The rate of reactions can be measured in two different ways:
- Measuring change in mass on scales
- Measuring gas produced with a gas cylinder
Both are equally good measures of the rate of reaction. To measure change in mass is relatively easy; you just add the reactants together and measure the change in mass. To measure the amount of gas produced with the equipment we have is slightly less accurate, you have to drop the reactant in and then place the bung in the hole. This obviously is not 100% accurate, however it does the job and gives a very definite reading.
The Collision Theory
In solution substances exist as millions of tiny particles in arbitrary motion. The particles frequently collide with one another. If a reaction is to occur, energy is required to break existing bonds and cause the reaction. For that reason, collisions need a minimum energy, called the activation energy, before a reaction can take place.
The collision theory used to predict the rates of chemical reactions, particularly for gases. The collision theory is based on the assumption that for a reaction to occur it is necessary for the reacting atoms or molecules to come together or collide with one another. Not all collisions, however, bring about chemical change. A collision will be effective in producing chemical change only if the molecules brought together possess a certain minimum value of internal energy, equal to the activation energy of the reaction. Furthermore, the colliding species must be oriented in a manner favourable to the necessary rearrangement of atoms and electrons. That’s why, according to the collision theory, the rate at which a chemical reaction proceeds is equal to the frequency of effective collisions.
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For a reaction to take place the reacting particles must collide with sufficient energy, or energy above the minimum activation energy. Therefore anything that can increase the number of collisions by reacting particles, or increase the energy of the collisions by the particles, or provide a reaction path with lower activation energy, increases the rate of reaction.
Surface area: when one of the reactants is a solid, the reaction can only take place at the surface of the solid. Breaking the solid into smaller pieces will increase the surface area exposed to the other reactant, so there is a higher chance that reacting particles will collide, which increases the reaction rate.
When reactant B is in single, solid form, only the outside particles can collide with particle A, so fewer successful collisions can occur.
When reactant B is powdered many more atoms \re exposed to particles A, so there are more collisions, and greater chance of successful collisions, so the rate is increased.
The Experiment:
Safety
To ensure safety, I must follow several rules when conducting my experiment. All long hair must be tied back to stop it interfering with the experiment, and getting caught in any apparatus. For the same reason laboratory coats, or tucking in any loose clothing, must be worn at all times during the experiment. This also stops clothes and skin from getting damaged. Safety goggles must be worn as Hydrochloric Acid will be used, and the goggles will protect the eyes. All bags and stools must be kept under the desks to stop people falling over with acid and injuring themselves. For the same reason, running is never permitted in the laboratory. When pouring the solutions, care must be taken to avoid spills, which could damage or irritate the skin.
Fair Test RAAID IQBAL
In any experiment it is important to make sure that the test is as fair as possible. This is especially true for scientific experiments. I have considered the following to make sure that my experiment is as fair and accurate as possible
- Concentration of acid
- Amount of acid
- Types of mg strips
- Repetition
- Condition – Every experiment will be carried out in the same conditions and in the same way. This includes things like temperature, pressure and light intensity etc.
Prediction:
I predict that the smaller the particle size the faster the rate of reaction will be.
As the reactant the solid has more surface area. This means the particles around it in the solution will have more area to work on so there’ll be more useful collisions.
As the diagram below shows
Apparatus
- Conical flask
- Measuring cylinder
- connecting tubes , cork
- tub
- Stop clock
Chemicals:
- Magnesium strips (all 12 cm)
- Hydrochloric acid (25ml)
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A reaction into the effects of surface area on reaction rates
During the experiment hydrogen escapes from the reacting mixture. This is what I will be measuring. In the Experiment I will be using three different magnesium strips with different surface areas the strips will all be 12 cm long but I will change the surface area by using TIPEX on the strips :
I will call these strips small , medium and large.
I would put these strips one by one in the same amount of hydrochloric acid in the same conditions.
I will be using a measuring cylinder filled with water in a tub of water, which will measure the amount of hydrogen filling in it from the experiment which I will use to find how the particle size effects the rates of reaction. This will be a little less accurate then the syringe test but it will do the job.
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“Reactions in which the reactant is solid take place faster when the solid is divided into small pieces. The smaller particle have a large surface area to mass ratio against the larger particles”
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Method:
THE EXPERIMENT:
Firstly set up all the apparatus and make sure the safety precautions are taken care off.
Then get the chemicals; first measure the magnesium strips accurately (12 cm)
Prepare them, put tipex on the strips then get the hydrochloric acid while the tipex dries . Measure and get exactly (25ml) of hydrochloric acid.
Once you have set up all the apparatus and gathered the chemicals pour the hydrochloric acid in the conical flask and get ready to put the magnesium in the hydrochloric acid but before you do that check the stop clock and that it is at zero. once you are all ready and have the safety equipment on put the magnesium strip in to the solution and quickly close it and at once turn the stop clock on. But making sure u don’t tip over the acid in hurry or some thing else it better to ask a partner because safety is always important. So as the hydrogen in the measuring cylinder is filled note the time after every 20 cm.
Do the same with the other two remaining Mg strips making sure it’s a fair test.
Mg(s) + 2HCl(aq) ----------> MgCl2 (g) + H2 (g)
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Then we can compare the results and show them on a graph to check how much particle size affect the rate of reaction.
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Conclusions
By looking at the results and the initial rate of reaction it becomes clear that there is a direct correlation between the particle size and the rate of a reaction. “The larger the surface area of the strips the faster the rate of the reaction.” This is because the larger the surface area of the particles the more area is available for collisions with the acid particles. This is collision theory. This creates more individual reactions at any one time and therefore speeds up the overall rate of a reaction. My prediction was correct, as you can see visually from the graph, the larger the amount of surface area the faster the rate of reaction (gas produced.)
Explanation
SCIENTIFIC EXPLANATION
SUFACE AREA
Reactions in which the reactant is solid take place faster when the solid is divided into small pieces. The smaller particle has a large surface area to mass ratio against the larger particles
Breaking the solid into smaller pieces will increase the surface area exposed to the other reactant, so there is a higher chance that reacting particles will collide, which increases the reaction rate.
Evaluation
The experiment was successful and I would say that there do not seem to be any major inaccuracies. The result is slightly lower than anticipated but still reasonable. The results are accurate as good care was taken while doing this experiment.
As with any experiment that is not done in a completely scientific laboratory with perfect equipment, there will be small inaccuracies that, although you may try to avoid will still be there.
The actual reading on the clock that we took fast cannot be 100% accurate.
Extension
We could extend the experiment in several ways; in fact you could carry on extending it. It would be a sensible start to decrease the amount of time that the samples are taken at. For example take readings every 10 cm cube instead of every 20, which would give us a better picture of how the initial rate actually occurs. We could investigate how other reactants occur with magnesium strips and compare this to our experiment.
BY RAAID IQBAL