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Which change will decrease the rate of the reaction between $\ce{I2(s)}$ and $\ce{H2(g)}$?

(A) Increasing the partial pressure of $\ce{H2(g)}$

(B) Adding the $\ce{I2(s)}$ as one piece rather than as several small ones

(C) Heating the reaction mixture

(D) Adding a catalyst for the reaction

I can eliminate D as a catalyst would speed up the reaction.

I am not sure about the others though. I could look up the enthalpy for this reaction and eliminate C, but this is from the Chemistry Olympiad and I can't look up enthalpies during the test. Is there any other way to determine that this reaction is endothermic?

Once you eliminate C, you are left with A and B. How would you go about finding the right answer?

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  • $\begingroup$ HINT 1: If you increase pressure in a fixed volume, what happens to temperature? HINT 2: What does surface area do to reaction rate? $\endgroup$ – user467 Mar 18 '14 at 0:20
  • $\begingroup$ By guy lussac's law increasing pressure will increase temperature. How would you determine if the equation is endothermic or exothermic though? Greater surface area results in faster reaction speeds. However, doesn't the larger piece have a greater surface area than the smaller pieces? $\endgroup$ – 1110101001 Mar 18 '14 at 1:29
  • $\begingroup$ Regarding the first hint, after increasing the partial pressure, even if the temperature is allowed to decrease to its previous value, an effect on the reaction rate is still observed. Why could this be? $\endgroup$ – Nicolau Saker Neto Mar 18 '14 at 1:30
  • $\begingroup$ "Greater surface area results in faster reaction speeds. However, doesn't the larger piece have a greater surface area than the smaller pieces?" Take a 1" cube and measure its surface area. Now cut the cube into 8 1/2" cubes and measure their total surface area. Note that the latter includes all the prior surface area PLUS new surface area from the cuts. We conclude? $\endgroup$ – user467 Mar 18 '14 at 1:37

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