[PoetryInMotion]

Hi!

Yesterdays we synthesized Mohr's salt as a lab excessive in school. One major step was the dissolution of iron in hot 2.5 M sulfuric acid. During a discussion about the reaction formula for this very step, some one raised the question about why H₂ is formed during the reaction. According to various tables of redox potentials (for instance at Wikipedia) the sulphate ions seem to be stronger oxidizing agents than hydrogen ions, which should mean that the SO₂ would be formed as the main gaseous product. Our lab instructors was not able answer this question, and we weren't allowed to investigate the gas further (for instance with pH paper). What do you people here believe? Most sources I have found says that H₂ is formed, so probably that is the case. But I cannot understand why.

[Borek]

Several things to consider.

1. Redox potentials are given for standard conditions - your solution was not in standard condition, so you should compare formal potentials (that requires knowledge about activities of all ions present in the solution - not a trivial task in the solution of high ionic strength).

2. Even knowing formal potentials is not enough, as some reactions are faster and some are slower - so it is possible, that the final effect depends more on the kinetics than on the thermodynamics.

3. Could be a mixture of gases is produced.

[Arkcon]

You could also have investigated the properties of the gas generated.

[Borek]

You could also have investigated the properties of the gas generated.

we weren't allowed to investigate the gas further (for instance with pH paper)

[AWK]

Practically - with diluted sulphuric acid only hydrogen is evolved. With concentrated one some acid can be reduced to sulphur dioxide.
2.5 M acid is rather still diluted.