To understand the potential-dependent kinetics of reactions at solid-liquid interface,we derive a constant-charge reaction theory for understanding the coupled charge transfer during the chemical bond making/breaking.The charge transfer coefficient(CTC)for reactions is shown to be linearly proportional to the electrochemical potential change from the initial state to the transition state as well the interface differential capacitance at the constant-charge model,and can be further related to the net dipole change normal to the surface during the reaction.Using the constant-charge theory,the CTC can be explicitly calculated based on the first principles calculations without the need to assume the redox behavior of the elementary reactions and thus provide a unique possibility to evaluate and compare the magnitude of CTC for different reactions across different surfaces..The role of surface dipole,solvation and molecular adsorption strength on the CTC can now be clarified from first principles calculations.