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According to:

https://www.webelements.com/calcium/chemistry.html

Calcium reacts with iodine upon heating as follows:

$\text{Ca}_{\,(s)} + \text{I}_{2\,(g)} \to \text{CaI}_{2\,(s)}$

I am effectively attempting this by dropping elemental calcium and iodine into a flask and putting it under a Bunsen burner. I have a tube leading from the flask to a beaker of water to safely condense escaping iodine gas.

However, this reaction is not occurring at any appreciable rate. I am wondering if I should be using powdered calcium instead of these larger pebble-sized chunks or if a Bunsen burner is insufficient to breach the energy barrier. Are these measures or others sufficient to increase the rate of reaction?

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    $\begingroup$ The trouble with having a big lump of calcium is that the reaction will only occur on the surface. And, if you heat the mixture, the iodine will sublime out of the mixture faster than the reaction occurs leaning you with little product. $\endgroup$
    – matt_black
    Jul 9, 2018 at 12:17
  • $\begingroup$ Your reactor should be sealed tight and probably fluidised bed would give fastest reaction. So flask with tube doesn't do it. $\endgroup$
    – Mithoron
    Jul 9, 2018 at 19:47

2 Answers 2

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Looks like the Bunsen burner is not providing enough energy for the reaction to occur. AFAIK, you need around 200-400°C of heat for the reaction to occur. Yes, you can always powder the calcium metal to increase surface area and see if the reaction is occurring or adopt simple(chemical) methods. You can react any calcium salt with hydroiodic acid to form calium iodide. I am using calcium carbonate as example[1]:

$$\ce{CaCO3 + 2 HI → CaI2 + H2O + CO2}$$

Or alternatively make a iodide of iron and make it react with a calcium salt[2]. There are many more ways to make calcium iodide in [2]. You can look at them but these two methods is by far the most simple methods.

References:

  1. https://en.wikipedia.org/wiki/Calcium_iodide
  2. Caty J. Braford; H. A. Langenhan; A preliminary report on the composition of syrup of calcium iodide N. F. V. DOI:https://onlinelibrary.wiley.com/doi/pdf/10.1002/jps.3080180218
  3. http://albumen.conservation-us.org/library/monographs/sunbeam/chap10.html
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    $\begingroup$ Hey Nilay! Thanks so much for your help. Your second link seems to not work. Also, I know I am getting WAAAY above 200-400C with my burner. $\endgroup$
    – David Reed
    Jul 9, 2018 at 5:12
  • $\begingroup$ @DavidReed I fixed the link. Hope it will work. The temperature seems to be fine in your case. Looks like the calcium needs to be powdered. You are asking to obtain HI from calcium iodide + any acid so you can't work with it. No problem. Make a iodide of iron and react with any calcium salt. Look in the 2nd link. They gave many ways of obtaining calcium iodide without using HI. If you say, I will add that part in my answer. $\endgroup$
    – Nilay Ghosh
    Jul 9, 2018 at 16:19
  • $\begingroup$ @DavidReed good enough $\endgroup$
    – Nilay Ghosh
    Jul 9, 2018 at 18:00
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--self answer--

Using the (quite ancient) links provided above by Nilay I was able to successfully isolate calcium iodide via an alternative method. I have posted it below for those who are curious.

1.) Mix excess iron filings with elemental iodine in water. Stir and heat. At first the solution will redden, indicating the formation of triiodide, and hence the oxidation/reduction of iron/iodine and the formation of iron (ii) iodide. Right around boiling, the solution will change color from red to light brown indicating completion.

2.) Slowly add calcium hydroxide to the solution, taking care to stay below saturation. Iron hydroxide will precipitate.

3.) Filter the precipitate, which should be bluish green. (This is consistent with the color of air-oxidized iron hydroxide).

4.) Recrystallize the resulting solution via evaporation.

Why I believe this worked---

1.) The resulting white compound was soluble, but a precipitate formed when mixed with sulfuric acid (calcium sulfate).

2.) The solution turned purple when electrolyzed, consistent with the presence of iodide.

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