Agar blocks

Effect of concentration on rate of diffusion

Procedure #2

1. Six agar blocks are cut into size 1.5cm³ using knife
2. 40ml cube of hydrochloric acid (0.75, 0.5 and 0.25 moles each separate) are put into a beaker, different beaker for each concentration.
3. 2 of the 6 agar blocks are put into the beaker containing 0.75 moles of Hcl. 2 blocks are put into beaker containing 0.5 and 2 blocks in beaker containing 0.25 moles of Hcl.
4. Time taken for agar blocks to go from pink to white is recorded using stopwatch.

Table 1

 

Table 2

Table 3

Graph for table 1

Figure

Graph for table 3

Figure 1

        

Discussion: The results obtained from this experiment are straight forward. When looking at the “size of the agar block” it can be easily seen that the bigger the agar block the longer it took for the phenolphthalein indicator to diffuse completely, no anomalies are present. The same can be said, when looking at the factor: concentration of Hcl, no anomalies are present whatsoever, and it can be easily deduced that the higher the concentration of acid the quicker the phenolphthalein indicator diffuses.

Evaluation of results: The results obtained from the above tables are-as previously mentioned-straight forward, in “Table 1” the reason for the increase in time as the agar block got bigger is mainly due to, as mentioned in the hypothesis, the fact that the bigger the object the smaller the surface area to volume ratio-as can be seen in “table 2”-and that is why it takes longer for the large agar blocks to diffuse. The results in “table 3” occur due to the factor concerning “concentration of Hcl”. Basically the higher the concentration of acid (e.g. Concentration 1 mol, time taken: 132.0sec) the quicker the rate of diffusion, this is because a high concentration causes a steeper concentration gradient for the particles to diffuse towards. Whereas in a low concentration, the concentration gradient (example 0.25 moles, average time: 286.5sec) is not as steep and hence it takes longer for the particles to diffuse.

Limitations and suggestions to improve: When doing the experiment one of the most difficult parts was cutting the agar blocks, for not only was it time consuming, but the separate agar blocks could not be cut into exactly the same size. I think that in order to improve this hindrance a more accurate instrument should be used instead of a knife and a ruler, as human error is very easy.  Another problem when doing the experiment was that the agar block itself contained different shades of colour ranging from very light pink to dark purple, meaning that the concentration of phenolphthalein indicator was different in some areas of the agar block ,and this obviously tampered  with the results. A solution to this problem would be to cut most of the light colour off and only use the darkest part of the agar block, however this would waste most of the agar block. So maybe dip the agar block into phenolphthalein indicator the second it’s been cut and then put it into the beaker of Hcl.

Conclusion: The hypothesis for both factors was correct, Surface area to volume size largely effects the rate of diffusion as does the concentration the acid being used.