Heinemann Chapter 26 Question

[coezooke]

Question 10 a)
Use the electrochemical series to predict what might be expected to occur if hydrogen gas were bubbled through a solution containing Fe³⁺ ions.
My answer was H₂ gas would be oxidised forming H+(aq) and Fe³⁺(aq) would be reduced forming Fe²⁺(aq)

.. I am confused when part c. of the question asks "When the reactants were mixed in an experiment, no reaction was observed. Suggest possible reasons for this" as a reaction could occur for part a of the question..

Please help! 

[psyxwar]

Hmm. I guess it's possible that just because it happens to be a spontaneous reaction doesn't mean it'll be a fast reaction -- rate of reaction could've been too slow to be observed?

[lzxnl]

Think about it this way. You can mix hydrogen and oxygen gas without them reacting. Hydrogen-hydrogen bonds are quite strong due to the small size of the hydrogen atoms and hence low bond lengths.
You need a spark to start the reaction. If you just mix hydrogen gas and iron metal...well...psyxwar is right, the reaction could be very slow. Just imagine hydrogen peroxide. The electrochemical series says that it reacts with itself. Well, it evidently doesn't do that very quickly because we can store H2O2 for a long time.

[coezooke]

Thank you!

[jgoudie]

Another thing to think about (although to a lesser extent than nliu's answer) could also be that the E.C. series is made using SLC (1atm,298oC and more importantly 1M solutions).  This is usually used when explaining electrolysis not producing the products you would expect it to. 

But yes I would say the E.C. does not tell you anything about the rate of these reactions.

Question 10 a)
Use the electrochemical series to predict what might be expected to occur if hydrogen gas were bubbled through a solution containing Fe³⁺ ions.
My answer was H₂ gas would be oxidised forming H+(aq) and Fe³⁺(aq) would be reduced forming Fe²⁺(aq)

.. I am confused when part c. of the question asks "When the reactants were mixed in an experiment, no reaction was observed. Suggest possible reasons for this" as a reaction could occur for part a of the question..

Please help! 

[lzxnl]

Another thing to think about (although to a lesser extent than nliu's answer) could also be that the E.C. series is made using SLC (1atm,298oC and more importantly 1M solutions).  This is usually used when explaining electrolysis not producing the products you would expect it to. 

But yes I would say the E.C. does not tell you anything about the rate of these reactions.

Yeah, using the electrochemical series to predict rates of reaction produces ludicrous results. For starters, lithium reacts less violently with water than potassium, even though lithium is a stronger reductant. Why? Lithium is smaller, so it holds onto its electrons a bit more tightly, leading to a higher activation energy and hence a slower reaction. The electrons are so loosely held in potassium that the activation energy is a fair bit lower.
Or think of hydrogen peroxide. The fact that we can store it for years shows that the electrochemical series doesn't predict rates of reaction.
The electrochemical series can be used to predict equilibrium constants, and we know those aren't directly related to rates of reaction.