Talk:Law of reciprocal proportions

First reaction

An interesting new article on a forgotten law which has no longer appeared in most chemistry textbooks for many decades, as is suggested by the age of 2 of the 3 references (although Holleman and Wiberg appears to be an exception).

It would help the reader to add an example with specific elements and numerical values, perhaps similar to the example given for the law of multiple proportions.

Also, it seems to me that the two statements by Richter and Berzelius are not equivalent. Richter considers 3 elements while Berzelius refers to 4. And Richter says simply related to but Berzelius says the same as. Are there in fact two distinct laws of reciprocal proportions?? Perhaps I have not understood something, which seems quite possible given the antique language used by Berzelius especially. I admit never having seen this law before in many years of chemistry teaching. Dirac66 (talk) 21:54, 12 April 2014 (UTC)

Yes, the key law of stoichiometry was the way I was taught it. Its an experimentally based proof of the concept of equivalent (combining) weights which underpins equivalent weights, and that lead to the concept of valency as an elements property.

The first statement of the law in the article is one from Holmyards old book, which is about the simplest I could find. Richter couched his work in very obscure language and didn't actually state the law. Partington, 1921, stated the law as "the weights of two or more substances which separately react chemically with identical weights of a third are also the weights which react with each other, or simple mutiples of them" this could be better- any thoughts? And yes there is only one law but as it is essentially an expression of ratios of weights of combining elements everyone seems to have formulated their own version of it. Freunds account is a typical formulation in ratios, and in it she express all of the laws, definite proportions, multiple proportions etc in terms of weight ratios. Would modern students be fluent enought with ratios to get it?

Examples of the law which use "modern" compounds are easy to "make up", for example Al + Cl -> AlCl₃; P + Cl ->PCl₃; law "predicts" AlP. Na+H -> NaH; Cl + H -> HCl; law "predicts" NaCl (this last is in Partington's 1921 book). Obviously the chemical formulas couldn't be used, and reacting weights would have to be mad up and quoted. Would such examples be adequate? I toyed with using the historical examples of Berzelius and Stas but they are difficult to access and from accounts of them in Freund, difficult to interpret in modern form. Axiosaurus (talk) 09:00, 13 April 2014 (UTC)

I looked at Holmyard and Freund on-line, and I think their examples could be used with their reacting weights. This could show how the law was actually described in the early 20th century, since it has disappeared from modern teaching. The formulas can be added if we indicate clearly what is part of the original example and what is added to help the reader.

Possible text for Holmyard's example, with added explanations in parentheses: 1 gram of sodium is observed to combine with either 1.54 grams of chlorine or 5.52 grams of iodine (corresponding to the modern formulas NaCl and NaI). The ratio of these two weights is 5.52/1.54 = 3.58. It is also observed that 1 gram of chlorine reacts with 1.19 g of iodine. This ratio of 1.19 obeys the law because it is a simple fraction (1/3) of 3.58. (In fact it corresponds to the formula ICl₃, which is a known compound of iodine and chlorine but not the only one.)

Note that I have worded this as an interpretation of observed data rather than as an imaginary prediction, since the predictive value of this method was probably limited, multiple formulas being possible for many pairs of elements. Dirac66 (talk) 15:10, 13 April 2014 (UTC)