# Does volume affect the reaction rate?

Jun 22, 2018

Does volume affect concentration?

If you answered yes to that, then it affects reaction rate, as you see in every rate law...

$r \left(t\right) = k {\left[A\right]}^{m} {\left[B\right]}^{n}$

for reactants $A$ and $B$ of reactant orders $m$ and $n$ respectively, with their rate constant $k$ at a given temperature $T$ over time $t$ for rate $r \left(t\right)$.

...and is not concentration the amount of substance divided by the volume?

$\text{Conc." = "Number of particles"/"unit volume}$

CASE I

• As container volume increases without adding more gas, reaction rates of gas-phase reactions decrease, as these gases will collide less frequently in a larger space.

$\text{Number of particles constant"/(uarr "unit volume") => "Conc.} \downarrow$

CASE II

• As the gas sample volume increases (e.g. as we add more gas into a flexible container, like a balloon), reaction rate stays about the same... The concentration effectively does not change due to the increase in mols of gas changing proportionally with the increase in container volume...

That is,

(uarr "Number of particles")/(uarr "unit volume") => "Conc. stays constant"

What is the distinction between the two scenarios?