The increase in temperature of an exothermic reaction decreases the amount of product.
The question is incomplete but the statement seems to refer to an exothermic reaction. An exothermic reaction is one in which increase in temperature favors the reverse reaction.
If the Cl2 is the product gas and its mole fraction decreases as temperature increases then the reaction is exothermic.
Learn more about exothermic reaction: https://brainly.com/question/14969584