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.

You:
DEF is deasil exaust fluid from walmart

Do not put DEF on your lawn. This is a bad idea, and here's why:

What DEF actually is

Diesel Exhaust Fluid is 32.5% high-purity urea + 67.5% deionized water. It's designed to be injected into diesel truck exhaust systems (SCR systems) to reduce NOx emissions — it is not a fertilizer product, even though urea is also used in fertilizers.

Why people get tempted

Urea (46-0-0) is a legitimate nitrogen fertilizer, so the logic seems to track: "It's just urea and water, right?" But the math is brutal:

The math on 2.5 gallons over 5,000 sq ft

• 2.5 gal DEF ≈ ~21 lbs of solution
• At 32.5% urea → roughly 6.8 lbs of urea
• Urea is 46% nitrogen → about 3.1 lbs of actual N
• Over 5,000 sq ft, that's ~0.62 lbs N per 1,000 sq ft

That N rate by itself isn't catastrophic (normal app is 0.5–1.0 lb N/1,000 sq ft). But the problems are:

1. Foliar burn. DEF is a concentrated 32.5% solution. Spraying or pouring it directly on grass blades will scorch them like spilling straight liquid fertilizer. Urea fertilizer solutions for turf are typically diluted to 3–10% and watered in immediately.
2. Uneven application. 2.5 gal spread over 5,000 sq ft is only ~0.5 oz per sq ft — nearly impossible to apply evenly without a calibrated sprayer, leading to striping and dead patches.
3. Not agricultural grade. DEF is manufactured to ISO 22241 purity specs for engines, not for plant/soil safety. It's not labeled, tested, or approved for turf use.
4. Volatilization losses. Surface-applied urea without immediate watering can lose 20–40%+ of its N to the atmosphere as ammonia gas.

What to do instead

• Return the DEF to Walmart, or save it for your truck.
• For ~5,000 sq ft, buy an actual lawn fertilizer. A bag of urea (46-0-0) is cheap (~$20–30 for 50 lb at a farm/co-op store) — you'd apply roughly 2 lbs of urea per 1,000 sq ft (= ~10 lbs over your lawn) for a ~1 lb N/1,000 application, spread with a broadcast spreader and watered in.
• Or grab a standard bagged lawn fertilizer (Scotts, Lesco, Anderson's, etc.) sized for 5,000 sq ft — the bag tells you the spreader setting.

What grass type and region are you in? I can recommend a specific fertilizer + timing if you tell me that.