Rates of Reaction

 

GRAPH PREDICTION

The reaction I am investing can be measured by:

• Rate of disappearance of Mg
• Rate of disappearance of HCl
• Rate of appearance of MgCl2
• Rate of appearance of H2 

I will measure the rate of appearance of H2.

I know from previous experiments the rate of reaction changes with time. When Dilute Hydrochloric acid reacts with Sodium Thiosulphate, as time increases the rate decreases and when the concentration increases the rate increases. The rate is fastest at the start of a reaction as the graph here is steepest. This is due to the fact that both reactants are at their highest concentration at the beginning of the reaction. Generally the rate decreases and finally comes to a stop, because the concentrations of the reactants start to decrease. The following is a sketch of what I think the graph will look like (i) Concentration v. time taken for 25 cm³ of H2to be collected. (ii) Concentration v. rate.

(i)

(ii)

PRELIMINARY WORK

I have decided to carry out some preliminary work. I will place a single piece of Magnesium Ribbon (5cm Long, 0.3cm wide) in Hydrochloric solutions. The first solution will be 50cm of 2Molar Hydrochloric solution (Strongest solution). The next solution will be a 50cm of 0.2Molar Hydrochloric solution (Weakest solution). I will time how long it takes for 25cm of H2 to be collected.

By carrying out this experiment I have gained some useful information. I have now got an idea of the range of results I will gain.

        I firstly used a Gas Burette to collect the H2, after the first experiment with the strongest solution; I became aware that it is very difficult to attain accurate results by using this piece of equipment. I have decided to use a measuring cylinder in the experiment instead.

        I was initially going to investigate how much time it would take for 25cm3 of H2 to be collected when the solution in which the Magnesium was to be submerged is 0.2 Molar, after doing the preliminary work, it has come to my knowledge that it is not vital to have this result and I would be wasting a lot of time if I were to carry it out 3 times, I have also decided not to experiment with the solution 0.4 Molar.

METHOD

EQUIPMENT

• Conical Flask
• Delivery Tube
• Trough
• Measuring Cylinder (50ml)
• Beaker 2 (250ml)
• Stand and Clamp
• Timer
• Thermometer

 

DIAGRAM

PROCEDURE

1. Set out apparatus (make sure the measuring cylinder is full of water, gas will be collected and this will push the water into the trough ‘Displacement of Water’) cut 10 pieces of Magnesium ribbon (5cm long and 0.3cm wide) ready for the first experiment.
2. Label two beakers one ‘HCL’ and the other ‘H2O’.
3. Measure 250 ml of each solution in two separate beakers.
4. Measure 50ml of HCL from the beaker into the measuring cylinder.
5. Put the solution from the measuring cylinder into the conical flask.
6. Drop the Magnesium ribbon into conical flask. As soon as the lid is on start the timer.
7. After 25 cm³ of H2 is collected stop the timer and record the time.
8. Repeat steps 4-7 but each time change the concentration of the HCL by adding water, as follows:

2 M        =        50ml HCl        :        0ml H2O

1.8 M        =        45ml HCl        :        5ml H2O

1.6 M        =        40ml HCl        :        10ml H2O

1.4 M        =        35ml HCl        :        15ml H2O

1.2 M        =        30ml HCl         :        20ml H2O

1 M        =        25ml HCl        :        25ml H2O

0.8 M        =        20ml HCl        :        30ml H2O

0.6M        =        15ml HCl        :        35ml H2O

      9.   The entire experiment will be repeated 3 times this will provide me with        accurate results, as the average* time for 25cm3 of H2 gas            

        collected for the various concentrations can be calculated.

        

FAIR TEST

I must make sure that I keep some factors in the experiments constant. If I don’t do this to the best of my ability I will ultimately have inaccurate results. The following are factors, which will have to be kept constant:

• The temperatures of the solutions must be kept constant as this affects the rate of reaction.

• The HCl and H2O are both measured in cm³.
• The HCl and H2O must be measured separately in case too much solution (more than 50 cm³) is poured into the measuring Cylinder.
• The magnesium ribbons must all have the same surface area (length 5cm, width 0.3cm).
• As soon as the Magnesium ribbon is dropped into the conical flask, the lid should be put on quickly and tightly. As soon as the lid is on start the timer.
• Make sure exactly 25cm³ of H2 has been collected and then stop the timer.
• *To find the average, we will calculate the sum of the 3 times for each

concentration, we will then divide this sum by 3. This is done so the results will be more reliable.

• I will record time in seconds and to one decimal space, for accuracy.
• I will repeat any results, which seem anomalous.

 

SAFETY PRECAUTIONS

I must give a lot of consideration to the safety aspects of this investigation for the sake of fellow students and myself:

• As I will be handling HCl, which is an irritant, I must have eye protection and when handling the more concentrated solutions should have some kind of skin protection. I must generally be careful when handling the acid.
• As I will be using a lot of glass apparatus which is sharp when broken I must be aware of this during the experiment and when putting all the apparatus back.
• I must take care when it comes to the disposal of chemicals.

RESULTS

The following table contains the time taken to collect 25cm³ for all three experiments and the average time for each concentration has been calculated.

        After observing the results I gained from the experiment, I can see some results, which seem anomalous, they will affect the shape of the curve and thus I will not be able to make accurate observations. The figures in the table above, which have been highlighted red are the ones which I don’t think are correct. I will repeat the experiments for each concentration and record the time taken for 25cm³ of H2 to be collected.

 This result seems to be more accurate.

After redoing this experiment I have found that the Time taken for 25cm³ of H2 to be collected is approximately the same as the 1st Experiment. Maybe the results for Experiment 2 and 3 were incorrect. I will redo the same experiment again for the third time to justify this result.

I now know for sure that the results gained from experiment 2 and 3 when concentration of HCL was 0.6M are incorrect. I will replace the incorrect figures in the initial table with the correct ones after redoing experiments. I will also change the averages accordingly.

 

I will display the results in a graph format; this will make it easier to see trends or patterns. I will initially plot a concentration versus time graph and after this will plot a concentration versus rate graph.

        To calculate the rate I will substitute the time taken for 25cm³ of H2 for each concentration into the following formula:

1

Time taken

        

The following table shows concentration, time take for 25cm³ H2 to be collected and rate (*100)

A graph to show Concentration v. Time

A graph to Show Concentration v. Rate

CONCLUSION

After observing the results, I have noticed that as the Concentration of HCl increases the rate of reaction also increases. Between each concentration the range for the time taken for 25cm³ of H2 to be collected increases.

 As I had to repeat some experiments because the results were anomalous, this provided me with more accurate readings. This helped because if they were not corrected I would ultimately have incorrect information, and would the conclusion would not be as accurate. The results gained by repeating some experiments seemed to fit in to the general pattern a lot better. The first experiment I had to redo was when the concentration of HCl was 0.8M; the result gained from experiment 2 did not fit in with the results gained from experiment 1 and 3. The ‘new’ results agreed with the general pattern for that specific concentration. I had to incorporate this in the final table of results. I also had to repeat experiment 1 for when the concentration of HCl was 0.6M; this result was extremely anomalous from the results gained in Experiment 2 and 3. After redoing this experiment I found that the ‘new’ results were very similar to the ones, which I initially thought to be incorrect. To make sure of this I had to repeat the experiment once more and this justified the result for the initial experiment 1 was correct. In actual fact the results gained from experiment 2 and 3 were extremely anomalous. Re-doing the experiments especially for when the concentration of HCl was 0.6M was very useful.

        The collision theory states that by doubling the concentration the chances of successful collisions are also doubled. Solutions, which have high concentrations and gasses of higher pressures, have more particles, which are closer together. Due to this the result is more ‘successful’ collisions, thus greater the rate of reaction.

The following is a table showing how doubling the concentration in this experiment had an affect on the rate of reaction:

After observing these results we are able to establish that the Rate of reaction does not actually double as the concentration doubles. However we are able to see that by doubling concentration the rate of reaction is approximately four to five times greater. My results do not comply with the prediction I have made.

        

EVALUATION

The results I gained from this experiment did not do any justification for the prediction. I believe that the method used to carry out the experiments was not efficient enough and there was a lot of potential for mistakes to be made. The following is what could have been partly to blame for the inaccurate results:

• Starting and stopping the stop clock may have been delayed.
• The Magnesium ribbon may have been exposed to water and so would have decreased the rate of reaction.
• The Magnesium may have not been cut accurately each time.
• The Concentrations of HCl may not have been measured out equally each time.
• The tubes where HCl was measured may have had some water in them thus making the solution more dilute.
• The conical flask was washed after every experiment and may not have been thoroughly dried.
• The measuring cylinders where the H2 was collected may not be accurate enough.
• Displacement of Water may not be efficient enough.
• There may have been inaccurate readings of when 25cm³ of H2 was collected.

If I was to improve the factors listed above I would have attained more accurate results. I also think the following factors have to be considered in order to make a proper conclusion:

• As the magnesium gives of H2 bubbles many of them stay on the surface of the magnesium thus decreasing the surface area of the magnesium and so decreasing the number of HCl ions reacting with the magnesium. This ultimately slows down the rate of reaction because there are fewer successful collisions.
• If we used larger concentrations and also larger volumes of concentrations we would be able to see a larger range of results, which would be easier to see with larger volumes.

• The reaction between Magnesium and HCl is an exothermic reaction. Heat is spread to the surroundings. Thus, raising the temperature of the solution, this adds to the rate of reaction.

The following are improvements I would make for any further investigation:

• Use Magnesium filings, as the impact of H2 molecules on the surface would not be as much as Magnesium ribbon.
• Take a lot more care when measuring all solutions.
• Making sure the containers are always dry on the inside.
• When measuring out concentrations I should double check whether the right amounts of HCL and water have been added together.
• I could use a gas syringe to collect the gas this would be more reliable than the displacement of water and their would be less faults in the timing of when 25cm³ of H2 is collected.
• I could use computers to construct graphs, which would give me more accurate best-fit lines, which I could read of.

To improve my investigation I can take other factors into account and observe how they would affect the rate of reaction, the following factors are worth considering:

• Surface area - If I were to carry out such experiment I would predict that as the surface area of the Magnesium increased, there would be a higher rate of ‘successful’ collisions, which would increase the rate of reaction.
• Temperature – I would predict that as the temperature increased the rate of reaction would increase. The higher temperature provides the particles with higher energy levels so more particles posses’ energy greater than the Activation Energy (Ea) so collide ‘successfully’.
• Use of a catalyst- a catalyst can alter the rate of reaction be reducing or increasing the Activation Energy (Ea) for a particular reaction. If a Catalyst, which reduces the Activation Energy (Ea) for particles in the reaction between Magnesium and Hydrochloric Acid. This would result in a higher proportion of ‘successful’ collisions. Thus increasing the rate of reaction.