I5 processor is second generation or first?


For most models:
nnn = 1st gen
2nnn = 2nd gen (SB)
3nnn = 3rd gen (IB)

Applies to i3, i5 and i7 except the LGA2011 i7 where Intel decided to introduce an unnecessary discrepancy in their numbering scheme by making SB-E i7 chips 38xx/39xx instead of 28xx/29xx for whatever reason.
The i5 comes in first, second, and third gen forms. The first, the core i5, the second is core i5 Sandy Bridge, and the third is Core i5 Ivy Bridge. The first gen uses socket LGA 1156 and is a 45nm chip, the second gen (Sandy Bridge) is a 32nm chip and uses LGA 1155, and the 3rd gen (Ivy Bridge) is a 22nm and uses the LGA 1155 platform like Sandy Bridge does.