Answer and Explanation: The types of reaction represented in the question are the following: S is Synthesis; D is Decomposition; SD is Single Displacement; and DD is Double Displacement.
The reaction showed is: HI ⇄ H₂ + I₂
This reaction's type is <u>Decomposition</u>, because it is a reaction in which a compund breaks down into 2 or more simpler compounds. Most of them requires energy that could be in form of heat, light or electricity.
To balance a chemical equation, remember that, according to the <u>Conservation</u> <u>of</u> <u>Mass</u> <u>Law</u>, matter can not be created or destroyed just transformed. Then, the amount of reagents that enters the reaction has to be same amount that reacts to become the products, i.e.:
For the decomposition reaction, there are 2 atoms of hydrogen and 2 atoms of iodine being produced while only 1 atom of each element enters the reaction.
The balanced reaction will be: 2HI ⇄ H₂ + I₂