Michael S. answered • 13d
B.S. in Chemistry, Indiana University; Organic Chem Teaching Intern
It depends entirely on which volume you shrink — and that ambiguity is the heart of the question.
X(s) + YZ(aq) → XY(aq) + Z(g)
Rate depends on the concentrations of the species that appear in the rate law, which means the reactants: the solid X and the aqueous YZ. Ask what shrinking a volume does to each.
Case 1: you shrink the container (less headspace above the liquid)
No change to the rate.
• X is a solid. Solids are essentially incompressible, and a solid has no meaningful concentration — what matters is its surface area, which squeezing the container does not alter.
• YZ is aqueous. Liquids are also nearly incompressible, so [YZ] is unchanged.
• The only thing compressed is the gas Z — and Z is a product. Products do not appear in the forward rate law.
This is exactly why your instinct from gas-phase reactions does not transfer. There, shrinking the volume raises the reactants' concentrations directly. Here it raises only a product's pressure.
Case 2: you shrink the solution volume (same moles of YZ in less water)
The rate increases.
Now [YZ] genuinely rises. More YZ molecules per unit volume means more collisions per second with the solid's surface, so the reaction speeds up. If the reaction is first order in YZ, halving the solution volume roughly doubles the rate.
This is the real mechanism by which "volume" affects a solution-phase rate — through concentration, not through pressure.
One caveat on Case 1
If the vessel is sealed and the reaction is reversible, letting Z build up to high pressure can slow the net rate by driving the reverse reaction (Le Châtelier). But that is an equilibrium effect on the net rate, not a change in the forward rate constant or the forward rate itself. For an irreversible reaction, or an open vessel where Z escapes, even that does not apply.
What actually controls this reaction's rate
• Surface area of X — powder reacts far faster than a single lump. This is the solid's equivalent of concentration, and it is usually the biggest lever.
• [YZ] — as in Case 2.
• Temperature — through the rate constant, via Arrhenius.
• Catalyst.
• Stirring — often overlooked. For a solid reacting with a solution, the reaction can become diffusion-limited: YZ near the surface gets used up and products accumulate there. Stirring refreshes that layer and can noticeably speed things up.
The rule to carry forward: compressing a volume changes rate only for species whose concentration the compression actually changes. That means gaseous reactants. Solids, liquids, and dissolved species in a fixed amount of solvent are unaffected by squeezing the container.
