How Does Rate of Concentration Affect the Rate of Reaction Between Marble Chips and Hydrochloric Acid?
How does rate of concentration affect the rate of reaction between marble chips and hydrochloric acid?
Introduction
We are going to investigate how the rate of reaction differs using varying concentrations of hydrochloric acid with marble chips.
Prediction
I think that the higher the molarity of the hydrochloric acid, the faster the reaction will occur because the less water molecules means the chances of acid and calcium carbonate particles colliding is greater. (If the acid was more dilute, it would contain more water molecules, meaning it would take longer for the particles to react with each other because the odds are reduced of them colliding.
Planning
CaCO3 + 2HCl(aq) CaCl2 (aq) H2O (l) + CO2 (g)
Calcium + Hydrochloric Calcium Water + Carbon
Carbonate Acid Chloride Dioxide
To ensure a fair test, the following must be considered:
* Temperature - to keep it the same or change it, reactions go faster under a higher temperature because the heat provides the particles with energy so they move faster and the collision factor is increased. However our experiment is to see if concentration affects the rate of reaction, not temperature so it must be kept as a controlled variable. Trusting the thermostat or central heating of the classroom, it should be kept at a regular temperature, suitable for a working environment.
* Volume of acid - this must be a controlled variable also, but should be in excess otherwise the rate of reaction cannot be measured if all the acid reacted before all the marble had reacted, we would never know where in the reaction the acid ran out.
* How we measure the rate of reaction - several methods were tried in our preliminary investigations, we tried using a boiling tube upturned and submerged, so that when the gas was tubed into it the water was displaced by carbon dioxide from the reaction. However the amount of gas a boiling tube can collect is very rapid and something bigger was needed... We chose a beaker upturned, (submerged in water) and we would stop when 200cm3 of gas was collected and then we would stop the stopwatch that recorded the length of time this took.
The reason why we found the glass syringes undesirable was because despite the smooth flow of the plunger there was too much room for blunder. For example gas could escape because the syringe has more than one part to it - unlike a beaker and inertia means the gas will have to collect until it compresses enough to depress the plunger, making a fault in the time recording.
* Balance - use the same balance to measure the chips, the room for error increases if we use different scales, to maintain a fair test the same balance must be used, and all equipment must be the same as well. The weight will be recorded in grams.
From our preliminary investigation, when we realised the acid should be in excess and not the chips, we decided to drop the value from 10g to 5g. This is for scientific and economical reasons, there were only so many marble chips available, we also decided before as a ...
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* Balance - use the same balance to measure the chips, the room for error increases if we use different scales, to maintain a fair test the same balance must be used, and all equipment must be the same as well. The weight will be recorded in grams.
From our preliminary investigation, when we realised the acid should be in excess and not the chips, we decided to drop the value from 10g to 5g. This is for scientific and economical reasons, there were only so many marble chips available, we also decided before as a class that varying the size of the marble chips was pointless, because what is big? And how can you regulate the size of chip - you can't. We chose to use medium size marble chips, there was powdered available but it would be harder to measure accurately and powder is easily displaceable and the weight could easily be altered by losing some. The rate of reaction would also be faster because the particles are already partially broken down, and have a better chance of colliding with acid particles, but the focus was on how does concentration affect reaction.
* Beaker - same beaker should be used, but all the beakers are standard, 200cm should be the same for any properly made beaker universally. Obviously we won't use a small beaker then swap it half way through or anything tedious like that.
* A stopwatch will be used to time the reaction accurately to the nearest second.
* Independent variable - this will be the concentration, the concentrations available are 0.25M, 0.50M, 1.0M, 1.5M and 2.0M a variation is needed to see if there is a pattern between them.
Safety
Safety goggles must be worn at all times, and care must be taken with the stronger acid especially as it is corrosive, if any is accidentally spilt it should be cleared up promptly and any on the hands must be washed off. With the vast amounts of glassware care must be taken especially as hands may be wet as well as surfaces too, which means no running and standing up is more appropriate than sitting down.
Equipment
Preliminary testing decided exactly what equipment we needed and its necessity in the experiment. The diagram demonstrates the set up we will use.
Results
TIME
Concentration
2
3
Anomalous
Average Time
0.25M
0.57
0.04
7.50
9.37
0.50M
8.56
9.06
9.59
2.44
7.41
.00M
3.34
3.47
3.05
8.35
4.55
.50M
2.00
2.00
2.27
2.09
2.00M
.36
.30
.45
.37
Conclusion
From my graph I can see that the rate of reaction decreased in time when as the concentration increased, in keeping with my hypothesis. I also plotted the graph of the gradients against concentration to show the linear proportional relationship. The graph I produced is pleasing because it coincides with a previous experiment we did with sodium thiosulphate and hydrochloric acid. The similarity is that we used 5ml of acid and varied the thiosulphate, but it still meant that the independent variable was changing concentration and the graph produced is the same parabola.
However, a couple of anomalous results occurred, but were included into the averages that were plotted on the graph, because although they don't fit with the other results, it could be that my results that seemed to be normal were actually the abnormal ones and the 'anomalous' was not, even if that wasn't the case they were still included.
These may have occurred because of the order that the concentrations were done in, the same concentration were not always in canon, and sometimes the acid from before would speed up or create a delay in events, but it was only a small amount coating the sides of the conical flask. Another reason for this was maybe because of this reason, after using 2M acid I washed the flask out with water, which is neutral and some was left in the bottom and coated the inside, so the acid was neutralized for a while before it could take affect on the marble chips.
Evaluation
I feel that to make the collection of data more accurate, taking readings of amount of gas produced at regular time intervals would have been better, so that individual graphs could be compared to others of different concentrations. The average speed could also be calculated for each concentration. (Amount of carbon dioxide produced per minute in centimetres cubed.)
As far as the practical collection of data goes, a systematic approach would have been better, because then the acid lining the conical flask would be at least similar. In ideal circumstances a new one should be used for every new concentration so there is no variation once the new concentration is added. For speed, instead of waiting for 200cm3 to be collected, 100 may have been more suitable because it takes a very long time for that much carbon dioxide to accumulate when there is only 0.25M of acid acting on the 5g of marble chips.
For speed measuring was done straight into a beaker, for optimum accuracy this should be done with a measuring cylinder, but in a way it is harder to pour into a narrow tall container and difficult to pour into such a small surface area from a large round bottomed flask, into a beaker, into the measuring cylinder, or straight from flask to measuring cylinder. There is also a greater chance of knocking over a measuring cylinder, and they cost more than a regular beaker whose centre of gravity is low, making it more stable and below elbow height.
To increase the accuracy with more time available, I would have liked to possibly repeat the concentrations again, or used concentrations within the existing ones, so I get a proper pattern, such as 0.25M, 0.50M, 0.75M etc with regular intervals, instead of keeping to the concentrations available.
To extend the investigation I could use powdered marble chips to see if this created a difference, or try adding a catalyst which would speed up the experiment, but would not be lost at the end, to compare with the results I have now, to see how the difference of speed of the reaction changes.
From internet sources I could see what other coursework assignments were done by GCSE students, some were the sodium thiosulphate and acid one we'd already done, but from out textbook it suggests that the independent variable be temperature not concentration, it says that the two reactants are controlled variables. The independent variable for our experiment could have been temperature if we wanted it to be, so perhaps we could do this as well as an extension. This would change the rate of reaction because:
"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.
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 reason for our apprehension to varying temperature as our first major investigation into rate of reaction was because of the room for error. If we were to keep temperature, as the independent variable then a water bath would be used, to get the acid to the same degree three times in succession would be difficult and the preliminary work would take longer than it would for concentration variation. For example if we were to choose temperature to vary, we would have to find out first which temperature's had an effect, room temperature is around 20-25 degrees which obviously the liquid would be a few degrees under this, depending on how long it had been exposed and to what intensity.
We would also have to decide which temperatures to use, 10, 20, and 30?? We would have to have good reason for our choices and have some correlation between them so it would be easy enough to plot a graph and see some relationship. Another thing is, the water bath would not possibly warm the acid up to the temperature we wanted, and HCl probably has a different boiling point to water so we couldn't heat it over that point otherwise the volume (controlled variable) would be altered and not be a fair test anymore. (We may even have to find this out for ourselves)
In contrast to raising the temperature, would it not be inversely true that if you decrease the temperature, the rate of reaction also lowers.
In reality we do this every time we put something in the refrigerator and harness this knowledge to our advantage. 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. However this is quite unscientific as it is immeasurable and the open room cannot be controlled it is open to whatever is in the air. This experiment would be more of a biology experiment because it is encouraging bacterial and microbes that secrete chemicals and toxins, so it is not strictly a chemistry experiment.
Another variation on the experiment we did is to use a base and acid, such as magnesium and hydrochloric acid, again to use particle size would be a waste of time because there is no scale of variation because the magnesium comes on a roll and is quite thin, the independent variable could be temperature or concentration of the acid.
Elisa Holbrook 08/05/07
P1