Chemistry

ArJoe SG

Why does copper react with concentrated nitric acid although copper is less reactive than hydrogen? why can hydrogen react with iron although it is less reactive(reduction of iron oxide by hydrogen)?

UltimaOnline SG

Why does copper react with concentrated nitric acid although copper is less reactive than hydrogen? why can hydrogen react with iron although it is less reactive (reduction of iron oxide by hydrogen)?

Regarding hydrogen reacting with iron :

First of all, you probably mean hydrogen reacting with compounds or oxides of iron (rather than iron itself). At room temperature and at standard molarities or partial pressures, the reaction will occur at a negligible rate. So at O levels, it's correct to say "iron is less reactive than hydrogen", at standard conditions.

However, when sufficient heat energy is supplied to overcome the Ea barrier, together with higher molarities or partial pressures of gaseous hydrogen, then the reaction may proceed (depending on the exact redox reaction). For industrial and pragmatic purposes, a transition metal catalyst is almost always employed to increase the rate of the redox reaction when employing gaseous hydrogen.

Moreover, if the reaction you're referring to, is the reduction of iron(III) oxide to iron(II) oxide, then note that the standard reduction potential of Fe3+ to Fe2+ is +0.77V, and thus the reaction with gaseous hydrogen is indeed expected to proceed under standard conditions. If the reaction you're referring to are the reduction of other metal oxides with hydrogen, then be aware that as long as the overall redox potential isn't too negative, some reaction can still occur with heat and at higher than standard molarities, it's only a matter of the % yield at equilibrium.

Regarding concentrated nitric acid :

This is because both the cation H+ and the anion NO3- are strong oxidizing agents, and when both are present in high molarities, can work together to oxidize even relatively unreactive metals (ie. with low or slightly negative oxidation potentials).

The NO3- in nitric(V) acid (ie. with nitrogen having the maximum OS of +5) can be reduced to species such as HNO2 (OS of +3) or NH4+ (OS of -3).

For an even more powerful oxidizing agent, which can oxidize even gold and platinum (which are even less reactive than copper), see Aqua regia :

http://en.wikipedia.org/wiki/Aqua_regia

The chemistry of Aqua regia involves both redox as well as coordination complex reactions, and can be used by Cambridge to craft an excellent and challenging H2 Chemistry A level exam qn.