Reactions where all reactants are present from the start can lead to a runaway event with rapid changes in temperature and pressure. What term best completes this sentence?

• A. Endothermic
• B. Cooling
• C. Isothermal
• D. Exothermic

Answer: (D)

The key idea is that runaway events are driven by heat being released during the reaction. When all reactants are present from the start and the reaction releases heat (an exothermic process), the temperature of the system rises. That higher temperature speeds up the reaction rate, which releases even more heat, creating a positive feedback loop. In a closed or poorly cooled system, this rapid, self-accelerating heat generation can push temperature and pressure up quickly.

Endothermic would absorb heat and slow things down, cooling or isothermal conditions describe removing or maintaining temperature to prevent runaway, not describe the rapid heat build-up. So the term that best completes the sentence is exothermic.