If we increase the volume the particles are more crowded so they collide more often. Even though the average amount of energy possessed by a particle does not change, there are more particles with each amount of energy - more particles with the activation energy.
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If reacting substances are heated, the rate of the reaction usually rises; conversely, if they are cooled, the reaction slows down. In order to react, the particles in the substances must collide with each other. gives them more to move around and so increases the chances of a collision. Also, when particles do collide, they are more likely to react, rather than just bounce off each other, if they are moving faster. Cooling has the opposite effects. For example, when sodium thiosulphate is mixed with dilute hydrochloric acid the mixture becomes cloudy as solid comes out of the solution (precipitates). If the mixture is heated, it becomes cloudier more quickly. If it is cooled, it takes longer to become cloudy.
Catalysts are very important in speeding up many chemical reactions. They are particularly valuable in the chemical industries, because their action in speeding up the can make a chemical reaction commercially viable. Catalysts create an effect by lowering the activation energy of the reaction they catalyse. This means that more of the colliding particles will overcome the activation energy barrier and react, speeding up the overall rate at which the reaction proceeds.
Solid reactants, like magnesium ribbons and marble chips are effected by surface area, the larger the surface area the more collisions that will take place. This will increase the rate of reaction and decrease the time taken.
Increasing the concentration of reactants (the amount dissolved in a given volume of solution) can have an effect similar to heating them, because the more particles present, the more likely a collision, and so the higher the reaction rate.
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The increase in the concentration should be directly proportional to the increase of the reaction rate at a given time. This is because by doubling the number of hydrochloric acid molecules present the chance of a collision should be doubled, as there is now twice the possibility of a collision-taking place initially
A stain can be removed by a bleach solution. It has been proven that the more concentrated the solution the faster the stain will disappear.
The graph below shows A and B. The acid in A is twice as concentrated as in acid B.
Magnesium + Hydrochloric acid Magnesium chloride + Hydrogen
Mg + 2HCL MgCl2 + H2
I didn’t measure the volume of hydrogen in my experiment because a gas syringe wasn’t available to me.
These diagrams show how the reaction rate increases as the concentration increases:
Apparatus
- Six test tubes
- Six strips of magnesium (2cm)
- Test tube holder
- Labels
- Stopwatch
- Hydrochloric acid (concentrations – 0.5m, 1.0m, 1.5m, 1.25, 1.75 and 2.0m)
- Thermometer
- Measuring cylinder
Method
Firstly I measured 10cm3 of hydrochloric acid, which had a concentration of 0.5m, using a measuring cylinder. Making certain that my eyes were protected using goggles.
I poured the hydrochloric acid into a test tube, which I then labelled, and then I put it in a test tube holder. I then measured the temperature of the hydrochloric acid, using a thermometer, to keep the temperature of each different concentration the same, 21˚C, ensuring a fair test.
I then filled another five test tubes with 10cm3 of hydrochloric acid, but changing the concentration. I labelled all the test tubes as to keep check of which test tube held which concentration. And I measured the temperature of the contents of each test tube, keeping it at 21˚C, ensuring a fair test.
I then dropped a 2cm strip of magnesium into the test tube containing 10cm3 of hydrochloric acid with a concentration of 0.5m and set the stopwatch immediately. I then closely observed as the hydrochloric acid and magnesium strip reacted and stopped the stopwatch when the magnesium strip had totally disappeared. I then recorded the time, in seconds, in a table of results.
I then added a strip of magnesium and one at a time observed the reaction and how long it took for the magnesium strip to disappear. I recorded the results in a table of results.
I then repeated the experiment as to achieve an average and check that the experiment was performed correctly and the results were eligible.
I will take all safety precautions to ensure my safety when carrying out the experiment, especially around the hydrochloric acid.
Fair Test
It is important that the experiment is performed correctly to ensure that the results are eligible and achieved correctly. To ensure the test will was fair I:
-
Measured 10cm3 exactly of hydrochloric acid using the measuring cylinder.
- Kept the temperature of the hydrochloric constant at 21˚C
- Kept the length of the magnesium strip constant at 2cm
Results
Analysis
A definite relationship can be noticed in the results. As the concentration of the hydrochloric acid increased the amount of time in which it took for the magnesium strip to disappear decreased.
When I refer back to the scientific knowledge I can observe that the pattern in the results can be explained by the collision theory, which describes how the rate of reaction increases (the time taken for the magnesium ribbon to disappear when it is reacted with hydrochloric acid) when the concentration of HCL increases. The theory states that if, the more concentrated the reactants, the greater the number of collisions between particles increase.
The below diagrams shows the particle theory and explains how the reaction rate is increased:
I predicted that if I increased the concentration of the hydrochloric acid then the reaction between the hydrochloric acid and the magnesium would take place more quickly. The results in which I achieved agree with my prediction, as the results clearly show that as the concentration is increasing the magnesium and the hydrochloric acid react at a quicker pace. When the concentration was 0.5m the time taken for the magnesium strip to disappear was 777.5secs. When the conⁿ was 2.0m the time taken for the magnesium strip to disappear was 27.5secs. There is quite a drastic difference between the two times and it clearly shows the effect of the concentration.
Evaluation
The observations in which I made were fairly accurate – they clearly supported the scientific knowledge and prediction that I made.
However accurate I believed the experiment to be there was a result that didn’t fit into the pattern. The curved line on the graph directly misses one point. The first set of results that I achieved supported the scientific knowledge collected, but when I did the second set of results I found they were very dissimilar – there was a great difference between the times of the reaction with the acid that had a concentration of 0.5m, I believe this was due to not sanding down the magnesium enough in the second attempt and poor observation skills and timing methods during the first attempt. When doing the second attempt the hydrochloric acid with a concentration of 1.25m produced an odd reaction time that did not fit in with the other results and perturbed the relationship slightly. This I believe to be due to poor sanding and bad timing methods.
I think that I could have drawn a better conclusion had I repeated the experiment, because my results weren’t entirely reliable. Due to the sanding down of the magnesium ribbons being inaccurate, the timing method being unreliable and my observation skills for withering slightly.
Perhaps the experiment would have been more accurate and therefore had given a more reliable conclusion if I’d used the gas syringe method to measure the hydrogen gas given off, but it was not available to me.
