3.1.1 Average rates and instantaneous rates of chemical reactions 1.Average rate the change in concentration of a reagent over a certain time period A t 1 AcB 名△t 3.1.1 Average rates and instantaneous rates of chemical reactions 1. Average rate the change in concentration of a reagent over a certain time period Δ t 1 B B Δ Δ = t c ν υ