To Investigate The Effect Of Varying Concentration On The Reaction Between Marble Chips And Hydrochloric Acid.

I know I will collect enough gas because:

Variable Factors

To make this a fair test I will need to keep the following variables the same throughout the experiment:

* Temp. of reactants

* Mass of reactants

* Presence of catalysts

* SA of solid reactants (as much as possible)

* Density of Gas/Solid reactants

* Amount of reactant

* Ea of reactants (presence of catalysts)

Prediction

I have done a previous experiment in which one molar HCL(aq) and two molar HCL(aq) were reacted and the two molar reaction was always twice the rate of the one molar acid. Based on this I predict that as concentration doubles then so does the rate of reaction. I think this because if the concentration doubles, then the number of ions of HCl(aq) doubles. Therefore collisions occur twice as frequently, and the rate of reaction doubles. Therefore if the concentration doubles, the time taken for 50cm3 of CO2 halves.

Obtaining Evidence

Moles of HCl(aq)

Time 1(s)

Time 2 (s)

Time 3 (s)

Average (s)

2

0.45

9.83

0.92

0.4

.5

5.29

4.57

5.08

4.98

9.75

20.65

9.66

20.02

0.5

41.03

40.63

40.38

40.68

Results

Analysis

You can see from this graph that as concentration decreases, the time increases.

This negative correlation means that so far my prediction is correct.

Here we can see that as concentration increases, so does the rate of reaction.

Here we can see all the results together. It is clear that as concentration increases, rate also increases an time decreases, meaning that reactions react faster if the concentration is higher.

In conclusion

In conclusion the collision theory seems to be correct. By increasing the concentration you therefore increase the number of ions in the solution. If you increase the number of ions you therefore decrease the distance between each ion and so you increase the frequency of which collisions occur. The increased frequency of collisions between ions means that they will react faster, thus increasing the rate. The evidence seems to match my prediction perfectly. Using this it is possible to see that as the concentration doubles so does the rate.

Evaluation

My method gave good general data but is not accurate. My method could be used to prove if predictions were true but because some results were quite far off (eg 41.03) my results should not be use for extreme accuracy. However it can be concluded that it takes about ten seconds for 50 cm3 of gas to be collected in a two molar solution and twice as long when the concentration is doubled. I think I have just enough evidence to draw a conclusion, I would prefer to have two or three times as much evidence but that is not possible.

As far as the method goes it was quite sound apart form the timing set-up. When I was starting the experiment, I had to add the marble chips, put the bung on, and start the watch at the same time. This gave inaccuracies in the times.

Other problems arose in practise that I could not help. Firstly, as the reaction occurred, it produced heat. This heat sped up the reaction therefore making the fast reactions faster. Also because I shook the reaction by hand some reactions were shaken more than others, causing them to react faster. Also the marble chips were of different sizes. This meant that the smaller ones had a larger surface area to volume ratio and reacted more quickly than bigger ones. Standard sizes chips would stop this.