Do we use activities or concentrations in the rate expression? The equilibrium constant of a reaction is a dimensionless quantity due to the fact that we use activities instead of concentrations(in moles/liter).
But, in the rate expression for a reaction, do we still use activities?

If this is so, then isn't the unit of the rate constant always (moles/litre-sec), since these are the dimensions of the rate of a reaction ?
The rate expression is always described in terms of molar concentrations. Units of the rate constant will change based upon the overall order of the reaction. This happens such that the rate is always given in M/s.