Bromine liquid reacts with sodium iodide to produce iodine and sodium bromide. As in all equations, it is important they be balanced out. It is not accurate to write it in the form, Br + NaI → NaBr + I. Reactants and products need to be written accurately, then the equation needs to be considered as a whole. This process is called stoichiometry
Balancing the Equation
Bromine actually occurs in the form of a diatomic molecule. That term means two atoms, so bromine is written Br₂. The same is true of iodine. It is written I₂. Plugging these results in our initial equation results in Br₂ + NaI → NaBr + I₂. Although this equation is slightly closer to reality, it is still inaccurate.
There are two atoms of bromine and one atom of iodine on the left side of the equation and one atom of bromine and two atoms of iodine on the right side. Further balancing is required. The next adjustment, the final result, is: Br₂ + 2 NaI → I₂ + 2 NaBr. All quantities balance; the reaction is accurately explained.
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