Reaction between magnesium and oxygen

  1. Today in class my chem teacher was insistent that 2Mg + O2 = 2MgO was a synthesis reaction. this I agree with. however, he was also adamant that this was not a combustion reaction, when it our text book it specifically gave us this very same example as a type of combustion reaction. Question: is the text book right (it is a combustion reaction) or is the teacher right (its not)?
  2. jcsd
  3. Borek

    Staff: Mentor

    I consulted with a group of chemistry teachers, and in general they treat magnesium reaction with oxygen as combustion. If it is slow for any reason, it will be just corrosion, but it is fast enough.

    That being said, there is no precise definition of combustion, so there is some room for interpretation.
  4. epenguin

    epenguin 2,499
    Homework Helper
    Gold Member

    Yes you can define, but anyone who said that is not combustion would also have to say not many things are.

    I think it was with magnesium in one of my first school chemistry lessons I first heard the word.
  5. It has to be a combustion reaction and as such it is endothermic. Otherwise how will Magnesium react will oxygen ?

    2Mg+O2 + Δ ----------> 2MgO

    Δ is heat taken by Mg to react with O2.
  6. Borek

    Staff: Mentor

    I have a rusty nail here. Obviously iron reacted with oxygen. Does it mean it was combustion?
  7. Here is the reaction of formation of rust :

    Fe+H2O+O2 ------------> Fe2O3.xH2O

    Obviously its not a combustion reaction.
    But can ever the reaction given below occur without combustion ?

    2Mg+O2 ----------> 2MgO
    Last edited: Dec 8, 2011
  8. Borek

    Staff: Mentor

    At low oxygen pressures it will be impossible for the reaction to proceed fast, so you will be just seeing slow oxidation.

    Conditions are always important part of the reaction.
  9. Actually the oxidation/combustion reaction for MgO is an endo/exo thermic reaction. Its endothermic with the reactants.The total first and second ionization enthalpies for Magnesium's 3s2 electrons are H =
    + 742 and +1450 kilo Joules / mole
    And exothermic for the products.
    While the net overall reaction is exothermic, - 602 kilo Joules/mole ( by way of Hess's law )

    Im not sure if the heat of formation can be measured directly with a calorimeter in which case the + 2192 kilo Joules/mole + negative heat of formation should = - 602 kilo Joules/mole
    Last edited: Dec 9, 2011
  10. It can't be combustion. Iron gets rusty in presence of catalyst like CO2. A catalyst never affects combustion.
  11. Borek

    Staff: Mentor

    No, iron doesn't need a catalyst to get oxidized. Iron burns quite nicely in oxygen. If it is fine enough it will even burn just in air, it is called pyrophoric iron then.

    So what happens in catalytic converters?
    Last edited by a moderator: Sep 25, 2014
  12. Rust is Fe2O3.xH2O Because you mentioned rust, I said that it requires a catalyst and can't be combination.
  13. Borek

    Staff: Mentor

    In colloquial use rust means iron oxides and discussion is about oxidation and combustion. Technically you are right, but this is nitpicking - and all it does it confuses earlier posters even more, as they still have problems understanding basic facts. You are not helping.
  14. Okay.
    The reaction between Mg and O2 is a combustion reaction because we observe a flame(that's what I remember from my textbook)
    Every combustion is oxidation but every oxidation is not combustion.
  15. Borek

    Staff: Mentor

    That's where the answer to the question becomes not so obvious. Do we observe the flame? We observe bright light in the place where magnesium reacts with oxygen, and we observe some dust of MgO that looks like smoke (and is quite bright, as it is very close to the very bright source of light). But I am not convinced we see a flame like the one in the candle or the burning wood.

    And here we agree.
  16. That's what I intended to say as a flame.
  17. Borek is correct. Chemistry is not so exact. For instance , we say that copper cannot displace sulphuric acid. This statement is not correct. Copper cannot displace dilute sulphuric acid but can react with concentrated sulphuric acid.

    Cu + H2SO4(dilute) ---> No reaction
    Cu + H2SO4 (concentrated)--------->CuSO4 + H2O + SO2(g)

    If oxygen level and the atmospheric energy like sunlight , percussion etc and the flame you are providing Magnesium is low , then you will only see a bright light instead of flame.
    You cannot call this combustion. Can you ? Its not so obvious as most of people think it is.

    In smaller classes study is lot generalized but the conditions affecting it are neglected. In higher classes people deal with temperature, catalyst and other conditions affecting a reaction.

    For example

    CH4 + 2O2 ----> CO2 + 2H2O not always because in less oxygen supply it forms CO instead of CO2 and in further less supply it only forms Carbon instead of CO or CO2.

    I think I am giving too many instances. I apologize that so many examples may confuse the readers.
    Last edited: Dec 11, 2011
  18. As magnesium burns with temperatures far above its own boiling point I would expect a flame. But it would be hard to observe with naked eyes because it is too bright to see any details.
  19. AlephZero

    AlephZero 7,248
    Science Advisor
    Homework Helper

    Wikipedia gives the melting point of magnesium as 1091C and the flame temperature as 1900 to 2000C

    If you do the experiment on a big enough scale (e.g. start with 100kg of molten magnesium in a metal foundry) you can definitely create things that looks like flames, even when viewed throgh dark goggles. Incidentally, the standard way to stop molten magesium catching fire when making castings used to be to shovel enough sulphur over it to react with all the available oxygen, and rely on the convection currents caused by the hot gas to stop the SO2 killing the worker using the shovel :eek:
  20. That bright light is called flame. Because it is not hydrogen or oxygen or any other gas burning. It is magnesium. You can call it oxidation because if you dissolve the ash formed in water and put few drops of red litmus solution, it will turn blue, indicating the the substance has formed a basic oxide with oxygen in air.

    Instances have added to our knowledge.
    Last edited: Dec 12, 2011
  21. Borek

    Staff: Mentor

    No, light and flame are two different things. Flame is often a source of light, but it doesn't have to - hydrogen flame is almost invisible.

    I don't see a logical connection between the previous phrase and this statement.

    So it is not oxygen, but it is oxygen? Please pay attention to what you wrote, as you are again more confusing than helpful.
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