[Foobarz]

Ok, question is:

If Magnesium is less reactive than Sodium, why does magnesium displace sodium from sodium hydroxide (NaOH)?

[UG]

Are you talking about magnesium metal? Or the ion?

[Foobarz]

Magneisum metal. This reaction here: http://www.youtube.com/watch?feature=endscreen&v=seSg_GWj1b0

Why does Magnesium (lower in the reactivity series) displace Sodium from Sodium hydroxide?

[UG]

Under non-standard conditions it is possible for the reaction to occur.

[Foobarz]

What do you mean by non-standard conditions?

[UG]

Well, I don't think there is a strict rule for defining the reactivity series but one way it can be done is by comparing standard reduction potentials (how easy an element is reduced compared to a reference element). Standard reduction potentials are measured under standard conditions and under these conditions, magnesium will not be able to displace sodium from a solution of sodium hydroxide. However, when you have non-standard conditions, the electrode potentials change and the reaction, as you have shown, can occur.

[Borek]

In general standard conditions mean activities of all substance involved equal to 1 - as a first approximation it means every ion in the solution in the 1M concentration, and every gas at pressure of 1 atm. Solids have to be just present.

When you mix solids outcome of the reaction starts to depend an additional factors, like lattice energy, and simple application of reactivity series - especially in the case of metals of similar activity - is not enough to predict the reaction result.