[FascinatingPens]

In real life, some gas particles would escape, but if the balloon was completely sealed, what effect do the collisions (between particles and balloon) have on the size of the balloon? I know larger/heavier molecules hit the inside of the balloon with more force, but I can't connect it back to the size thing. If there were two balloons filled with different gases, and both started at the same temperature, pressure, and volume, and absolutely no gas could escape, which would be smaller after a day (the one with the heavier or lighter molecules)?

[Schrödinger]

I don't think the balloons will become small if they are completely sealed. I think the only reason for that is the escaping of gas molecules. I mean, where would all the energy of the gas molecules go?
Please correct me if I am wrong.

[FascinatingPens]

The energy of the gas in both balloons would be the same to begin with (right?), but wouldn't the large molecules (and greater force) mean more energy released with each collision? (and if so, would the smaller molecules collide more frequently with the balloon so everything balances again?)

[billnotgatez]

In real life, helium seeps out through the pores in the latex.

[Yggdrasil]

The energy of the gas in both balloons would be the same to begin with (right?), but wouldn't the large molecules (and greater force) mean more energy released with each collision? (and if so, would the smaller molecules collide more frequently with the balloon so everything balances again?)

Assuming the balloons are at the same temperature, the gas molecules would have the same average kinetic energy.  Therefore the more massive particles will also be moving more slowly than the less massive particles.  Therefore, although they do impart more force on impact, they collide with the wall less frequently.

Also, for a balloon you also need to consider that the gasses on the outside of the balloon are also colliding with the balloon walls to add energy back into the system.  The rate of loss of energy due to collisions between the wall and particles inside the balloon equals the rate of gain of energy due to collisions between the wall and particles outside of the balloon when the temperatures inside and outside of the balloons are equal.