The reaction rate between iron and hydrochloric acid solution will decrease when

The speed of a chemical reaction depends on the nature of the reacting substances.
The reaction rate increases with increasing temperature, increasing the concentration of starting materials (for gaseous and dissolved reagents), increasing the contact area of ​​the reagents (for heterogeneous reagents – which are in different phases, for example, liquid and solid; solid and gaseous), with increasing pressure (for gaseous reagents). Also, the reaction rate increases under the influence of catalysts – substances that accelerate the reaction, but are not part of the reaction products.
For these aggregate states of the reactants, a decrease in the temperature and concentration of the reactants (hydrochloric acid) contributes to a decrease in the reaction rate.