Exothermic vs Endothermic Reactions: Sign of ΔH and Energy Profiles
Chemistry · Thermodynamics · NEET
An exothermic reaction gives out heat, so the products have less energy than the reactants and ΔH is negative (minus). An endothermic reaction takes in heat, so the products have more energy than the reactants and ΔH is positive (plus). Memory hook: "Exo = Exit heat = ΔH minus; Endo = Enter heat = ΔH plus."
Left: exothermic reaction, products lower than reactants, so ΔH is negative (heat released). Right: endothermic reaction, products higher than reactants, so ΔH is positive (heat absorbed). The hump in each is the activation energy and does not change the sign of ΔH.
Your doubts, answered
Is ΔH positive or negative for an exothermic reaction?
For an exothermic reaction ΔH is negative (a minus number). The reaction lets heat leave the system into the surroundings, so the system loses energy. Since ΔH = H(products) − H(reactants), and the products end up with less energy, the answer comes out negative. Example: burning of methane, ΔH = −890 kJ/mol.
Why is ΔH negative when heat is released? It feels backwards.
Think from the reaction's point of view (the system), not yours. When heat exits the system, the system's stored energy goes down. A drop in energy is written as a negative change. So even though you feel the flask get hot, the system itself has lost energy, and that loss is shown by the minus sign in ΔH.
In an energy profile, are the products higher or lower than the reactants?
In an exothermic reaction the products are drawn LOWER than the reactants, because energy was released. In an endothermic reaction the products are drawn HIGHER than the reactants, because energy was absorbed. The up-and-down gap between reactant level and product level is ΔH.
What is the difference between ΔH (enthalpy of reaction) and the activation energy hump?
They are two different heights. The hump (activation energy, Ea) is how high the curve climbs before falling to the products. ΔH is only the difference between the START level (reactants) and the END level (products). NEET often gives both numbers to trick you, but the sign of ΔH depends only on reactant vs product levels, not on the hump.
How do I quickly pick the right energy diagram in a NEET MCQ?
Read the sign of ΔH first. If ΔH is negative (like −74.8 kJ/mol), pick the diagram where products sit BELOW reactants. If ΔH is positive, pick the diagram where products sit ABOVE reactants. Ignore how tall the barrier looks until you have already used the sign.
Is bond forming exothermic or endothermic?
Forming a bond releases energy, so it is exothermic (ΔH negative). Breaking a bond needs energy, so it is endothermic (ΔH positive). This is why a reaction like 2Cl(g) → Cl₂(g), which makes a new Cl–Cl bond, has ΔH < 0.
⚠️ The NEET trap ✗ The reaction has a large activation hump, so ΔH must be positive (endothermic). ✓ The size of the activation hump does not decide the sign of ΔH. ΔH is only reactant level minus product level. A reaction can have a tall barrier and still be exothermic if the products end up below the reactants. 🧠 Hump = how hard to start; ΔH = where you finish. Compare only the two flat ends.
Real NEET questions
NEET 2021
For a reaction A → B, the enthalpy of reaction is −4.2 kJ/mol and the enthalpy of activation is 9.6 kJ/mol. Which potential energy profile is correct?
A · A at low energy, curve rises to a high peak, B higher than A (endothermic, large barrier)
B · A at low energy, curve rises over a barrier, B higher than A (endothermic)
C · A and B at roughly the same level with a barrier in between (thermoneutral)
D · A at higher energy, small barrier, B lower than A (exothermic, ΔH negative) ✓
Solution: ΔH = −4.2 kJ/mol is negative, so the reaction is exothermic and product B lies BELOW reactant A. The curve first climbs by the activation enthalpy (9.6 kJ/mol) to the peak, then falls to B at a level lower than A. Only option D shows products below reactants, so D is correct. Notice the large activation value does not make it endothermic.
NEET 2025
C(s) + 2H₂(g) → CH₄(g); ΔH = −74.8 kJ/mol. Which energy diagram accurately represents this reaction? (R = reactants, P = products)
A · R and P at nearly the same level, P only marginally below R
B · P at higher energy than R (endothermic)
C · R higher, an activation hump, P lower than R (exothermic) ✓
D · P at a higher peak than R
Solution: ΔH = −74.8 kJ/mol < 0, so the reaction is exothermic: energy is released and the products (CH₄) sit lower than the reactants. The correct picture shows R higher, a hump for activation energy, and P clearly below R. That is option C. The released energy equals 74.8 kJ/mol.
NEET 2020
For the reaction 2Cl(g) → Cl₂(g), the correct option is:
A · Δ𝑟H < 0 and Δ𝑟S > 0
B · Δ𝑟H < 0 and Δ𝑟S < 0 ✓
C · Δ𝑟H > 0 and Δ𝑟S > 0
D · Δ𝑟H > 0 and Δ𝑟S < 0
Solution: Two chlorine atoms join to form a Cl–Cl bond. Forming a bond releases energy, so the reaction is exothermic: Δ𝑟H < 0. Also two gas particles become one gas particle, so disorder falls and Δ𝑟S < 0. Both are negative, giving option B. This shows bond formation is always exothermic.
Solved Thermodynamics NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the sign of ΔH for exothermic and endothermic reactions?
Exothermic reactions have a negative ΔH (heat is released). Endothermic reactions have a positive ΔH (heat is absorbed). This single rule answers most NEET questions on this topic.
Does exothermic mean the products are more stable?
Usually yes. Lower energy products are generally more stable, and exothermic reactions form lower-energy products. That is why many exothermic reactions happen easily once started.
Can a reaction be exothermic but still not start on its own?
Yes. Even an exothermic reaction may need activation energy to begin (like a spark to light petrol). The sign of ΔH tells you the energy change from start to end, not whether it starts by itself.
Is melting ice exothermic or endothermic?
Melting ice is endothermic. It absorbs heat from the surroundings to break the solid structure, so ΔH is positive. Freezing water is the reverse, so it is exothermic.
Why does NEET give both ΔH and activation energy in one question?
To test if you can separate the two. Activation energy is the barrier height; ΔH is only the difference between reactant and product levels. Use ΔH's sign to place products above or below reactants.
