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There are 3·2 = 6 H and 1·2 = 2 N on the left. These are combined as 1N+3H on the right, so the sum needs to be multiplied by 2 to make the numbers match. (6H+2N) = 2(3H+N)
3H₂ + N₂ ⇔ 2NH₃
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