Investigation on Rates of a Reaction.

• Surface area – The greater the surface area visible, the more molecules on the solid there are available to react, therefore the chance of particles colliding with enough kinetic energy is increased and so the rate of a reaction is increased.

During this experiment I will be looking at what effect the concentration of an acid has on the rate of a reaction. I have chosen concentration because it is one of the easiest to monitor. I will ensure that I keep the other variables constant.

I conducted a pilot experiment to gather preliminary results and to ensure that the decisions I make for the actual experiment are wise and will help me gather a reliable set of results,

During the experiment I used the following concentrations of Hydrochloric acid: -

• 0.1 m
• 0.5 m
• 1.0 m
• 1.5 m

And used magnesium with length of 2 cm

My results were as follows

Based on my preliminary results I have found the following helpful towards my actual experiment: -

• I will not be using the concentration of 0.1 moles because it was very slow at reacting and so I will now be starting at 0.5 moles.
• 2 cm of magnesium has a large surface area and so takes a long time to react.
• I will use a 5th concentration to give myself a broader basis to back up my prediction.
• I will record all times to .1 of a second

Key Factors

Here is a list of variables that can affect the experiment: -

• The length of the reactant (magnesium) – The larger the surface area the longer it will take to react
• The concentration of acid – The higher the concentration the denser the acid particles are and so the greater the chance that the acid particles and the magnesium particles will collide.
• The surrounding temperature and temperature of the solution – If the temperature is high then this large amount of thermal energy will convert into a large amount of kinetic energy, the acid molecules will travel faster and so they will collide with the magnesium particles more often. The reaction therefore will occur much faster.

We are changing the concentration of the acid solution to see which has a quicker reaction with the metal.

To keep it a fair test I will be keeping the following constant: -

• The length of the magnesium
• The volume of the acid
• The temperature of the solution and the laboratory in which we carry out the experiment
• I will not be using a catalyst

Prediction

Based on my scientific knowledge I predict that as the number moles in a solution (unit at which we measure concentration) increases the speed at which the reaction occurs will also therefore the higher the concentration the faster the reaction takes place. In this view by doubling the concentration theoretically the time taken should half.

Apparatus

Here is a list of equipment that I will be using during this experiment: -

• 25 x test tubes
• 5 x test tube racks
• 500 ml x hydrochloric acid (HCl)
• 160 ml x distilled water
• 25 cm x magnesium ribbon (mg)
• 1 x medium beaker
• 1 x bung
• 1 x measuring cylinder
• 1 x scissors
• 1 x stop watch
• 1 x pencil, pen (to label the test tubes) and table for results

Method

Here is an account of how I will be doing the actual experiment (based on the pilot experiment)

• Collect the test tubes and place them in the racks, cut magnesium ribbon and collect the acid and distilled water, measure out the following: -

• 20 ml of acid (2 moles of acid)
• 17.5 ml of acid and 2.5 ml of D. water (1.75 moles “)
• 15 ml of acid and 5 ml of D. water (1.5 moles “)
• 10 ml of acid and 10 ml of D. water (1.0 moles “)
• 5 ml of acid and 15 ml of D. water (0.5 moles “)

Prepare each 5 times and pour into test tubes

Use one rack per concentration.

• Once each is measured place a bung in the starting test tube and shake vigorously to thoroughly mix the solution
• Place one piece of magnesium ribbon into the solution and start the stop clock.
• Stop the clock once the solution has completely finished reacting (no bubbles are being produced) with the magnesium and record.
• Repeat this for all 25 test tubes
• Calculate the average for each set of results
• Plot this on a graph

NB. D. stands for distilled.

In order to ensure that this experiment is safe I will be doing the following: -

• Any bags must be placed securely under the desk.
• The experiment must be performed in the centre of the table.
• I will only add acid to water.
• We will be wearing safety goggles at all times.
• Towels and running water will be nearby in case of any acid spills.
• Ms Pinion will be supervising all of the experiments.

Below shows a diagram whilst the experiment is being performed: -

Results

Analysis

Looking at the graph I can see that as the concentration increases the time taken for the reaction to occur decreases therefore the rate of reaction increases. As I said in my scientific knowledge this occurs because as the concentration increases obviously the number of particles within the solution also increases. With more particles the chances that the HCl molecules will collide with the magnesium molecules and have enough activation energy increases and so the rate of reaction will increase.

You could also read the graph another way, as the time taken for the reaction to occur increases the concentration decreases. The reason behind this is that with fewer molecules of HCl in a weaker acid the fewer the chances that the particles will collide with the magnesium molecules and so the rate of reaction decreases.

Between the concentrations 0.5 moles and 1.0 mole the line was quite steep; there was a rapid decrease in the time taken for the reaction to complete. A possible reason for this could be that 1.0 moles is double 0.5 moles and so because there is double the amount of molecules theoretically the time taken at 1 mole should be half of that taken at 0.5 moles.

My prediction was correct because as the concentration increased the time taken for the reaction to occur decreased therefore resulting in a higher rate of reaction.

Evaluation

Looking at my results I think that they are quite accurate, the graph is a smooth curve joining 4 out of the 5 points, I received one anomalous result. This was for 1.5 moles, the result I got was 48 seconds when it should have been 32 seconds. A possible reason for the anomaly could be from the following:-

• Too little acid was placed in the solution and so the concentration could have been less than originally expected.
• Human reaction time is always delayed and so the reaction may have stopped seconds earlier.
• The equipment I used may not have been accurate enough, for example I used a measuring cylinder when I could have used a burette to measure the acid and the water.
• I judged whether the reaction had finished by looking when it had stopped bubbling; obviously this can be difficult to keep accurate as it is not always easy to tell, in a slow reaction for example bubbles can be produced very slowly, if there is a large gap in time when no bubbles are produced I could assume that the reaction had stopped when it hadn’t making the reaction unfair. To over come this, instead I could measure the amount of hydrogen produced in a certain amount of time; to do this I would need a gas syringe.  

I believe that I repeated my results a suitable amount of times to get a reliable average and I think that my range of results was enough to complete the investigation with suitable proof. Yet based on the information above I could say that the data may not be sufficiently accurate to support a conclusion, despite the fact that my results were as I had expected the question arises that if my results had have been more precise would my results have been different.

My result didn’t follow a specific pattern but they did decrease as I increased the concentration. If I were to develop this investigation further I would almost certainly repeat the experiment taking into account the information above (possible reasons for the anomalie), by doing this I will be able to establish the whether or not the investigation that I performed this time was inaccurate.  If I were to explore other areas of this topic I would look at different metals and how they affect the rat of a reaction, different acids and see what effect they have and I would possibly look into one of the aspects of my scientific knowledge.

END OF INVESTIGATION