For aA + bB -> cC + dD: rate = -(1/a)d[A]/dt = (1/c)d[C]/dt. Rate is always positive. The negative sign accounts for decreasing reactant concentration. Instantaneous rate = slope of tangent to concentration-time curve. Average rate = delta[concentration]/delta_t (over a time interval). The rate of a reaction decreases with time (for most reactions) because reactant concentration decreases. Important: the rate of disappearance of A and rate of disappearance of B are NOT equal unless a = b. Always divide by the stoichiometric coefficient to get the overall reaction rate. For 2N2O5 -> 4NO2 + O2: rate = -(1/2)d[N2O5]/dt = (1/4)d[NO2]/dt = d[O2]/dt.