You:
should i use the entire 2.5gal def on my 5k sq feet lawn

Before I can give you a safe answer, I need to clarify a couple things — getting this wrong could damage your lawn or waste a lot of product.

What is "DEF"? That's not a standard lawn care abbreviation I recognize. A few possibilities:
- A typo for a specific product (e.g., Spectracide Weed Stop, Ortho Weed B Gon, Tenacity, Roundup for Lawns, etc.)
- A regional brand name
- Diesel Exhaust Fluid — which absolutely should not go on a lawn (it's urea-based but at industrial concentrations that will burn/kill turf)

A few questions so I can advise properly:
1. What's the exact product name on the label?
2. Is it a concentrate (needs dilution with water) or ready-to-use (RTU, spray straight from the jug)?
3. What does the label say for application rate (e.g., "X oz per 1,000 sq ft" or "X oz per gallon")?
4. What grass type do you have, and what are you trying to treat (weeds, fungus, insects, fertilizing)?

General rule of thumb: Almost no lawn chemical should be applied at 2.5 gallons of concentrate on 5,000 sq ft — that would be a massive overdose. If it's a ready-to-use product, 2.5 gallons typically only covers a few hundred to ~1,000 sq ft of spot treatment, not a whole 5k lawn.

Share the product name (or a photo of the label info) and I'll calculate the exact amount you need.