The Effect of Hydrochloric Acid on the Rate of Reaction

The Effect of Hydrochloric Acid on the Rate of Reaction
Sodium thiosulphate + hydrochloric acid â??Â? sodium chloride + sulphur + water + sulphur dioxide Na S O + 2HCl â??Â? 2NaCl + F + H2o + SO2 Preliminary Investigation: Before we carried out our investigation in full, we needed to do some preliminary work to prove that the variables we were changing were suitable, the concentrations of hydrochloric acid would not take too long or short a time and to discover what range of values would be needed on apparatus such as the conical flask. The results of our preliminary work were as follows: Concentration (%) Time taken for cross to disappear (secs) 80 121 50 133 20 153 We also realised that we needed to use 25 ml of each sodium thiosulphate and hydrochloric acid to make 50 ml. This seemed like an appropriate amount to work with as it would offer significant results without being over-large. Although we were only using 50 ml of chemicals, we decided to use a 100ml conical flask. This allowed room for reactions to take place without it spilling over.
To change the volume of our eventual solution, we would use a measuring cylinder to measure the appropriate volumes of each, and we would not re-use the solutions as once they had been reacted they could not be re-reacted. The thermometer we used ranged from -10 to 110 degrees Celsius. Although if this experiment is done correctly the temperature should not change, we must make allowances for if it does, and that entails allowing for negative temperatures. We chose a thermometer that was accurate to 1 degree Celsius as we needed only that level of accuracy, no more. We used a 25ml measuring cylinder as that was the maximum amount we would be measuring out each time. It was accurate to 0.5 ml. We are intending to repeat this experiment several times in order to gain accurate, conclusive results. Proving it is a suitable variable: The preliminary investigation proves that the reactants are going to fully react in a reasonable timescale, which in this case is 129 seconds. Another factor showing that these concentrations are suitable is the temperature. The temperature is not meant to change and it doesnÂ?t in the experiment so it is a success. To prove that the concentration of the acid is a suitable variable to change we have to measure the time for the strongest and weakest acids to prove that the graduation in time would not be too large. We have also decided to use the hydrochloric acid rather than the sodium thiosulphate because the hydrochloric acid will react more quickly and therefore give us our results in a smaller time. Apparatus: Goggles Sodium Thiosulphate (25ml) Hydrochloric acid (25ml) 100ml conical flask (20ml intervals) Thermometer (-10 to 110 degrees Celsius accurate to 1degree Celsius) Stop-clock accurate to 100th of a sec 25ml measuring cylinder accurate to 0.5 ml Method:
Draw a cross in pencil on plain, white paper.
Measure out the reactants (Hydrochloric acid and Sodium
thiosulphate)
Pour sodium thiosulphate into conical flask
Add the hydrochloric acid
Start the stop-clock as soon as the first drop of acid touches the
sodium thiosulphate
Take the temperature
Stop the stop-clock when you cannot see the cross any more.
Re-take the temperature.
Repeat with the other concentrations of acid
A table to show the concentrations of hydrochloric acid we will be using HCl (ml) Water (ml) Percent (%) 5 20 20 10 15 40 15 10 60 20 5 80 25 0 100
We will repeat the experiment three times for each of the varying
concentrations of acid shown above.
Fair Test: The variable we are changing is the concentration of the hydrochloric acid. To make it a fair test we have to keep all the other variable constant throughout. The variables we are keeping the same are;
The overall volume of the two reactants Â? this must be kept the
same so there is the same number of overall particles in each
solution as a larger number of particles could affect the rate of
reaction.
The temperature Â? the temperature must be kept the same because if
it is changed the rate of reaction could be increased therefore
affecting the experiment.
The concentration of sodium thiosulphate Â? this must be kept the
same to ensure a fair test.
Prediction: I predict that as the concentration of hydrochloric acid increases, the rate of reaction will also increase. I think this because, according to the collision theory, as the concentration increases there is a bigger ratio of hydrochloric acid particles to the other particles within that substance (for example, water) and so there is a bigger chance that the reactant particles (hydrochloric acid and sodium thiosulphate) will collide and react. For any reaction to occur, the reactant particles must collide with at least the base activation energy. If they collide but do not have this amount of energy, they will simply Â?bounceÂ? off. As we increase the concentration of the hydrochloric acid the cross will disappear more quickly as the precipitate (sulphur) is being made more quickly. This is the substance that makes the solution become cloudy. I also predict that the time taken for the cross to disappear will decrease by approximately 15 every time the concentration increases by 20%. Safety:
During this experiment, you should wear goggles throughout because
some of the materials used could cause irritation.
HCl is the material that could cause irritation, so be careful
when handling and if any is split, clear it up immediately.
Concentration of hydrochloric acid (%) Temp. (ËšC) before Temp. (ËšC) after Time taken for X to disappear (secs) Average time (secs) 20 20 20 153 153.66Â? 20 20 20 155 20 20 20 153 40 20 20 145 146.00 40 20 20 146 40 20 20 147 60 20 20 128 128.66Â? 60 20 20 130 60 20 20 128 80 20 20 118 118.66Â? 80 20 20 120 80 20 20 122 100 20 20 98 99.33Â? 100 20 20 98 100 20 20 102 Conclusion: As the concentration of hydrochloric acid increases, the time taken for the cross to disappear decreases. This happened because when the concentration of hydrochloric acid was increased, there were more hydrochloric acid particles although there was the same volume of liquid altogether. This means that there is more chance that the two reactant particles (hydrochloric acid and sodium thiosulphate) will collide with each other, causing a reaction. To react, the particles must collide with the Â?activation energyÂ? which is the minimum amount of energy the particles need to react, when they collide. As we increase the concentration of the hydrochloric acid the cross will disappear more quickly as the precipitate (sulphur) is being made more quickly. This is the substance that makes the solution become cloudy. Because of all these things, the rate of reaction should increase as the concentration increases. We know this is correct when we look at our results. For example; at the concentration of 20% hydrochloric acid to 80% water the average time taken for the cross to disappear is 153.66Â? seconds. We worked out the rate of reaction for this and found it to be 0.0065 using the formula rate of reaction = 1/time. At 80% hydrochloric acid and 20% water the average time taken for the cross to disappear is 118.66Â?. We worked out the rate of reaction for this and found it to be 0.0084 using the formula rate of reaction = 1/time. This shows a marked increase in the rate of reaction, we which calculated to be 0.0019. This shows that my prediction was correct and the rate of reaction did in fact increase as the concentration of hydrochloric acid increased. As the concentrations increase, there is a definite but steady decrease in the time taken for the cross to disappear. The time decreases by an average of 13.583333Â? each time the concentration increases by 20%. This is very close to my prediction, which was that the time taken for the cross to disappear would decrease by 15 seconds every time the concentration increases by 20%. This matched my prediction because I did fairly extensive preliminary work which gave me an accurate indication of what to expect from the full investigation. Evaluation: My experiment was reliable because I took every reading three times which lessened the likelihood of a misleading anomaly. We recorded the temperature before and after each experiment and it stayed constantly at 20Ëšc throughout which also proves it was reliable. We only changed one variable (the concentration of the hydrochloric acid) which made it a fair test and also reliable as we took care to ensure that the variables we didnÂ?t change were constant. The table below shows how far apart the results in each concentration are; Concentration (%) Time taken for X to disappear (secs) Range between times (secs) 20 153 2 20 155 20 153 40 145 2 40 146 40 147 60 128 2 60 130 60 128 80 118 4 80 120 80 122 100 98 4 100 98 100 102 All these results are fairly close together (no more than 4 seconds apart) which proves that my experiment was very reliable and followed the pattern expected of them. We found no anomalies in out results which also indicates accuracy. The results we got from this experiment followed a pattern of time decreasion as the concentration increased. This was a correct pattern and what we expected. Our results, when plotted on a graph, followed a straight line of best fit with negative correlation. To improve the reliability of my investigation I could repeat each reading 5 times instead of 3. This would virtually eradicate any chance of getting all wrong readings. I could also go up in 10% inclinations of the concentration of hydrochloric acid rather than 20% inclinations. This would give us more readings, again reducing the chance of getting several wrong results in a row and believing them to be correct. With these extra results we could also plot a more detailed graph which would then in turn give a more accurate line of best fit. In the table below are the concentrations I would do if I was to re-do this investigation. Concentration of hydrochloric acid (%) Volume of hydrochloric acid (ml) Volume of water (ml) Time taken for cross to disappear (secs) 10 2.5 22.5 My results would go here. 20 5 20 30 7.5 17.5 40 10 15 50 12.5 12.5 60 15 10 70 17.5 7.5 80 20 5 90 22.5 2.5 100 25 0 My investigation was accurate because I measured all of the volumes of the hydrochloric acid and sodium thiosulphate very carefully. I also used as clean as apparatus as I could find and I cleaned it before I used it. To maintain accuracy, I used a thermometer to 1Ëšc. To improve the accuracy of my experiment I could use a digital thermometer, or even better a data-logger with a temperature probe. This would automatically take the temperature every 10 seconds and is accurate to 0.1Ëšc. Another way I tried to be as accurate as possible was I got the same person out of me and the person I was working with to see if the cross had disappeared each time. I did this because one personÂ?s perception could be different to anotherÂ?s, as one could have better eyesight than the other. To improve upon the accuracy of this I could use a light sensor on a data-logger underneath the conical flask. This would automatically stop the timer when it could not sense any light getting through the solution. To be accurate, I used a measuring cylinder accurate to 0.5 ml. To improve upon this I could use a papet which is more accurate. Summary: At the end of this investigation and taking into account all the evidence I have gathered, I believe I can draw a firm conclusion. This is; as the concentration of the hydrochloric acid increases the time taken for the cross to disappear decreased at a steady rate. This concurs with my prediction. I could have improved the accuracy of my investigation as stated previously but I trust that I can draw a strong conclusion as my results only varied by a maximum of 4 seconds which is very accurate and they followed the pattern expected. I did not have any anomalous results which was also a good sign that they were accurate. The temperatures did not change either, which indicated accuracy. To maintain accuracy, we scrupulously didnÂ?t alter any other variables as much as we could. Overall, our investigation was a success with good, reliable results being the outcome and a clear, definite conclusion.

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