The sign of ΔH: exothermic vs endothermic
Negative ΔH means heat leaves the system — exothermic.
The common mistake
Treating a negative ΔH as heat being absorbed, or writing a positive ΔH for a reaction that warms its surroundings.
Instead: ΔH is from the system's point of view. Heat leaving the system makes ΔH negative (exothermic, surroundings warm up). Heat entering makes ΔH positive (endothermic, surroundings cool down).
Worked example with Guide me
A reaction in a beaker makes the water get hotter. Is ΔH positive or negative?
- Sugar · Where is the heat going?Student · From the reaction into the water.
- Sugar · So is the system gaining or losing heat?Student · Losing it.
- Sugar · What sign does losing heat give ΔH?Student · Negative.
Answer: ΔH is negative — the reaction is exothermic.
Try 3 practice questions
1. An ice pack gets cold when activated. Sign of ΔH?
2. Combustion has ΔH = −890 kJ/mol. Exothermic or endothermic?
3. If A → B has ΔH = +40 kJ, what is ΔH for B → A?
Quick answers
- What is the most common mistake with the sign of δh: exothermic vs endothermic?
- Treating a negative ΔH as heat being absorbed, or writing a positive ΔH for a reaction that warms its surroundings.
- How do I get the sign of δh: exothermic vs endothermic right?
- ΔH is from the system's point of view. Heat leaving the system makes ΔH negative (exothermic, surroundings warm up). Heat entering makes ΔH positive (endothermic, surroundings cool down).
- Can you show a worked example?
- A reaction in a beaker makes the water get hotter. Is ΔH positive or negative? ΔH is negative — the reaction is exothermic.
Related concepts
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