When ethyl chloride is added to Sodium tert-butoxide, ethyl tert-butyl ether is obtained as the major product (shown below):
When the concentration of ethyl chloride is doubled, the reaction rate:
doubles
Clearly, the reaction is an SN2 – a bimolecular reaction. Since no further data is given, let us assume an order of 2, so the rate expression is:
Reaction rate=[(CH3)3O−]×[C2H5Cl]
Clearly, the rate would double if concentration of ethyl chloride is doubled.