Hello!
I have a question.
Concerning gases, we say that decreasing the volume leads to particles to collide ore often. ( here is an explanation that I got from the teacher: The number of collisions with the walls per unit time increases because the molecules are moving in a smaller volume but with the same average speed as before. Under these conditions they will strike the walls of the container more frequently).
However, when we are learning the reaction rate, when talk about increasing the size of the reaction container, it says that it has no change, how? I though that increasing the container size leads to less collisions of particles which leads to decreasing the reaction rate no?
In addition, we say that increasing the volume has no change, but increasing the volume of one of the substances, does that not add more particles even if the concentration is the same?
Thank you so much!
Explanation from Alloprof
This Explanation was submitted by a member of the Alloprof team.
Hi!
This confusion comes from the fact that pressure and reaction rate do not depend on exactly the same things, even though both involve particle collisions.
First, when you talk about gases and pressure, decreasing the volume while keeping the same number of particles and the same temperature forces the particles into a smaller space. They still move at the same average speed, but because the space is smaller, they hit the walls more often. That is why pressure increases. Here, we care specifically about collisions with the walls of the container.
Now, with the reaction rate, you are no longer interested in collisions with the walls, but in collisions between reacting particles. What matters most for reaction rate is the concentration of reactants, meaning how many particles are present per unit volume.
This is why increasing the size of the reaction container does not automatically change the reaction rate. If the container gets bigger but the amount of gas stays the same, the particles are more spread out, so the concentration decreases, and the reaction rate would indeed decrease. However, when it says that increasing the container size has no effect, it means that the concentration is kept constant. If concentration stays the same, then the number of collisions between particles stays the same, so the reaction rate does not change.
This also explains your last question. If you increase the volume of one substance but keep the concentration the same, you must also be adding more particles. Since concentration is particles per volume, keeping it constant means both the number of particles and the volume increase proportionally. In that case, the reaction rate still does not change.
Hope this helped!
PoutineOrange5768