In a runaway reaction, the rate of heat generation exceeds the rate at which heat can be removed.

• A. Rate of heat generation exceeds heat removal
• B. Pressure decreases
• C. Temperature remains constant
• D. Reaction stops

Answer: (A)

The main idea is heat balance in an exothermic process. A runaway reaction happens when the heat generated by the reaction cannot be removed fast enough by the cooling system. That shortfall creates a temperature rise, and as the temperature climbs, the reaction rate often increases, generating even more heat in a positive feedback loop. That’s why the situation is described as the rate of heat generation exceeding the rate of heat removal.

So the best statement is that the rate of heat generation exceeds heat removal. If cooling can’t keep up, temperature rises, not remains steady, and the reaction tends to accelerate rather than stop.

Why the other ideas don’t fit: a decrease in pressure isn’t characteristic of runaway; heat buildup and possible gas generation typically raise pressure or at least do not cause a drop. Temperature staying constant would require heat removal to match generation; in runaway, removal is insufficient, so temperature rises. A runaway doesn’t imply the reaction stops; it implies uncontrolled, accelerating reaction due to insufficient heat control.