The aim of this experiment is to see how reactions change when altering the temperature and concentration of the acid.

GCSE Science

Rates of Reactions

Rates Of Reactions

Aim

Factors Affecting Reaction Times

There are various things we need to take into account on what can affect reaction times. There are four main factors:

- Temperature

By heating up the mixture the particles gain more energy, so they will then move faster, making a faster reaction time.

- Concentration

When the mixture has a bigger concentration there will be more particles, which will mean a faster reaction.

- Particle Size

The bigger the size of the particles the slower the reaction, so to get a fast reaction you will need a lot of small particles rather than one big one.

Catalyst

A catalyst will speed up the reaction by acting as a template for the to substances, it will hold them together long enough for the substances to react without itself being involved in the reaction. It will speed up any reaction.

Apparatus

The apparatus we used was:

                        - Three Beakers
                        - Hydrochloric Acid
                        - Sodium Thiosulphate
                        - Bunsen burner

- Tripod

- Safety Mat

- Gauze

- Two Thermometers

- Water

Method

The first thing we need to do is to collect in the apparatus and set up the experiment. Once that is done we need to collect separately 25ml of Hydrochloric acid and 25ml of sodium Thiosulphate. Place the 25ml of acid into the beaker with the Sodium Thiosulphate, and place the beaker on top of the card with the cross on it. Start the stopwatch as soon as the two are mixed together. Have one person constantly looking into the mixture and when they can no longer see the cross, when the mixture has turned opaque you stop the stopwatch and record the time. Then collect another 25ml of acid

and Sodium Thiosulphate. This time change the concentration of the acid by adding 25ml of water. Then pour 25ml of the 0.5mole acid into a third beaker. Add the remaining 25ml to the Sodium Thiosulphate and start the stopwatch. Then repeat as before, have someone looking into the mixture until the cross on the card beneath is no longer visible, then stop the clock and record the time. Then collect another 25ml of Sodium Thiosulphate. Then half the concentration of the 0.5mole in the 3rd beaker, the one from before, by adding 25ml of water. Then pour 25ml of this 50ml, 0.25 moles in strength, into another beaker and then add that to the Sodium Thiosulphate and start the reaction again. Repeat the method of weakening the acid until all concentrations are done, the concentrations are:

1.0 mole
0.5 mole (1/2)
0.25 mole (1/4)
0.125 mole (1/8)
0.0625 mole (1/16)

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1.0 mole
0.5 mole (1/2)
0.25 mole (1/4)
0.125 mole (1/8)
0.0625 mole (1/16)

Once that has been fully finished. Repeat it fully four more times, but this time heat the acid before mixing with the Sodium Thiosulphate. We do this so we can see the affect it has on the rates of reaction. We will try to get the mixture up to the temps of 22˚C (already done), 26˚C, 30˚C, 35˚C, and 40˚C. But to get the mixture to be that temp we need to heat the acid to a certain temperature, we need to do this because the Sodium Thiosulphate is already at room temperature so we need to heat it further so when the two mix it will even out and hopefully match our target temps we set. The day we did the experiment the room temp remained at a constant temp of 22˚C. To get the mixture to a temp of 26˚C we will need to heat the acid to 30˚C before adding it to the Sodium Thiosulphate. To get a mixture temp of 30˚C we nee to heat the acid to 38˚C then add. To get a mixture temp of 35˚C we heat the acid to 48˚C, and then add. To get a mixture temp of 40˚C we heat the acid to 58˚C before adding.

When recording the results I will put them into a table like this. This one big table is a lot easier to look at to collect patterns than several small ones.

Preliminary experiment

Mr Edmead showed us a basic scaled down version of the experiment we will do. He set up five different beakers with the same amount of sulphuric acid in each one. He then added, with help, five different amounts of water and Iodine in each beaker at the same time. By doing this the concentration changed in each beaker. Due to the concentration being changed, all five reacted at visibly different speeds. We could see this because the iodine in the water turned the acid black once it had reacted. By doing this preliminary experiment I can tell that when we come to do ours concentration will affect the reaction time of the Sodium Thiosulphate mixed with the Hydrochloric acid, as it has affected this scaled down version.

Prediction

By doing a preliminary experiment I can now draw up my predictions. Looking at the preliminary experiment I can see how concentration will change the rate of the reaction. I could see that the weakest acid, in terms of moles, reacted slower. The stronger acid reacted a lot quicker. Looking back at earlier work it is explained, because the bigger the concentration there is the more particles there will be. Due to there being more particles, there will be more collisions ending in a faster reaction. So I can now make my first hypothesis for concentration. I think that the stronger the concentration the faster the reaction.

For temperature all I have to go on is scientific knowledge and previous work. Looking at previous work on reactions and looking at how temperature affects it, it says that as the particles are heated they will move quicker, meaning they will collide more, so that will make a faster reaction. There is an everyday example of this, an oven. As the oven gets hotter, the food inside will cook quicker, the colder it is the slower it will cook. So looking at the work and the example, I can now draw up my second hypothesis. I think that as the temperature increases, the reaction will be quicker.

I now have my two hypotheses, one on each variable; here is both my hypothesis together.

-The stronger the concentration, the faster the reaction.

-The higher the temperature, the quicker the reaction.

I have now done all the preparations and I’m ready to start the experiment.

Results and analysis

I have now completed the experiment. I then drew up the following table and entered the results.

By looking at the table you can see that the stronger concentration is quicker than the weaker concentration, this then proves my first hypothesis –

‘The stronger the concentration, the faster the reaction.’

I have also entered the results into a graph, where it is then easier to pick up on patterns. Looking at the graph I can again see that my first hypothesis is indeed correct.

Again looking at the graph I noticed there were individual results, which did not fit the pattern of the table. I have redone the table with the specified results highlighted. These results are anomalous to the table.

The result, which is highlighted in orange, doesn’t fit the table when looking across it, as it is quicker than the previous result by a full eight seconds. The reason for this is unknown but could be down to an uncertainty from the observer as to whether he could actually see the cross or not. It wasn’t a major mistake but did harm are results slightly.

The results, which are highlighted in red, do not fit the pattern of the table when looking down the table, as the results should be getting quicker. This is a small and uncanny error in our results. Although the last results, one hundred and fifty five seconds is a full seventeen seconds slower than the result previously, when it should have been quicker than the previous one and slower than the one after it.

I don’t have, or could find an explanation for this mistake being made so I feel that they are purely anomalous results. All the anomalous results fall into the same attempted temperature, so that could mean something to do with heating up the acid was done incorrectly, that may have caused the anomalous results.

Without the anomalous results included I can see that my second hypothesis is proved correct -

“The higher the temperature, the quicker the reaction.”

You can see this when looking down the table, when the temperature does increase the reaction time drops. You can also clearly see this when looking at my graph.

To check for any mistakes and to check the accuracy of my own results I compared them to the results of another group.

Looking at the table I can see that the results are similar to mine, so I feel, by ignoring the anomalous results that my results as well as the other group’s are accurate enough to support both my hypothesis.

Conclusion

Looking back at the experiment I can see that I have proved both my hypothesis. That the higher the temperature the quicker the reaction, and the stronger the concentration the faster the reaction. This proves that the concentration and temperature will always have an affect on the speed of a chemical reaction, as you can see in the table of results. This can be double-checked by comparing my groups results with the results of another group, that again proves my hypothesis are in fact correct.

I feel that the experiment was completed very well and that the method was followed exactly. We only encountered one problem, which was quite important but didn’t jeopardize what we found out, and that was our anomalous results. Looking back I could find no exact reason as to why the results turned out the way they did.