:
. I will get 12 boiling tubes and set them into two Test/Boiling tube racks. I will set up the scales to measure 0.00 grams with a spatula on them. I will take a pestle and mortar, and grind pieces of calcium carbonate into small powder. I will then check this powder under a microscope to make sure it is of a reasonable particle size.
2. I will measure out 0.10 grams of powered calcium carbonate onto the spatula on the scales. When the display reads 0.10 grams, I will stop adding more calcium carbonate.
I will take the 1 molar hydrochloric acid and measure out 30ml using the measuring cylinder [I am using a 50ml measuring cylinder as it will measure out 30ml to a good accuracy, a 100ml would be unsuitable, as it would not be as accurate as a 50ml]. I will add this into a test tube, and then add the 0.10 grams of calcium carbonate I had measured out.
I will start timing this with the stop watch as soon as the calcium carbonate is added, and stop when the reaction has stopped (going by the reaction has stopped when: There is no more powder at the bottom; The bubbles have all, or very nearly, dissipated).
3. After recording the result, I will then repeat it for the same concentration two more times.
When these are finished and the results recorded, I will move onto a different concentration. I will measure the rate of reaction at the following concentrations:
* 0.5 molar
* 0.6 molar
* 0.7 molar
* 0.8 molar
* 0.9 molar
* 1.0 molar (already measured at this stage)
To measure out a correct concentration for each step (0.5, 0.6 molar etc.) I will have to mix water with the 1 molar hydrochloric acid I have. To do this, I use the following sum:
30*[molar] = Amount (in ml) of hydrochloric acid needed.
So for 0.5 molar, I would do: 30*0.5 = 15.
Therefore I would use 15ml of hydrochloric acid, and fill the rest of the 30ml with water (making 15ml or water in the solution).
4. I will repeat step two, but repeating it thrice for each concentration, until I have three times for each of these concentration values recorded.
[I am repeating the readings at each value three times so that I can form averages, and also so that I can eliminate any anomalous results. This will make my results more reliable].
5. I now have three results for each concentration, and can begin to form a conclusion from these results.
Safety is a factor in this experiment, as I am dealing with acid. I should be aware that acid in dangerous and handle it carefully. I will also wear safety goggles to protect my eyes. I will make sure that any loose clothing or similar is held back in some way, so that nothing gets knocked.
I am also dealing with a fine powder, which could be dangerous if placed into a person's eye, or ingested, so I will take care as to not blow the powder, as it would then be out of my control where it goes. Once again, wearing safety goggles will help prevent this.
Here are the results I got from my investigation:
Concentration of Acid
Attempt Number
Time Taken for Reaction
Average Time (sec)
(Molar)
(sec)
First Two Attmepts
All Results
0.5
745
} 660
} 778.3
0.5
2
575
0.5
3
015
0.6
389
} 396
} 572.3
0.6
2
403
0.6
3
925
0.7
460
} 511.5
} 568
0.7
2
563
0.7
3
681
0.8
339
} 387.5
} 439.7
0.8
2
436
0.8
3
544
0.9
419
} 370.5
} 425.7
0.9
2
322
0.9
3
536
.0
282
} 290
} 336
.0
2
298
.0
3
428
[Anomalous Results are Shaded in Blue with Red Text.]
I have shown what I believe are anomalous results on the graph, however I believe that all of the third attempts at the results could be anomalous.
For this reason, I have included an average of all the results, and also an average of only the first two attempts, as if all the third attempts are anomalous, then I should not include them in calculating the averages.
I calculated the averages by totalling either all three times for the events or the first two (depending on which average I was calculating) and then dividing by either 2 or 3.
On the following page is the graph that I have drawn from these results.
I plotted the average of all three results on the graph, however I eliminated the anomalous results (highlighted in blue in the table) from calculating the averages.
The points that I believe are anomalous I have circled in red.
From these results, I believe that I can assume a reasonable conclusion.
The graph shows that at a lower concentration, it takes longer to react than at a higher concentration.
The graph is also curved, which tells me that the reaction time decreases more when you are going from a low concentration to a slightly higher one, however when you are already measuring high concentration, the drop in reaction time to the next is not so much.
These results fit my prediction, and my prediction graph, although my prediction graph was steeper than the actual graph was.
The lower concentration of hydrochloric acid takes longer to react because there is a lower amount of acid particles in the solution.
The acid particles must collide with the calcium carbonate particles for a reaction to occur, and so when there are not many particles, the chance of a reaction is lower than when there are lots of acid particles. When the concentration is high, there are lots of particle collisions, and therefore the reaction is faster.
During this investigation, I met a few difficulties, which would explain some of my anomalous results.
It was difficult to get an exact amount of calcium carbonate. Although I tried my best to get 3 grams, the weight on the scales was affected by other things, such as air currents, and so the weight changed, making the amount slightly inaccurate. Also, it was extremely difficult to get the particles of calcium carbonate the same size after grinding them, and this probably affected the results.
The amount of hydrochloric acid was also probably inaccurate due to human error, and some drips of the liquid could have been left in the measuring cylinder, which would lower the level by a few millilitres. Another difficulty was getting the proportions of water to hydrochloric acid correct, which was once again probably human error.
I could not control temperature due to a lack of resources; I had no equipment available that I could use to keep the temperature constant.
Due to these difficulties, I would estimate that my results were reasonably accurate, perhaps about 70%, based on how well they fitted with the line of best fit. They are no lower than 50%, as they do show a pattern, and I could draw a curved line from them, however most were not very consistent with each other, and this could affect the averages.
There are ways in which I could amend some of these difficulties, if I had the ability to do so. I could try and get the correct amount of calcium carbonate by restricting the amount of movement in the room I was working in, and making sure all windows, doors and similar were closed.
I would also have to make sure I directed my breath away from the calcium carbonate, as the light powder could easily be blown away.
I would also need to have an accurate way of distributing the powder onto the spatula on the scales. The method I used proved to be fairly inaccurate, and some of the powder was spilled onto the scales, and not onto the spatula. This could not only have affected that weighing, however a build up of powder on the scales could have affected all results. I did try to combat this problem, however, by tearing the weight on the scales before I began adding the powder each time.
Andrew Lyons GCSE Chemistry Investigation