Out of ${Rb}$ and ${Sr}, {Sr}$ will have greater first ionisation enthalpy because of its small size.
As we move from left to right in the periodic table, the atomic size decreases due to increase in effective nuclear charge and more energy is required to remove electron from the outermost shell.
Out of ${Cl}$ and ${Br}, {Cl}$ will have greater ionisation enthalpy because of its small atomic size.
As we move down a group in the periodic table the atomic size increases due to addition of an extra valence shell so, it is easier to remove an electron from the outermost shell.
