A chemical reaction involves the chemical transformation of reactants into products. As the reaction progresses, the concentration of reactants decreases while the concentration of products increases. This variance in concentrations of reactants and products can be plotted in a graph as a function of time.

The speed at which a reaction progresses is called the reaction rate. It measures the rate of reactants’ disappearance or the rate of products’ appearance and is expressed in units of molarity-per-second. 

The average reaction rate can be calculated from the change in concentrations of reactants or products during a specific period of time. The molar concentration value is denoted in brackets, t stands for the period of time, and the delta symbol represents ‘change in’.

Since reactants are depleted during a chemical reaction, the value of concentration change of reactants is always negative. Therefore, a reaction rate calculated in terms of reactants is supplemented by a negative sign, to make the overall value positive.

The reaction rate is not uniform throughout a reaction. The rate at the start of the reaction, at ‘time-zero’, is called the initial reaction rate. As the concentrations of reactants reduce, the reaction rate decreases, or the reaction slows down.

The reaction rate at a given time point, or the instantaneous reaction rate, is measured by calculating the slope of a tangent drawn to the reaction curve, for either reactants or products, at the time of interest.

The slope value or the instantaneous rate at a specific time-point is equal for all reactants and products.

The reaction rate of a chemical reaction also reflects the actual stoichiometric coefficients of reactants and products.

So, for any balanced reaction, where a moles of A reacts with b moles of B, producing c moles of C and d moles of D, the reaction rate can be expressed and calculated using the generic formula: −1/a × Δ[A]/Δt = −1/b × Δ[B]/Δt. These values are the same as +1/c × Δ[C]/Δt, which is the same as +1/d × Δ[D]/Δt.

Determining reaction rates is fundamental to studying chemical kinetics, which helps to understand how quickly a drug, catalyst, or a synthetic reaction works so that it can be better controlled and optimized in its function.