Rate of a reaction decreases with the passage of time as the concentration of reactants decrease. Conversely, rates generally increase when reactant concentrations increase. So, rate of a reaction depends upon the concentration of reactants. 

Consider a general reaction

aA + bB → cC + dD

where a, b, c and d are the stoichiometric coefficients of reactants and products.

The rate expression for this reaction is

Rate ∝ [A]x [B]y

where exponents x and y may or may not be equal to the stoichiometric coefficients (a and b) of the reactants.

Above equation can also be written as Rate = k [A]^x [B]^y

-\(\frac{dR}{dT}\) = k [A]^x [B]^y

This form of equation is known as differential rate equation, where k is a proportionality constant called rate constant.

-\(\frac{dR}{dT}\) in the rate equation refers to which of the following?

Decrease in concentration with time

-\(\frac{dR}{dT}\) in the rate equation refers to decrease in concentration with time.