How do you know whether or not a reaction would occur

For the first 3, there is supposed to be no reaction, but can anyone tell me WHY? And how come there ARE reactions for the last 3?

i. I2 + 2Cl- → N.R.
ii. I2 + 2Br- → N.R.
iii. Br2 + 2Cl- → N.R.
iv. Br2 + 2I- → 2Br- + I2
v. Cl2 + 2Br- → 2Cl- + Br2
vi. Cl2 + 2I- → 2Cl- + I2

Have a look at the halogen group on the periodic table That is Group 17.
The most reactive halogen is at the top , and the least reactive halogen is at the bottom
Highly reactive halogens can replace less reactive halogens.

Therefore : In reaction i)
I2 is below Cl in the group so it cannot replace Cl - no reaction

Reaction ii)
Similarly : I2 is below Br - no reaction

Reaction iii) Br2 is below Cl - no reaction

Reaction iv)
Br is above I - can displace I so reaction takes place

Reaction v) Cl is above Br - can displace Br - reaction can take place

Reaction vi) Cl is above I - so it can displace I - reaction can take place.

This is due to the electrochemical Series.
The first three don't occur because it has do with the oxidising power of these elements.
The oxidising power of halogens(Group 17) decreses down the group so that's the reason why this is happening

Because there was no change in the molecules in the first 3 while the type of molecules changed in the last three