Concentration – Increasing the concentration means there are more molecules in the acid so there is more chance of the molecules bumping (Brownian motion) into each other with enough force to react. This makes it more possible for more reactions to occur every second.
Collision Theory – For two substances to react, their particles have to collide. It is what affects how they collide is what we will be changing and investigating. However, there is a required amount of energy in the collision between the two molecules for a reaction to occur. This is called the activation energy.
Formulae used to calculate the rate of reaction:
Change in Amount of Substance
Reaction Rate =
Time taken for Change
When we react the two substances, magnesium ribbon and hydrochloric acid, the formula is:
Magnesium + Hydrochloric Acid Magnesium Chloride + Hydrogen
Mg + 2HCl MgCl2 + H2
Prediction
I predict that as the concentration of the hydrochloric acid increases so will the time taken for the reaction to occur.
I predict this because the collision theory states that this should be true, however, we are just investigating it to see for ourselves. When two substances react, their molecules will collide with each other with enough energy for them to react. However, when you put more molecules together they will bounce off each other more frequently, giving them more motion and force. If you put more energy into the molecules movement, there will be more chance of the molecule meeting another one, as there are more in the acid and surrounding the magnesium, with more force, therefore, the time taken for the reaction to occur decreases. Therefore, the more molecules there are in a solution, the greater the chance of the molecules colliding with enough activation energy for them to react. This will apply to the concentration of the hydrochloric acid I am using in this investigation. Theoretically, if I increase the concentration of the acid, the reaction should take less to time. If I decrease the concentration, then the opposite should happen and the reaction time should be less.
Apparatus
- Beakers
- Gas syringe
- Rubber Cork
- Magnesium Ribbon Coils
- Scissors
- Ruler
- Different concentrations of hydrochloric acid
Stopwatches
- Hydrochloric Acid – and water to dilute it with
Method
- I gathered all my equipment and laid it out on the lab desk and mixed the necessary concentrations of acid I needed, using water.
- I then cut all the magnesium ribbon I had into 5 cm lengths, ready to be dropped into the beaker.
- For example, I get a beaker of 100ml of hydrochloric acid, diluted down to 1 molar of concentration.
- When the magnesium ribbon is dropped into the beaker and a rubber cork is put in the neck of the beaker. The rubber cork is attached to a tube that leads from inside the beaker to the gas syringe.
- Immediately as the magnesium is dropped into the beaker and the rubber cork put in place then the 3 stopwatches are started.
- The stopwatches are stopped at different times; the first is topped when the syringe has reached 20mls of hydrogen. The second when the syringe has reached 40mls and the third, when it has reached 60mls.
- The results are taken down on a table drawn by pencil and ruler, but then transferred onto a spreadsheet on a laptop.
- This is done 3 times for each of the concentrations so we get a variation in results. Averages are then calculated.
Fair Test
These are the factors I looked at to make this a fair test:
- I decided to keep everything that I used in the experiment constant, for example:
- I decided to try out each concentration of acid three times so that if one wasn’t accurate then there were other results to balance it out.
- Every time I put reacted the two substance, I made sure that the gas syringe was put back into place and the line was in line with 0 mark.
- When the stopwatches had to be started, I tried to start them all at the same time so one person had two of the stopwatches and the other had one of the stopwatches, while putting in the magnesium in the beaker and putting in the rubber cork.
Diagram
Results
The results for how long it took that reaction with the specified concentration to fill up 20 ml of hydrogen:
The results for how long it took that reaction with the specified concentration to fill up 40 ml of hydrogen:
The results for how long it took that reaction with the specified concentration to fill up 60 ml of hydrogen:
I can immediately see form the results that as the concentration of the acid went up, the time it took for the reaction to occur decreased.
This is also made apparent here, as the line curves, up from 0.5 molars down to the 3 molars, showing clearly the decrease in time. However, the time does not decrease in a constant matter. For example, there is a huge difference in the results for 0.5 molars and 1 molar, you can see this because of the steep gradient the line has. This also applies to the other results, except the difference gets smaller as the concentration increases. The reason that the first reaction (0.5 molar) took so long was because the reaction, over time, uses up acid molecules and so the concentration of the acid goes down.
These results follow a similar pattern to those of the 20 mls, with the steep gradient of line form 0.5 to 1 molar showing the difference in time. Then, however, the difference gets gradually smaller, as does the time for each concentration (as the concentration increases). For all of the results, for the most concentrated acid, the reaction happens as soon as the reactants are put in the beaker. The reason the difference between each results of each concentration gets less and less because there is probably a fixed time in which the reaction can take place, it cannot keep going faster and faster at split second times. This is why the graph curves towards being a straight line.
These results follow a similar pattern to those of the 20 mls, with the steep gradient of line form 0.5 to 1 molar showing the difference in time. Then, however, the difference gets gradually smaller, as does the time for each concentration (as the concentration increases). You can also see that our results were fairly correct, as the results for the 40 molar tests were approximately double to that of the 20 molar results.
Conclusion:
I conclude that my prediction was correct in stating, when the concentration of the acid increases, increase in the number of molecules per 100 ml3, the time for the reaction to take place decreases. As stated by the collision theory and in my prediction, having more molecules in the same volume of solution will increase the chances of the molecules colliding with enough activation energy to react.
