Op,

imagine a similar box with one half hydrogen and the other oxygen. Outside the box is the vacuum of space. Combine the gases causing a violent explosive reaction generating great heat and largely water to form, then release the pressure of this reaction out of a hole in the box to space. What escapes is water vapor, steam and even ice crystals at very high velocity. Can you not see how the result will transfer momentum via the pressure waves transmitted through the escaping gas/vapor column to the box.