Procedure
Diagram of Apparatus
Method
- Set up apparatus as shown above with the conical flask on top of the tile with the ‘X’ on it.
- Using the 10ml measuring cylinder, measure out 10ml of HCl.
- Using the 50ml measuring cylinder, measure out 50ml of Sodium Thiosulphate.
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Add the 50ml of Na2S203 to the conical flask, followed by the 10ml of HCl.
- When the HCl is added, start the stopwatch.
- Look down the neck of the conical flask, and when the ‘X’ is no longer visible, stop the stopwatch.
- In a table record the volumes in the solution and the time it took for the ‘X’ to disappear.
- Wash out the conical flask three times, until there is no solution remaining and it is clean.
- Replace the conical flask on top of the ‘X’ tile and add 10ml HCl to the measuring cylinder.
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To the 50ml measuring cylinder, add 10ml of water followed by 40ml of Na2S203.
- Add these to the conical flask and repeat steps 5 through 9.
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Now add 10ml less of Na2S203 to the 50ml and 10ml more of water. Add 10ml of HCl to the 10ml measuring cylinder and add these to the conical flask.
- Repeat steps 5 through 9.
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Repeat steps 12 and 13 until you have readings which include a solution which contains 10ml Na2S203 and 40ml of water reacting with 10ml HCl.
- Repeat the entire experiment a second time.
- Find the average time it takes for the ‘X’ to be obscured in each case by adding the two times and dividing by two. Rate is calculated by finding the inverse of this time.
Safety Precautions
Wear goggles to stop acid getting into eyes.
Wash hands immediately after contact with substances.
Pour solution down fume cupboard sink to prevent sulphur escaping into the atmosphere as it is poisonous.
Plan of Table
Below is a plan of the table I will record my results in.
Range of Values
I will always use 10cm3 of HCl in each reaction and volumes of Na2S203 from 50cm3 to 10 cm3 and volumes of water from 0cm3 to 40cm3.
Explanation
As we are mixing two solutions, the substances are dissolved therefore giving ions. There a number of different ions in solution and these can be divided into two categories; spectator ions and ions which cause and take part in the reaction. Here are the equations of the reaction including the ionic equation of the ions that take part in the reaction:
Na2S203 (aq) + 2 HCl (aq) 2NaCl (aq) + H2O (l) + SO2 (g) + S (s)
2Na + (aq)+ S2032- (aq) + 2H+ (aq) + 2Cl+ (aq) 2Na + (aq) + 2Cl+ (aq) + H2O (l) + SO2 (g) + S (s) + SO2 (g) + S (s)
S2O32- (aq) + 2H + (aq) H20 (l) + SO2 (g) + S (s)
From this we can see that the S2O32- (thiosulphate) and 2H + (aq) (hydrogen) ions are the ones which take part in the actual reaction. We can draw from this that the sodium and chloride ions are the spectator ions, the play no part in the reaction and remain in solution.
Why rate of reaction will increase as the concentration of the sodium thiosulphate increases can be explained by the collision theory. The collision theory states that for a reaction to take place between reacting particles (in this case ions) it is necessary for them to collide. Furthermore they must have sufficient energy to react otherwise they will simply bounce off each other. The minimum amount of energy that reacting particles must have to react is called the activation energy; this is the amount required to break the bonds and form new products.
In my diagram below, I have illustrated that at low concentrations of sodium thiosulphate, there are relatively few thiosulphate ions to react with the hydrogen ions provided by the hydrochloric acid. Therefore not many collisions will take place between them in a set time and as only a few ions will have the required activation energy to react, the rate of reaction is slow. As the concentration is increased, there are more S2O32- ions in solution to collide with the H+ ions so the rate of reaction will increase as there are more successful reactions in a set amount of time.
Diagram showing effect of Concentration on Rate of Reaction
Accuracy of Measurements
I decided to use a 10ml measuring cylinder to measure out the hydrochloric acid as I needed to have the most precise amount of it possible to keep it a fair test. The small measuring cylinder allows for very accurate readings. The 50ml measuring cylinder was used as it too was a very easy and accurate way to measure out the larger volumes required for the sodium thiosulphate solutions. I will measure to the same point each time in each test tube, i.e. exactly to the line meaning I will always have precise measurements. The stopwatch allowed us to make time reading accurate to a second which will be more than accurate enough for our investigations. They have all been calibrated to be very accurate.
Improving Reliability
By repeating the experiment I will get a more reliable result as I can find the averages of the time for the ‘X’ to disappear at the different concentrations. This reduces the effect of small anomalies in the results.
Calculating Concentration of Sodium Thiosulphate
Concentration is the amount of a solute dissolved in a certain volume of a solvent. The amount of solute is usually measured in moles and the volume in dm3 so the concentration of a solution is measured in mol dm-3. To find the concentration of sodium thiosulphate in each solution I must use the equation
Moles = Concentration (mol dm-3) x volume (dm3)
I first find the number of moles in each volume of sodium thiosulphate as shown in the table:
Then I must find the concentration of sodium thiosulphate in the entire solution of it, water and hydrochloric acid which will always be 60ml (0.06 dm3) . By rearranging the above formula I can find the concentration:
Concentration (mol dm-3) = Moles ÷ Volume (dm3)
For example, to find the concentration of 30cm3 Na2S203 in 60cm3 solution (mol dm-3), we take the number of moles of Na2S203 in 30cm3 and divide this by the total volume (0.06dm3).
0.0048 ÷0.06 = 0.08, therefore in 60cm3 of solution, Na2S203 is at a concentration of 0.08 mol dm-3.
It is clear that that the higher the volume of Na2S203, the concentration of Na2S203 is higher. They are directly proportional as the concentration of Na2S203 at 20cm3 is half that of 40cm3 of Na2S203 in 60cm3 of solution.
Time and Rate
When the time taken increases, the rate is slower, as rate of reaction is how fast a reaction occurs. The rate will be inversely proportional to the time taken; as the time increases the rate of reaction decreases:
Rate of Reaction α 1/time
Results
Table of Results
Table showing Time and Rate
As is shown in the table above, when the time increases the rate increases so therefore they are inversely proportional. To prove this I will take two sets of results. When the time taken is 30.5 seconds the rate is 0.0328. When the 94 seconds the rate is 0.0106.
30.5 x 3 = 91.5
0.0106 x 3 = 0.0318
It is clear that the rate is a third when the time is a three times bigger. Therefore they are inversely proportional.
Interpreting Results
As the concentration decreases the rate decreases also, showing that they are proportional to each other. I will now plot my results in a graph so as to evaluate them. On the x-axis I will plot the concentration of sodium thiosulphate in solution and on the y-axis the rate at which the reaction occurred.
Because the numbers I am working with are so small, I will multiply them to give numbers which will be easier to plot and read from a graph. Below is a table of the figures I will plot. The general shape of the graph is changed in no way when I do this.
Conclusion
From the graph I am able to draw the conclusion that rate is directly proportional to concentration of Na2S203 in solution. I can prove this by looking at two examples from the graph and comparing them. When the concentration of Na2S203 is 0.05 mol dm-3 the rate of reaction is 0.0115. When the concentration of Na2S203 is 0.10 mol dm-3 the rate of reaction is 0.0228.
0.10 is twice 0.05
0.028 is twice 0.0115
Therefore rate is directly proportional to the concentration of sodium thiosulphate.
Explanation
Why rate of reaction will increase as the concentration of the sodium thiosulphate increases can be explained by the collision theory. The collision theory states that for a reaction to take place between reacting particles (in this case ions) it is necessary for them to collide. Furthermore they must have sufficient energy to react otherwise they will simply bounce off each other. The minimum amount of energy that reacting particles must have to react is called the activation energy; this is the amount required to break the bonds and form new products.
In my diagram below, I have illustrated that at low concentrations of sodium thiosulphate, there are relatively few thiosulphate ions to react with the hydrogen ions provided by the hydrochloric acid. Therefore not many collisions will take place between them in a set time and as only a few ions will have the required activation energy to react, the rate of reaction is slow. As the concentration is increased, there are more S2O32- ions in solution to collide with the H+ ions so the rate of reaction will increase as there are more successful reactions in a set amount of time.
Diagram showing effect of Concentration on Rate of Reaction
In my prediction I stated that I believed that as the concentration of sodium thiosulphate decreases, the time taken for the ‘X’ to disappear will increase proportionally. Thus they will be inversely proportional to each other. My results have proven that this is the case.
Critical Evaluation
I was able to plot a linear graph where all my points were on or very close to the line of best fit which suggests my method was relatively good. There are ways to improve it however. The human eye is not always the most accurate so a better way to test whether a certain amount of sulphur had been produced instead of the human eye waiting for an ‘X’ to be obscured would be to use a light sensor which would record a point at which no or very little light was getting through the solution. The benefit of this is the timer will stop at a definite point every time instead of me stopping it. Apart from the eye not being totally accurate, my reaction would be slower than that of a computer. Unfortunately, using light sensors is impractical as they are expensive and would take far too long to set up and use. I could also use a pipette to measure out the HCl which may give more accurate measurements but I think the 10ml measuring cylinder was very accurate anyway. I could also have used a burette for the Na2S203 and water solution but this is quite impractical for the amount of reactions I required to take results of. To further improve accuracy, using a new conical flask every time would help, but again this is completely impractical as it would require far too many for what a school can provide.
Diagram of Possible Apparatus
I would then have been presented with a graph such as the one shown below, which would have shown how quickly the light became obscured. Notice it does not go all the way down to zero as there are other light sources in the room which the light sensor registers. The quicker there is a steep line, and the steeper it is, the higher the rate of reaction.
I had no conspicuous anomalies suggesting my results were very accurate and reliable.