The concentration of reactants can significantly affect the selectivity of a reaction between molecule X and molecule Y when carried out at different temperatures. Selectivity refers to the preference for the formation of one product over another in a chemical reaction. The factors that influence selectivity include reactant concentrations, temperature, and the presence of catalysts.1. Reactant concentrations: The concentration of reactants can influence the rate at which a reaction proceeds. According to the law of mass action, the rate of a reaction is proportional to the product of the concentrations of the reactants, each raised to the power of their stoichiometric coefficients. If the reaction between molecule X and molecule Y is competing with other reactions, a higher concentration of one reactant may favor the formation of a specific product, thus increasing the selectivity.2. Temperature: The effect of temperature on selectivity is related to the activation energy of the reaction. If the reaction between molecule X and molecule Y has a lower activation energy than competing reactions, increasing the temperature may favor the formation of the desired product. Conversely, if the activation energy is higher, a lower temperature may be more selective. The relationship between temperature and reaction rate is described by the Arrhenius equation, which states that the rate constant k is equal to the product of a pre-exponential factor A and the exponential of the negative activation energy Ea divided by the product of the gas constant R and the temperature T .3. Presence of catalysts: Catalysts can also influence the selectivity of a reaction by providing alternative reaction pathways with lower activation energies. If a catalyst selectively promotes the reaction between molecule X and molecule Y over competing reactions, the selectivity of the desired product will increase.In summary, the concentration of reactants, temperature, and the presence of catalysts can all influence the selectivity of a reaction between molecule X and molecule Y. To optimize selectivity, chemists often need to carefully control these variables and consider their interplay.