Changing the concentration of reactants in a chemical reaction can significantly affect the reaction rate. In the case of the reaction between hydrochloric acid HCl and magnesium Mg , the reaction can be represented by the following balanced equation:Mg s + 2 HCl aq MgCl2 aq + H2 g According to the collision theory, the reaction rate depends on the frequency of collisions between the reacting particles and the fraction of collisions that have enough energy to overcome the activation energy barrier. When the concentration of reactants is increased, the number of particles per unit volume also increases. This leads to a higher frequency of collisions between the reacting particles, which in turn increases the reaction rate.In the case of the reaction between HCl and Mg, if the concentration of HCl is increased, there will be more HCl molecules available to collide with the magnesium atoms, leading to a faster reaction rate. Similarly, if the concentration of HCl is decreased, there will be fewer HCl molecules available to collide with the magnesium atoms, resulting in a slower reaction rate.However, it is important to note that other factors, such as temperature, pressure, and the presence of catalysts, can also affect the reaction rate. In practice, the relationship between concentration and reaction rate can be more complex and may not always be directly proportional. Nonetheless, in general, increasing the concentration of reactants will lead to an increase in the reaction rate, while decreasing the concentration will result in a decrease in the reaction rate.