Magnesium Ribbon

         

According to the collision theory, there are four factors, which affect the rate of reactions; temperature, catalyst, surface area and concentration.

Temperature is a variable that can change the rate of reaction as it causes more collisions to occur and when these collisions occur, there is more chance that the collision will lead to a reaction. The particles will collide more often and thus the rate at which the product is being formed will also increase. This is because the amount of energy is more likely to be greater than the minimum amount of energy needed (activation energy)

Surface area is another variable that can cause a change to the rate of reactions. Solids with a smaller particle size (e.g. shorter magnesium strips) react more quickly than solids with a larger particle size (e.g longer magnesium strips).

This is because reducing the size of particles increases the rate of a reaction because it increases the surface area available for collisions to take place. This increases the number of collisions. It has no effect on the energy of the particles.

Catalyst is a substance that can speed up chemical reactions, without being used up itself. It does this by lowering the activation energy so more particles can react. Catalysts provide surface on which the reaction can take place. This increases the number of collisions between the particles of the substances that are reacting. An example of a catalyst:

Hydrogen peroxide decomposes to form water and oxygen gas:

Hydrogen peroxide —> water + oxygen

This reaction only occurs very slowly and it would take 500 days to produce 50 cm3 of oxygen. However by adding the compound manganese oxide that acts as a catalyst for this reaction, the reaction speeds up greatly but the manganese oxide never runs out. Therefore catalysts have the ability to get the product in much less time and can cause an effect on the rate of reactions.

Concentration is the strength of a solution, for example the reaction between marble chips (calcium carbonate) and hydrochloric acid:

Calcium carbonate + hydrochloric acid —> calcium chloride + water + carbon dioxide

A strong acid contains more particles and less water particles than weaker acid. Increasing the concentration of a solution will lead to more collisions and thus a change in the rate of reactions.

The concentration of the acid is low, thus there are less collisions taking place which means slower rate of reaction.

High concentration of acid, more collisions thus faster rate of reactions.

Apparatus:

1. Hydrochloric acid                        6. Funnel

2. Magnesium Strips                                7. Conical Flask

3. Beakers                                        8. Goggles

4. Measuring cylinders                        9. Water

5. Scale                                        10. Pipette

Fair testing:

This experiment has to be kept fair in order to achieve the correct results. There are various ways in which fair testing will be implemented:

1. The experiment will be repeated three times to obtain reliable results.
2. It will be made sure that the apparatus is not contaminated.
3. All variables will be kept constant except the concentration variable

Diagram:

Done by hand

Method

Dilution:

In order to carry out the experiment, the hydrochloric acid needed to be diluted.

Therefore a dilution table was used to produce the acid of the correct molarity.

The dilution table below shows the measurements of the hydrochloric acid and the amount of water needed to create each molarity.

 

The dilution table:

This is the method used for the dilution process using the dilution table.

1. Firstly the apparatus used for carrying out the dilution process was set up. This included:

• Measuring cylinder
• Goggles
• Hydrochloric acid
• Water
• Beakers 

2. Then due to safety precautions, gloves, goggles and an apron was worn.
3. Next for producing the correct molarity of acid, the correct amount of water, which is stated in the dilution table, was measured in a measuring cylinder.
4. Then using the dilution table the correct measurement of hydrochloric acid was measured in a measuring cylinder and mixed with the water.

The experiment:

1. First of all the following apparatus was set up:

• Beakers
• Magnesium strips 2cm each
• Hydrochloric acid of concentrations:

• 0.5M
• 1M
• 1.5M
• 2M
• 2.5M
• 3M

• Conical Flask
• Scale
• Test tubes
• Test tube holder
• Measuring Cylinder
• Stop Clock
• Pipette

2. Secondly, as safety was important a glove, goggle and apron had to be worn.
3. Next a 2cm length of magnesium strip was measured
4. Now by using a pipette, 6cm³ of diluted 0.5M-concentrated acid was poured into a test tube.
5. One magnesium strip, rolled up, was put into the test tube containing the acid.
6. As soon as the magnesium strip was put into the test tube with the acid, the stop clock was turned on to record the rate of the reaction.
7. When all chemical reactions had stopped in the solution, the stop clock was turned off and the time was recorded.
8. This same procedure was repeated two more times to achieve reliable and the average results
9. To see whether concentration has an effect on the rates of reactions, the above method was repeated with all the other molarities, three times each.
10. Now all the results were compiled together for observation.

OBTAINING EVIDENCE

Results:

Shown below are the results of the experiment.

Table

This Table shows that as the concentration of Acid molarity increases, the time taken for magnesium ribbon to dissolve decreases. The results match with the prediction, and thus the collision theory is correct.

ANALYSIS

Graph

Average time taken for magnesium ribbon to dissolve:

EVALUATION

Conclusion:

The results of this experiment show that concentration does have an effect on the rate of reactions. Furthermore it also proves that the stronger the concentration, the faster the speed of reactions will be and the weaker the concentration, the slower the speed of reactions will be. In this experiment, the magnesium ribbon reacted faster and more visibly in the acid molar 3.00M than it did in 0.5M.  The results show a clear pattern to support this theory.

Evaluation:

The results and the conclusion obtained in this experiment prove that the prediction made was correct. It was predicted that the rates of reaction of the magnesium ribbon would be affected by the concentration of the acid.

The higher the molarity of the acid the faster the speed of the reaction will be.

The prediction made was based on the collision theory, which clearly states that concentration does have an effect on the rate of reactions. Moreover by carrying out this experiment it is further proven that the collision theory principle is true.

According to the collision theory there are four factors that affect the rates of reactions, temperature, surface area, catalyst and concentration. The aim of this experiment was to prove that this theory was correct, and thus one of these factors was chosen to do so. This factor was concentration and there are various reason why it as chosen. First of all, it was the easiest for obtaining quick results. Secondly making it a fair test was far less difficult as only one variable was changed and the rest were kept constant. Thirdly, if any other factor was chosen for example, surface area, then two variables would have to be looked at, the size of the ribbon and the amount of HCl. This would have taken more time and yet the risk of the magnesium ribbon being the required size would have been high as such apparatus was not available. If any other factor that affects the rate of reactions were chosen such as temperature, then concentration would have been involved in the experiment too, in order to get the correct results. This simply would have just complicated the experiment even further.

The results obtained are according to the prediction, however there are certain factors, which could have caused anomalous results.  

The magnesium ribbon was put in the acid as an unfolded strip. This strip often got stuck at the top and slowed down the reaction. Thus the ribbon was rolled up so it settled at the bottom of the acid instead of the top. The experiment was done with the unfolded strip of magnesium ribbon and the time taken for it to react completely was double of the time taken for a folded strip of magnesium ribbon to react completely with the acid. This affect in the rate of reaction is because the more exposed the magnesium ribbon is to the acid, the more collisions will take place thus the faster the reaction will be. Therefore surface area determines how soon the magnesium ribbon comes to contact with the acid.

 

 Furthermore there is a possibility that the length of all the magnesium ribbons may not have been exactly the same. This can cause anomalous results

Also, the test tube in which the reaction took place was open. This can cause a layer of magnesium oxide to develop on the magnesium ribbon as it reacts with oxygen. Sand paper should be used to remove it as it affects the rate of reaction.

Moreover the beakers used were most probably contaminated, this is another factor, which can cause anomalous results.