# 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 ?

• For the rate expression we use concentrations instead of activities. But, the of the rate constant unit is (moles/litre-sec) only when you have a first order reaction. – Yomen Atassi Mar 25 '16 at 6:38