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Name: Sam
Status: student
Grade: 9-12
Location: TX
Date: December 2007


Question:
Is the reaction time between calcium carbonate and 5% acetic acid dependent on the amount of calcium carbonate that is used? Is a larger amount of calcium carbonate supposed to take longer for the chemical reaction to complete when the amount of the acetic acid (10 ml) stays the same? Our experiment had mixed results.



Replies:
Sam,

I do not know the order of the reaction as a function of calcium carbonate for the reaction between calcium carbonate and acetic acid. However, there are only three possibilities, that the order is 0, 1, or 2. If the order of the reaction as a function of calcium carbonate is zero, then the reaction rate will not speed up as a function of an increase in concentration of calcium carbonate. If the order of the reaction is 1, the rate will double; if the order is 2, the rate will quadruple.

As you can see, under no conditions will the rate decrease as a function of an increase in the concentration of calcium carbonate. In fact, in any reaction, either the rate will not change (zero order) or increase as a function of an increase in a concentration of a reactant. The rate never decreases with increasing concentration.

What is more likely is that there may have been an error in the design or execution of the experiment. How did you observe the rates? How did you change the concentrations?

Greg (Roberto Gregorius)


"Mixed results" is exactly what you should expect. The reaction between solids and either liquids or gases is very difficult to follow experimentally. There are a low of factors that come into play that you usually cannot control -- particle size and particle size distribution. "Wetting" the solid phase by the liquid phase -- the list is pretty long. "On paper" the rate should not depend upon the amount of calcium carbonate, but experimentally, it is difficult to control all of the contending factors.

Vince Calder



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