Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Combustion: where does the heat come from?

  1. Feb 9, 2012 #1

    I have some silly questions about combustion and the Wikipedia page (http://en.wikipedia.org/wiki/Combustion) doesn't explain all I would like to know. Can you please help to answer these questions?

    Q1. What exactly makes combustion an exothermic reaction?
    Is it because there is a tiny loss of mass which had been converted into energy under the form of light & heat?

    Q2. From the Chemical Equation: CH4 + 2 O2 → CO2 + 2 H2O
    What had happened between the atoms and molecules so that heat was produced?

    Q3. Put a sheet of paper on an anvil and slam on it with a hammer.
    In spite of the great supplement of energy added to the paper at the point of impact, the combustion could not be initiated. Why?

    Q4. What are the conditions for a material to be combustible?
    Burning a rock with a flame, like in a camp fire and the rock is just warmed up but still remains a rock. Ultimately, if I use higher temperature, the rock will probably melt but I guess it took more energy to melt the rock than the energy I could recuperate from the melting rock. So what are the condition to be a good combustible?

    Thanks in advance for any advice.
  2. jcsd
  3. Feb 9, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    2018 Award

    1. Yes, the molecule that is formed from combustion has less mass than the combined mass of the fuel particles before combustion.

    2. Not sure what you want to know exactly. The atoms have bound differently and released energy, as their previous state was less favorable for them to be in. AKA that means the new combination are more strongly bound than their previous ones were.

    3. A couple of different reasons. For one, there isn't that much energy applied to the paper. Second, the energy that is applied is spread out all over the impact point. Third, alot of the energy is simply passed through to the anvil or used to tear the paper up, not converted directly into heat.

    4. Combustion requires a chemical reaction to take place. Heating a rock does nothing but melt it because the atoms in the rock are already bound in stable forms and heating them up doesn't do anything. For example, many rocks contain Silicon Dioxide, which is Silicon bound with 2 Oxygen atoms. Since the atmosphere doesn't contain any other elements that are better than oxygen at forming bonds with silicon, heating the rock up does nothing to it. The best combustive chemicals are generally right before or after the Noble Gas column in the periodic table of elements. This is because their electron shells either need 1 electron to make them full or need to give up one electron to be full. For example, Oxygen, while being highly reactive with Hydrogen and other substances, is not as reactive as the next element Flourine, which is extremely electronegative and a strong oxide and occupies the column right before the Noble Gas column. Luckily flourine isn't found naturally in a free state and is always bound to something else.
  4. Feb 10, 2012 #3
    Is it correct to say that a supplement of energy in the form of heat must be supplied to start a combustion. This heat is used to split H and C from the HydroCarbons and allow them to be oxidized. The Oxidation is exothermic which allows the combustion to keep going.

    From your explanation, I understood that the H-C bonding had more energy than that of H-O and C-O bondings. If so, what is the reason for nature to bother to create less stable structure? Photosynthesis is probably the explanation for Earth as a kind of solar energy storage. But on Titan where there is a lake of methane, would it be easier for nature to form C-O and H-O bondings at the beginning of Titan rather than a lake of CH4 ?
  5. Feb 10, 2012 #4


    User Avatar
    Science Advisor
    Gold Member

    I don't think the E= mcsquared but really needs to come into this.
    The single step answer is surely that some of the Chemical Potential Energy of the Reactants is transferred to Kinetic Energy in the Product(s). Has anyone actually MEASURED the mass defect in a Chemical Reaction?

    Most exothermic reactions need some energy (activation energy) in order to initiate them. e.g. the match flame to get the bonfire going. Without the flame, the reactants can stay in contact for ever without anything happening. Even Nitroglycerine needs a bit of a physical nudge to make it go off.
  6. Feb 10, 2012 #5
    Combustion, fire, needs some energy to initiate it as noted above.
    But 'slow combustion' or plain old oxidation proceeds on it's own.

    Wikipedia confirms that 'chemcial reactions' involve electron exchanges which alter
    chemical bonds....so it seems like some electron orbital energy must change.

    The thermodynamics of chemcial reactions is explained here:

    I'm not familiar with details and no time now to follow up further..
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook