[ricky101]

Hello,

I am currently doing a chemistry EEI that involves determining the orders of reaction for a particular reaction. The reaction chosen for the experiment is:

   Mg + 2HCl  MgCl₂ + H₂

The EEI asks students to hypothesis what the orders of reaction might be based on secondary data. I was able to locate some secondary data, which is shown below:

Concentration of HCl 0.8   1.5   2.0   3.0
Time take (s)           36    18    11     8

By graphing the data I was able to determine that the overall order of reaction followed a roughly that of a second order. When it came to hypothesise what the individual orders of reaction were (for Mg and HCl) I thought that it could be achieved by finding two concentrations that were double. Because there was a concentration of 1.5 M and 3 M i thought this could be used to see if the order of reaction was first, second etc...

Because there is roughly half the time taken between the two concentrations I assumed that it was a first order with respect to HCl and therefore a first order with respect to Mg.

To verify this however, I thought that I would do a quick trial experiment to ensure that my predictions were indeed correct. This is where I ran into problems. Upon trying each of the test, I encountered the following results:

Concentration: 1   0.5
Time taken:   174 723

According to my data, there is roughly a 4x increase in time taken for a concentration increase of 2x. This would indicate a second order, which would go against my original prediction.

What I would like to know is, would anyone be able to confirm what the orders of reaction are with respect to each of the reactants?

Thanks a lot for your help,

Ricky

[Astrokel]

hey ricky101,

What graph did you do? Concentration against time and calculate the initial rates?

Concentration: 1   0.5
Time taken:   174 723

According to my data, there is roughly a 4x increase in time taken for a concentration increase of 2x. This would indicate a second order, which would go against my original prediction.

I would say your method is wrong but not entirely. If you want to vary your [acid] while keeping your mass of Mg the same(and without plotting graph), then you should use EXCESS acid instead.

For example using 1g of Mg, first you start off with 1M of excess acid and measure the time taken to complete the reaction. second, vary the concentration of excess acid like 0.8M then measure the time taken. Repeat until you obtain many datas and you know the initial rates in all the experiment should be proportional to 1/time right, since [acid] remains approximate constant. Then of course you can compare and work out the order. If you want to confirm your order, you can plot initial rates against concentration to see the shape of the graph.

of course this is only 1 of the many method to calculate the orders. and and pls be careful when handling the acid(never pour water to a concentrated acid, do the opposite)

hope it helps some..