What drives double displacement reactions?

Question

I have two questions:

1. I understand that in a displacement reaction the more reactive element displace the less reactive element. But why? In the reaction with Zinc and Copper Sulfate, we form Copper and Zinc Sulfate. But both Copper Sulfate and Zinc Sulfate are soluble in water so how does Zinc Sulfate stay together?

2. What drives double displacement reactions? They are not redox reactions so the reactivity series doesn't apply, so how can we explain why the anions and cations switch places. Is this energetically favorable?

Answer 1

Dissolved salts exist as separated, hydrated ions, moving and reacting independently. (*)

Like for $\ce{Zn}$ and $\ce{CuSO4}$:

• The reaction is $\ce{Zn(s) + Cu^2+(aq) -> Zn^2+(aq) + Cu(s)}$.
• The ion $\ce{SO4^2-(aq)}$ is called a spectator/bystanding ion in the reaction context, not participating on the reaction.
• The reaction can be formally written using full salt formulas, which should not be confused with molecules, as they express just ion stoichiometric ratios: $\ce{Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s)}$.
• The driving force is here different ion affinity to electrons and related redox potential and reaction Gibbs energy.

The example for "double displacement" reaction - precipitation of $\ce{AgCl}$ from $\ce{AgNO3}$ and $\ce{KCl}$:

• The reaction is $\ce{Ag+(aq) + Cl-(aq) -> AgCl(s)}$.
• The spectator/bystanding ions are $\ce{NO3-(aq)}$ and $\ce{K+(aq)}$.
• The reaction can be again formally written using full salt formulas: $\ce{AgNO3(aq) + KCl(aq) -> KNO3(aq) + AgCl(s)}$
• The driving force is here the difference in mutual ion affinity for various cation + anion combinations, leading to precipitation of the least soluble salt - $\ce{AgCl}$ in this case.
• It is closely related to energy released by ion hydration versus energy released by ions forming solid ionic lattice, hold by coulombic forces.
• It again depends on the reaction Gibbs energy.
• The same case is when all 4 ion combinations form more or less well soluble salts.

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(*) At some conditions, hydrated ions may not move independently, but they can have various degree of mutual connection of their hydration envelope, forming ion pairs.