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Name: Lydia
Status: student
Grade: 9-12
Country: Canada
Date: Summer 2009

My chemistry class conducted a lab on the rates of reaction of different acids and concentrations. 10.0 mL of each acid was diluted to 100 mL, and reacted with a 3.0 cm strip of magnesium ribbon: 3.0 mol/L HCl 1.5 mol/L H2SO4 The reaction rate for the production of 15 mL of H2 gas from each trial was timed. I hypothesized that the HCl would react faster than the H2SO4, since the second H+ would not fully dissociate, but in the experiment the H2SO4 reacted much faster than the HCl (by about 2 minutes). I was wondering if there is something wrong with my reasoning, or it my results were off. If someone could please explain the rates, I would greatly appreciate your help.

I do not know the answer to your question, but I can suggest a possibility. Check the temperatures of your two solutions when you run the reaction. The products of the two reactions, MgSO4 and MgCl2, have different enthalpies of solution. MgCl2 gets quite cold when it is dissolved in water. It is possible that your reaction making MgCl2 gets colder than the reaction making MgSo4, and the lower temperature makes the reaction proceed at a slower rate.

I do have one question about your setup: what do you mean by "3.0 mol/L HCl 1.5 mol/L H2SO4" at the same time as "10.0 mL of each acid was diluted to 100 mL?" Did you start with 10 mL of 3.0 M HCl and 10 mL of 1.5 M H2SO4 and dilute each to 150 mL?

Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming


Whenever something surprising happens to me in the lab, the first thing I do is check my samples. In this case, I might titrate my acid samples to ensure that they were in fact truly 3M and 1.5M.

Another thing you might consider trying is to use a different acid. If your intention is to show that the partial dissociation of an acid affects the kinetics of a reaction, then you might try to use a 3M acetic acid in comparison to the 3M HCl. Or, if you want an acid that has two or more protons, you might try a 3M phosphoric acid.

Greg (Roberto Gregorius)
Canisius College

Measuring rates of reaction, especially rates of reaction involving multiple phases -- in this case all three -- solid, liquid, and gas. Some reasons are: particle size of the solid (a major effect), adsorption of gas bubbles on the metallic surface, interfacial tension of the solid and liquid as well as the liquid to air interface, change in temperature. And this is the short list. The rate of reaction is essentially instantaneous if the surface area is large.

My recommendation is to reconsider the reaction you want to use as a model. The one you are testing is not going to give consistent results for reasons having nothing to do with the chemistry. There must be numerous web sites with detailed procedures.

Vince Calder

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