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B. Cl₂ + NaBr → NaCl + Br₂

Explanation:

The single replacement chemical equation that is unbalanced is Cl₂ + NaBr → NaCl + Br₂.

A look at the equation and you will discover that the chlorine on the reactant side is not balanced with the one on the product side. The chlorine at the reactant side has 2 atoms of chlorine but just 1 atom of chlorine in the product side. Therefore, it is unbalanced.   Also, you will discover that the bromine on the reactant side is not balanced with the one at the product side. The bromine on the reactant side has 1 atom of bromine while at the product side, it has 2 atoms. Therefore, it is unbalanced.

For a chemical equation to be balanced, the number of atoms and molecules at the reactant side must be equal to the number of atoms and molecules at the product side.  A single replacement reaction takes place whenever there is a replacement of one element by another element in a compound during a chemical reaction.

definition

                    entropy is a measure of the randomness or the extent of disorder of a chemical process.

                      enthalpy is a measure of the heat change of a reaction occurring at a constant pressure.

measurement units:

                        entropy is measured in joule per kelvin (jk⁻¹)   whereas enthalpy is measured in joule per mole (jmol⁻¹)

requirements:

                            entropy has no requirements or limits, and its change is measured by the division between the heat change of the chemical process and the temperature.

                              in contrast, enthalpy can only be related to the heat change of the reaction under standard conditions.