Which expression represents a reaction rate?

The expression that represents and calculates a reaction rate is defined as a change in the reactant by the change in time. A reaction rate signifies how slow or fast a reaction takes place over time.

The rate of reaction of any reactant is always positive. However, this rate decreases consistently until an equilibrium is reached where there is no net change in the concentration of the reactants. It is at this point that the reaction appears to stop unless more concentrated substance is added into the reaction. Consistent negative values indicate that the reaction rate is decreasing, while positive values denote an increase in the rate.