My experiment will be conducted to find out at what concentration does sodium thiosulphate with hydrochloric acid have the most reaction and how the concentration generally affects the rate of reaction.

I had to think about factors which would impact on our experiment and then find out if they impact our results, then I had to neutralise these and keep recordings and monitor them during the investigation.

• I found out that the temperature affected our rate of reaction, (increasingly) this altered my results therefore I monitored the room temperature throughout the final investigation.  You can make particles move more quickly by making the temperature rise because heat energy turns into kinetic energy. The faster the particles are going, the more energy they have. Fast moving particles are more likely to react when they collide.
• Also swirling the liquid increases the rate of reaction therefore it would make the results inaccurate, it does this mean it moves the particles giving them more kinetic energy so they can bump into each other.
• The concentration of hydrochloric acid was kept at 2 mols because this would neutralise the effect quicker if it was too strong changing my results.

 On our initial test I thought I would lack time therefore I chose to do only four concentrations with ten tries these are 100%,75%, 50% and 25%. After I had completed this task I still had some excess time thus, I studied our results. Shortly after viewing our results I realised there was an unusual, big difference between the 50% concentration and the 25% concentration. I wanted to investigate if this was a mistake that has occurred or if this was a pattern. I thought this because the 100% concentration and 75% concentration biggest difference was only 13.62 seconds on the other hand the biggest difference between 50% concentration and 25% concentration was 122.06 seconds. I then created a new concentration of 40% to see if the difference would be majorly different to 50%.

When I we carried out the 25% concentration, it took too long to react therefore we decided to set up two concentrations at the same time. When we started this the roles doubles so the person who was timing was busy washing up so pressing the ‘stop’ button was delayed resulting in outliers in our results. We did most of the outliers again but some were outliers again then we ran out of time so we couldn’t do it again.

Then we dedicated each member to a task, one member had to measure the water and the THIO. The other had to measure HCL and the final member had to wash up and controlled the stop watch. We did this because, the judgement of X’s disappearance varied across the group so we dedicated the most accurate member to complete this task.

Every time we washed the beaker there was water left in it. This could have diluted the THIO changing the accuracy of the results therefore we set up a system where we dried the beakers before using them for the final investigation.

From the initial method results we can tell the more concentrated the THIO the faster the rate of reaction. This matches our prediction therefore seems to be accurate.

The changes we made from the initial experience:

• Keeping an eye on the temperature
• introducing a new concentration
• Decreasing the number of tries from 10 to 8. ( or else we would have time to replace outliers if we had any)
• dedicated roles to each member

then dried the beakers

 The Apparatus

 

We used measuring cylinder to measure water, we used a 50ml one because we needed a maximum amount of water, of 36ml. It’s easier to measure it with a smaller measuring cylinder because the scale is more detailed giving us precision in the amount we use.

We used two 100ml capacity beakers to carry out our experiment because we would have mad 80ml maximum of solution in a beaker. Also the smaller the beaker the easier it is to see the ‘X’ sign if the beaker was bigger the ‘X’ becomes harder to see resulting in the member pressing the ‘Stop’ button on the stop watch later which would impact our result’s precision.

A Pasteur pipette was kept to measure the smaller amounts of water and THIO where 25ml and 36ml are necessary to be accurate.  It is easier to measure 6ml in a Pasteur pipette therefore we measure 6ml with that, and 30ml with the measuring cylinder, this is a more efficient use of time.

A stop watch kept to measure the time because it says the time and has milliseconds. The digits stop on screen so the member who watches the time will not miss the correct integers.

Collecting Data

From the initial method we found out the factors that affected our results were temperature and excess water from the washed up beaker. For these reasons we monitored the room’s temperature which was 18.5 degrees Celsius while we were doing 100%, 75% and 50% concentrations, when we began to do 40% the temperature increased by 0.5 Celsius  possibly affecting the rate of reaction,  making our results less accurate.

From initial experiment we had results to compare to the results we were having. These results were similar for 100 % they stayed within the 32 range, but then we realised when we had an outlier on try number seven because we had results to compare to, we did try seven again however it still gave us an outlier so we had to exclude it from our results.

We decided to use 8 tries instead of ten because we would rather have accurate results rather than more results. This also enabled us to watch out for outliers and to redo them.

The variables we watched:

• The same person timed each time so then different judgements wouldn’t affect the results.
• We used the same amount of HCL each time so then the sodium thiosulphate had the same amount of HCL atoms to react with.
• We dried the left over water in the beaker so there wasn’t any excess water to slow down the reaction so it wouldn’t affect our results.

Interpreting Data

The biggest gradient is 3.2 for the 100% which tells me that the fastest rate of reaction and there is a trend:

My data shows that the more concentration there is the faster the rate of reaction. The more dilute the THIO the slower the reaction. The change of rate of reaction is biggest when the concentration is more concentrated, as the concentration goes down so does the difference of rate of reaction.  The graph shows a negative correlation time and concentration which also supports my theory. The percentage slightly varies but this could have been caused by different factors but this limits us from making a definite conclusion.

The explanation behind my findings would be the particles in the higher concentration will bump into each other more frequently because there are no obstacle water molecules to delay the collision.

There are more collisions in a period of time which means there are more successful collisions. Increasing the concentration increases the number of particles in a given volume of solution, and so makes successful collisions more likely. If you keep increasing the concentration eventually it stops, because if there are enough HCl particles to collide with every THIO one; adding more will have a small or no change will occur. This explains what occurs at 100%, therefore there isn’t a 100% difference between the 75% and 100% concentration. The trend shown is that as the input variable is decreased, concentration of sodium thiosulphate, the outcome variable increases; time taken for a reaction. 

Evaluating the data that you have collected

In total, including the outliers, we collected 40 results which were sufficient to come to a conclusion. Most of the results were accurate however 3 of them were outliers and were excluded from the average.

The results match my prediction and it has a negative correlation without any odd parts. There was no difficulty in drawing the line of best fit. This shows that my results are accurate.

I think the three outliers should be repeated although this isn’t particularly necessary because I had enough results to come to an average.

My results have small error bars which show most of my results are accurate, but the 25% and 50% concentration are much bigger showing that these results could be replaced with more accurate ones.

There were only minor issues with the results therefore I think it is sufficient to come to a conclusion.