“Have shown below that the rate of reaction is proportional as 1st order.”
Page: 315.
Book: Chemistry in action.
Author: Michael Fremantle.
First order means if I was to half the concentration of hydrochloric acid, there will be half the amount of hydrochloric acid in a set volume. This halves the chance of the hydrochloric acid colliding with the magnesium, therefore this will half the rate of reaction.
Graph showing a proportional rate on the effect of using different concentrations of hydrochloric acid on the rate of reaction with magnesium.
Diagram:
Pilot Study:
- Gather the equipment out shown in the diagram above.
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Mix 20cm3 of 0.5mol dm-3 of hydrochloric acid with 3cm of magnesium strip.
- Start timing when the cork hits the flask.
- Measure the volume on the cylinder every 10 seconds and record the results.
- Stop timing when there are no more bubbles forming in the measuring cylinder.
- Wash the flask out three times thourally.
- Repeat this experiment until you have a set of reliable results.
In order to see if my method works and to ensure I have a set of reliable results I will pilot my investigation first. For my pilot study I used 20cm of 2mol dm -3 hydrochloric acid and no water, and I also used 20cm3 of 5mol-3 hydrochloric acid and no water.
I collected these results shown below
I can conclude that 5mol dm-3 reactions are too fast to record, as all the gas escaped before I put the cork on the flask.
However 2mol dm-3 was successful so I used this as my strongest acid and then made it weaker by dillluting the acid with water to make other concentrations.
Method:
- Assemble the equipment out shown in the diagram above.
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Mix 5cm3 of mol dm-3 of hydrochloric acid and 15cm3 of water with 3cm of magnesium strip.
- Start timing when the bung hits the flask.
- Measure the volume on the cylinder every 10 seconds and record the results.
- Stop timing when there are no more bubbles forming in the measuring cylinder.
- Wash the flask out three times thourally.
- Repeat this experiment until you have a set of reliable results.
- Repeat for the other concentrations such as 1.0, 1.2, 1.5, 1.6, and 2.0.
- The table below shows how to make these concentrations.
Fair test: Throughout the experiment the temperature must remain at a constant heat, the reason for this is that if the temperature was to increase or even decrease there will be more collision, or either less collision and the results would be incorrect and it would not be a fair test.
The amount of magnesium must also be kept the same because this will cause the surface area to increase or decrease and there will be more or less collision as there is more magnesium. Therefore there must be three magnesium strips cut to 1cm used throughout the experiment.
The magnesium strips should also be cleaned before they are placed in the flask, for the reason that the magnesium may react with something else for example oxygen and this may cause the rate of reaction to either speed up or either slow down.
Conclusion: In conclusion my prediction was correct as my graph shows that when the concentration doubles the rate of reaction doubles. In my prediction I explained the more concentrated my hydrochloric acid was there will be twice as much collision; this is shown in my graph as the concentration is proportional to my rate of reaction. However the last point is very high this was because of an exothermic reaction, this also may be the result of not washing the flask out carefully.
Analysis: As the concentration of hydrochloric acid increases, I found that as the acid was more concentrated, the quicker the reaction was. This is explained in a lot more detail by using the collision theory, the collision theory is when there is a higher concentration of acid therefore there will be more acid particles in a given area to react.
Factors such as changes in temperature, changing the pressure, changing the surface area, adding a catalyst, and changing the concentration can increase the rate of reaction. For particles to collide successfully there must be a correct amount of activation energy, the more the particles collide there will be a higher chance of a successful collision. If there are more hydrochloric acid particles there will be a faster reaction in short space of time. In this diagram below you can see if you double the amount of hydrochloric acid, you get twice as much collision.
By looking at my results I can see that as the concentration increases the rate also increases however there is one point that I have highlighted that is very high I expected this point to be lower, this may be because the rate then increases more because there is more heat released due to the exothermic reaction taking place. The heat gained increases the rate of reaction which results with the particles then colliding as they are moving around more rapidly. Therefore my prediction was correct as I have my graph for evidence.
My results show a straight line pattern because my graph is proportional to the concentration and rate of reaction; however the last point is too high so it is out of line and this changes the pattern of my graph as it is not 1st order.
My graph shows that the concentration of hydrochloric acid and rate of reaction are directly proportional and shows a linear relationship. As the concentration of hydrochloric acid doubles from 0.5mol dm-3 to 1.0mol dm-3 the rate of reaction also doubles from 1s-1 to 2 s-1. Therefore the hydrochloric acid must be 1st order.
In my graph when my concentration is at 0.5mol dm-3 the rate of reaction is at 0.5 s-1 when it is at 1.0mol dm-3 the rate of reaction is 1s-1, when the concentration is at 1.2mol dm -3 the rate is at 1.2 s-1, when the concentration is at 1.5mol dm 3 the rate is around about 1.5s-1, when the concentration is at 1.6mol dm -3he rate is slightly higher than I expected and this is the same for the concentration of 2.0mol dm 3 was at 5s-1 however I would of expected it to be approximately around 4s-1. This is because when the concentrations where higher the temperature increased and caused the rate to go up and this had a major impact on the last 2 points of my graph especially the last point. In my evaluation I have explained in more detail how the reaction was exothermic and how I can prevent this from occurring in future.
Evaluation: My results are fairly accurate however the last point should be lower approximately around 4 s-1, I have highlighted this point to show that it is inaccurate. My conclusion is reliable as explained that in my graph as the concentration doubles the rate of reaction also doubles this was also explained in more detail in my prediction. The last point in my graph was not in proportion to the rest of the graph this may be out of line since the reaction was exothermic due to the fact that some particles may have reacted faster in a short amount of time, this has caused the temperature to rise and makes more intense heat and this causes more collisions, this produces kinetic energy making molecules vibrate faster and cause more successful collisions increasing the rate of reaction.
In future to prevent this from happening I will use a water bath.
The diagram above represents a water bath, this is a piece of equipment designed to keep the reaction at a constant temperature throughout the experiment and will keep it a fair test. This is an accurate way to improve my experiment instead of using the method I used for my experiment; however in future if I was to use the water bath it will decrease the errors in my experiment.
Also the experiment might have not been accurate because when I put the hydrochloric acid and magnesium particles in the flask they reacted too fast and I was unable to begin the stopwatch accurately before I put the bung on the flask. This is a problem because you lose some gas and also a lot of heat. In future to prevent this from happening I will use a divided flask.
The diagram above represents a divided flask, this separates the magnesium particles and hydrochloric acid particles from each other, this helps so you can put the bung on the flask first then shake it before the reaction takes place and releases heat. This makes my method more reliable as it gives me more time to record and time the results, therefore I will have a set of reliable results.
If I was to repeat this experiment again in future I would clean the magnesium before I placed it in the flask, because as I mentioned in my fair test the magnesium may of reacted with another gas in the air for example oxygen, this would then give me magnesium oxide and this will make a huge effect on the results. Therefore in future to make my results more reliable I will clan the magnesium before placed in the flask.
To make my results more reliable in future I would repeat the experiment three or four times instead of twice, this will give me more replicates and I will a set of accurate results. In future I will also give myself a longer fixed time to do the experiment in, as I only had a minimal amount of time to do this experiment in. I would also use more concentrations of hydrochloric acid to see the better effect it has on the concentration and the rate of reaction.
