Why is hot concentrated sulfuric acid preferable to cold?

Question

Why is hot concentrated sulfuric acid sometimes used to react with metals instead of concentrated sulfuric acid which is at room temperature?

I have ascertained that concentrated sulfuric acid has quite strong oxidizing properties. Still, some metals which are below the hydrogen in electrochemical series do not react with concentrated sulfuric acid which is kept at room temperature. When the acid is warm/hot, the reaction takes place.

All I want to know is how does temperature affects its oxidizing properties?

Answer 1

A really short answer is kinetics. Heating up chemicals adds more energy to the system, allowing molecules to colide with more force increasing the probably of a reaction taking place.

Answer 2

Firstly these metal are not non-reactive with concentrated sulfuric acid- rather they form protective layers whether sulfate or oxide which reduce the speed of the reaction (kinetics).

The effect of heating the sulfuric acid depends on the metal (e.g. steel is good for up to 25C 60-95% sulfuric acid) but there the trans-passive region where metals such as iron no longer have a protective layer. This is due to instability of the oxide/sulfate layer so it dissolves or falls off.

Answer 3

When we say that a metal falls below hydrogen in the electrochemical series, we typically refer to the oxidizing power of hydrogen ion at 1 molar concentration in water solution. The oxidizing power of concentrated sulfuric acid, forming sulfur dioxide and water when it reacts, could be greater than that of the aqueous hydrogen ions; if so, then thermodynamically some noble metals could react with concentrated sulfuric acid. Then it's a matter of kinetics: getting the redox reaction to go reasonably fast and getting enough solubility of any passivating films that might form, which is where heating the acid comes in.