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The above reaction can be balanced in many ways. For example, the coefficient of $\ce{I2}$ is two when coefficient of $\ce{I4O9}$ is one. But the coefficient of $\ce{O3}$ and $\ce{O2}$ can be 5 and 3 respectively or 7 and 6 or 9 and 9. What will be correct balanced equation?

Rassank
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  • There is "formally correct" reaction equation and "reality reflecting" reaction equation. Formally correct are all where are balanced charge and atom counts. They can be formally even any linear combination of several formally correct equations. "Reality reflecting" correct equation must reflect observed stoichiometry, relating to observable chemistry and known reaction mechanisms. // All 4 provided reaction equations in Maurice's answer are formally correct, but only (1) is "reality reflecting" correct, if the assumed reaction mechanism is correct or if stoichiometry has been verified. – Poutnik Jan 19 '24 at 09:17

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The basic equation of the ozone reaction on iodine is $$\ce{2 I2 + 9 O3 -> I4O9 + 9 O2\tag{1}}$$ because it is the sum of the following two equations: $$\ce{2 I2 + 9 O -> I4O9}$$ and $$\ce{9 O3 -> 9 O2 + 9 O}$$ It should be mentioned that the following reaction never happens : $$\ce{O3 -> 3 O}$$ But this original equation ($1$) may be modified by adding one or many times the following equation :$$\ce{2O3 -> 3 O2}$$ which produces one of the following possible equations : $$\ce{2 I2 + 11 O3 -> I4O9 + 12 O2}$$ $$\ce{2I2 + 13 O3 -> I4O9 + 15O2}$$ $$\ce{2 I2 + 15 O3 -> I4O9 + 18 O2}$$ etc. These equations are all correct.

Ian Bush
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Maurice
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  • But @Maurice, how can a single equation be balanced in multiple ways? Should not there be a single unique equation which is the correct one? – Harikrishnan M Dec 12 '23 at 15:29
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    @HarikrishnanM, People write rules; the universe is not obligated to follow them. Consider nitrogen + oxygen, which forms various nitrogen oxides: https://en.wikipedia.org/wiki/Nitrogen_oxide – DrMoishe Pippik Dec 12 '23 at 16:33
  • Yes @DrMoishePippik I am familiar with that example. But same equation with different stoichiometric coefficients? – Harikrishnan M Dec 12 '23 at 16:55
  • I can form N2O5, NO2, N2O, N2O4 and so on... but I4O9 having many equations with different stoichiometric coefficients! Looks a bit new – Harikrishnan M Dec 12 '23 at 16:56
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    This addition of reactions look like a similar situation in elementary mathematics. Suppose you have two equations $\ce{f_1(x)} = 0$ and $\ce{f_2(x)} = 0$. You cannot prevent the following combinations to be true : $\ce{f_1(x) + f_2(x) = 0}$, or ; $\ce{3f_1(x) + 2f_2(x) = 0}$ – Maurice Dec 12 '23 at 17:47
  • You may want to use \tag{number} to enumerate or name equations. The way you're trying to do it breaks in many browsers, especially on mobile. – Martin - マーチン Dec 13 '23 at 02:20
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    I disagree with this a bit. Just adding another reaction, which may happen at the same time, to the initial (or minimal) reaction is questionable. If these are indeed linked, there would be, given postcode conditions, still only one stoichiometry coming out. At least in that end, there answer is but very satisfying. – Martin - マーチン Dec 13 '23 at 02:29
  • @HarikrishnanM https://chemistry.stackexchange.com/questions/58069/can-a-chemical-reaction-have-two-balanced-equations?noredirect=1&lq=1 – Nilay Ghosh Dec 13 '23 at 10:59
  • It is not a good idea to gratuitously add in side reactions to an equation unless perhaps they are somehow linked to the reaction mechanism ie I atoms somehow hijack an O3 in the process of its decomposition to O2. – jimchmst Dec 13 '23 at 20:27