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Can pressure stop a reaction?

  1. Jun 3, 2009 #1
    So i am mixing Magnesium and water (the MRE heaters) in about a .3 mL container with a ball valve. It's reacting but not as well as usual, like in an unpressurized container such as a 2 liter coke bottle. It's just not producing enough Hydrogen (yes my calculations and ratios are correct) to get a good enough pressure of about 2.5 atm or 50 psi.

    Please let me know if you have ANY ideas as to what I can do.
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  3. Jun 4, 2009 #2


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    Of course it does. p*V is energy. If a reaction forms gas, then increased pressure will force the equilibrium towards in whatever direction that lowers the pressure.

    That's basic high-school chemistry, really.
  4. Jun 4, 2009 #3
    Do you mean that you have tried with a 2 liter coke bottle at controlled temperature? If the T is not controlled, then it's normal that in a small container the T don't rise as much as in a larger one (because of the ratio volume/container surface)
  5. Jun 4, 2009 #4
    We are using metal piping with a volume of .03 L. Is it that the volume is smaller so therefore we are not getting enough temperature out of the reaction and therefore not getting enough pressure? We can get the temperature of the reaction to about 100 degrees Celsius through calorimetry and we can feel the temperature increase inside the metal piping but we cannot get nearly enough pressure to launch a projectile (We are creating a chemical based propellant for our cannon in our high school Chemistry class).
  6. Jun 4, 2009 #5


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    In the case where gas is generated ... yes , there may be more instances , some physical processes such as boiling point depend on the pressure since the liquid needs to expand for cavities - or bubbles - to arise. We once had a massive hydrogen peroxide incident at the workplace within production and wanted to replicate the situation in the lab for investigation ... the lab demo was not on the scale of what happened simply because in the latter situation the tank was closed and the heat was confined within the tank.
  7. Jun 9, 2009 #6


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    Hydrogen can easily pass through very tiny pores, so maybe there are losses. H2 molecules can pass even through metal, and it dissolves in most substances.

    How much water an magnesium is in that 3ml container anyway?
  8. Jul 3, 2009 #7
    In case of an increase in pressure, the equilibrium will, according to Le Châtelier counter this increase in pressure. It does so by shifting to the side with the least particles
    This is because less particles cause less collisions and thus less pressure?

    But according to p = nRT / V an increase in pressure can also be countered by a decrease in temperature, so will an equilibrium also shift to the endothermic side?

    By the way, it seems to be a general nature law (Le Châtelier, Lenz, ... ) that a sytem will try to counter change, why is that?
  9. Jul 4, 2009 #8


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    System has some amount of internal energy that it can spend during reaction - the harder it gets, the shorter it can proceed.

    That's what (important part of) thermodynamics is about.
  10. Jul 5, 2009 #9


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    This reaction only happens vigorously if you have hot water or steam, so this may be the reason you are getting very little hydrogen.

    magnesium reacts with water at room temperature, though it reacts much more slowly than calcium. When it is submerged in water, hydrogen bubbles will almost unnoticeably begin to form on the surface of the metal, though if powdered it will react much more rapidly. The reaction will occur faster with higher temperatures.
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