For a first-order reaction, the reaction rate is directly proportional to the concentration of the reactant. This means that if you increase the concentration of the reactant, the reaction rate will also increase, and if you decrease the concentration of the reactant, the reaction rate will decrease.Mathematically, this relationship can be represented by the rate law equation for a first-order reaction:Rate = k[A]where Rate is the reaction rate, k is the rate constant, and [A] is the concentration of the reactant. As you can see from the equation, the reaction rate depends on the concentration of the reactant raised to the power of 1 hence, first-order . So, any change in the concentration of the reactant will have a direct impact on the reaction rate.