Diagram
Method
- Put a 250 ml conical flask on a magnetic stirrer.
- Insert the open end of a delivery tube into a one-hole plastic stopper and plug the mouth of the conical flask with the plastic stopper.
- Attach the other end of the delivery tube to a gas syringe and clamp it horizontally, making sure it doesn’t oppose gravity. (Set apparatus should look like the diagram above)
- Using an electronic balance, weigh out 0.2g of zinc dust in a piece of paper.
- Place the 0.2g of zinc dust in the conical flask.
- Using a 100 ml measuring cylinder, measure out 20 ml of 1M hydrochloric acid.
- Pour the 20 ml of hydrochloric acid into the conical flask with zinc dust and re-insert the stopper immediately.
- Turn on the magnetic stirrer to the maximum value and simultaneously start a stopwatch.
- Record the volume of hydrogen gas collected in the gas syringe every 30 seconds until the reaction is complete- end to effervescence and volume stops increasing.
- Record all observations.
- Repeat steps 1-11 for 2M and 3M hydrochloric acid.
- To keep all controlled variables constant and/or to minimize their effects on the experiment, make sure the room temperature, mass of zinc dust used for each trial, temperature of the hydrochloric acid stays the same throughout the entire experiment. Also, use the 100 mL measuring cylinders with precision to ensure that the volume of hydrochloric acid used each time is constant for all three concentrations.
Data Collection and Processing (DCP)
Results Volume of H2 collected every 30 seconds of the reaction
Observations
As the reaction proceeded, there was effervescence as hydrogen gas was being produced. The lowest concentration was 1M, and thus a minimal amount of effervescence occurred at the concentration. The reaction at 1M hydrochloric acid was also very slow to start. The rate by which hydrogen gas was produced and rapidity of effervescence had increased by a lot at 2M of hydrochloric acid.
As hydrogen gas was produced, it was being collected in the gas syringe. This could be distinguished by the slow yet noticeable movement of the piston being pushed outwards.
Since the hydrochloric acid was in excess, the zinc dust gradually disappeared as it reacted with the acid to give a colourless solution of zinc chloride mixed with remaining acid.
The zinc in the flask disappeared quickest at 3M hydrochloric acid.
The Reaction
Zn(s) + 2 HCl → ZnCl2(aq) + H2(g)
Data Processing
Gradients of all three curves in the graph attached represents the reaction rates of each. To determine the reaction rate:
Reaction rate= Change in volume
Change in time
Examples of how the reaction rate was determined are shown on the graph attached.
Uncertainty= ±0.5
Reaction rate of 1M hydrochloric acid= 2.0ml min-1
Reaction rate of 2M hydrochloric acid= 5.0ml min-1
Reaction rate of 3M hydrochloric acid= 7.5ml min-1
How the average was calculated:
Value of Trial 1 + Value of Trial 2 = Average value
Number of trials
Example:
To calculate the average value for the 30th second of the reaction process-
8.0 + 9.0 = 8.5
2
Conclusion and Evaluation (CE)
Conclusion
In conclusion to the graph and the experimental results, it is safe to say that my stated hypothesis is correct. The graph attached suggests that as the concentration of the acid is increased, the rate of reaction increases with it until an increase in the acid no longer effects the reaction rate. The gradient of each curve gives the reaction rate of each concentration- the steeper the gradient, the faster the rate. Clearly, the steepest curve is the curve for 3M hydrochloric acid and this again supports my hypothesis. Curves 1 and 2 illustrate the effect of concentration on 1M and 2M hydrochloric acid respectively. Curve 1 has the slowest rise thus with the smallest gradient. This insists that the lowest concentration has the least effect on the reaction rate of zinc.
The collision theory suggests that for a reaction to occur between two particles, the three following conditions must be met: the particles must collide; they must collide with the appropriate geometry and orientation so that the reactive parts of the particles come into contact with one another and that they must collide with sufficient energy to bring about the reaction. The same argument applies whether concentration involves more collision, fastening the reaction rate, between two different particles. In order for any reaction to happen, those particles must first collide. This is true whether both particles are in solution, or whether one is in solution and the other a solid. If the concentration is higher, the chances of collision are greater. An example of this is the experiment undergone.
With reference to the results from this experiment, it is understandable that the collision theory is correct in practice and my hypothesis is correct. Both my graph and my experimental data explain the effect of increasing concentration on the reaction rates of zinc at constant mass.
Evaluation
This experiment consisted of errors from many different sources. It is clear that the experiment proved my point however; the results obtained were merely as accurate as expected. The presence of outliers in the set of raw data hints towards the unlimited sources of errors in the experiment. The maximum volume of gas collected in this experiment was merely 19 ml, whereas if calculated with the aid of stoichiometry, it is evident that the final amounts of gas collected should have been a lot higher in volume.
Sources of error: The small volumes of gas collected mean that our raw data involved high percentages uncertainties, in turns making the results of this experiment quite unreliable. The apparatus used may not have been of such accuracy as to which it would provide us with exact and precise values. One main problem encountered in the experiment turned out to be the use of the gas syringe. The gas syringe was initially introduced to the method to help collect and measure the amount of hydrogen gas produced. When hydrochloric acid was first introduced to the zinc dust in the conical flask, effervescence was visible. However, it was not until a minute or so after until the syringe piston had started moving. There may be several reasons behind this, such as; the apparatus used may not have been completely leak-proof and airtight. Another reason behind this may have been because the syringe piston was not lubricated sufficiently, so that a certain volume of gas had to be collected before the piston was pressured to move outwards. The scale of the syringe was not very precise and so the data obtained accordingly is not very precise and accurate.
An electronic balance was used to measure out the zinc dust prior to the experiment. It is possible that the improper calibration or fluctuation of the scale disabled us from having a constant mass of zinc dust for all trials. To measure out the hydrochloric acid, a 100 ml measuring cylinder was used. It is again possible that the scale of the measuring cylinder was not accurate enough. However, a significant amount of human error is also involved here. It is vital to note the reading to taken according to the position of the meniscus. Thus an error whilst taking the reading may be caused the volume of hydrochloric acid used throughout the experiment to not remain constant.
Due to time limitations, trials could only be undergone twice using all three concentrations. Though two trials for all three concentrations is good enough to prove the point, using higher and lower concentrations of acid and repeating trials for each would provide better experimental data to prove the hypothesis correct.
Improvements
One of the very first actions that I would take in order to improve the experiment is modify the design of the experiment. Clearly, the use of the gas syringe was not the best possible way to measure the amount of gas collected. An easier way to investigate the effect of concentration on reaction rates would be by weighing the conical flask whilst the reaction was progress, in order to determine the decrease in mass. For this method to be successful, it would be vital to ensure that the mouth of the conical flask is open. Since this reaction is an exothermic, it would give hydrogen off to the surrounding, in turns decreasing the overall mass of the final product. This method involves fewer sources of errors, and the experimental data would accordingly make more sense.
Since the hard-to-move syringe piston was a reason to the inaccurate data, it would be ideal to make sure that the inside of the syringe is lubricated sufficiently so that no extra force is needed to move it outwards. Another way to minimize errors is by ensuring that the syringe, delivery tube, plastic stopper and the conical flask is completely airtight and leak-proof. This will stop the dependent variable, hydrogen, from escaping in turns giving us a more reliable and precise set of raw data. The electronic balance would also be tested prior to the experiment to ensure that it is providing accurate reading, without any doubting fluctuations.
All in all, for all methods of measurement it would be best if the experiment could be carried out more than once with more than at least 6 concentrations. This would provide us with a wide enough range of values with which to plot an accurate graph to compare reaction rates and to test the hypothesis with.