TL;DR:
Exothermic processes release heat to the surroundings (ΔH is negative). Endothermic ones absorb it (ΔH is positive).
Breaking bonds takes energy in. Forming bonds gives energy out. The balance decides the sign.
In an example cold pack,
q = m c ΔTgives about 3.2 kJ absorbed by 0.125 mol of dissolving salt, so ΔH is about +26 kJ/mol.
Squeeze an instant cold pack and it turns icy within seconds. Crack open a hand warmer and it heats up. Both are chemistry doing the opposite thing with energy. The two words for it, endothermic and exothermic, are easy to mix up, so it helps to see what they mean in terms of energy.
Heat in, heat out
Chemists split the universe into the system (the reacting chemicals) and the surroundings (everything else, including the water in the pack and your hand). Energy moves between them as heat.
Exothermic: the system releases heat. The surroundings get warmer.
ΔHis negative.Endothermic: the system absorbs heat. The surroundings get colder.
ΔHis positive.
The symbol ΔH is the enthalpy change, which for reactions at constant pressure is the heat exchanged. The sign is from the system's point of view: heat leaving the system counts as negative. A tip for memory: exo means out, endo means in.
Why it happens: bonds breaking and forming
Breaking a chemical bond takes energy, because you must pull atoms apart. Forming a bond releases energy, because atoms settle into a more stable arrangement. A reaction does both.
If the energy released by forming the new bonds is more than the energy spent breaking the old ones, the reaction is exothermic. If it is less, the reaction is endothermic. Burning methane is a clear exothermic case, with ΔH ≈ −890 kJ for each mole of methane burned.
An energy diagram plots energy against progress of the reaction. In an exothermic reaction the products sit below the reactants and the difference is |ΔH|. In an endothermic one the products sit above. In both, there is a hump in between, the activation energy, as described in what speeds up a chemical reaction.
Worked example: an instant cold pack
Many cold packs hold water and a bag of ammonium nitrate, NH₄NO₃. Breaking the bag lets the salt dissolve, and dissolving absorbs heat. Say you dissolve 10.0 g of NH₄NO₃ in 100 g of water, and the temperature falls by 7.0 °C. Treat the solution as 110 g with the specific heat of water, 4.18 J/(g·°C). These are example figures that match typical behaviour.
Heat taken from the water:
q = m c ΔT = 110 × 4.18 × 7.0 = 3218 J, about 3.2 kJ.Moles of salt: the molar mass of NH₄NO₃ is
14.01 × 2 + 1.008 × 4 + 16.00 × 3 = 80.05 g/mol, so10.0 ÷ 80.05 = 0.125 mol.Heat absorbed per mole:
3218 J ÷ 0.125 mol = 25,700 J/mol, about 26 kJ/mol.Sign: heat was absorbed by the system, so
ΔH = +26 kJ/mol.
Check the direction: the solution got colder, which means the surroundings lost heat to the dissolving salt, so the process is endothermic and ΔH is positive. Check units: g × J/(g·°C) × °C = J.
A hand warmer is the reverse. Some use iron powder reacting with oxygen from the air, which is a form of rusting (see oxidation and reduction). It is exothermic, so the pack warms up. Others dissolve a salt that releases heat as it dissolves.
Packs are not toys: Do not cut open cold packs or hand warmers, mix their contents or heat them. The contents can irritate skin and eyes and are harmful if swallowed. Do not put a hand warmer directly on bare skin for long, since it can burn. Keep them away from young children.
Here is a rough bond-energy picture. For H₂ + Cl₂ → 2 HCl, say breaking one mole of H-H takes 436 kJ and one mole of Cl-Cl takes 243 kJ, a total of 436 + 243 = 679 kJ in. Forming two moles of H-Cl releases 2 × 431 = 862 kJ. The net is 679 − 862 = −183 kJ, negative, so the reaction is exothermic. Bond energies are averages from data tables, so such estimates are approximate.
How to check your answer
Temperature test. If the surroundings got colder, the process is endothermic with positive ΔH. If they got warmer, it is exothermic with negative ΔH.
Per mole. Divide by moles of the reacting substance, not grams.
Units. Convert J to kJ at the end if the question asks for kJ/mol.
Sign of q. The heat gained by the water is the negative of the heat gained by the reaction.
Mistakes that cost marks
Mixing up the sign: an exothermic reaction has negative ΔH, even though the surroundings get hotter.
Thinking exothermic reactions need no energy to start. Many need a spark or heat to get over the activation energy.
Saying that breaking bonds releases energy. It absorbs energy.
Using the mass of the salt alone, instead of the mass of the whole solution, in
q = mcΔT.Quoting ΔH without units or without 'per mole'.
Where you will use it next
Enthalpy leads to Hess's law, bond energies and spontaneity. The heat arithmetic is the same as in heat vs temperature, and the way temperature shifts an equilibrium is in Le Chatelier's principle with examples.
Frequently asked questions
Is melting ice endothermic or exothermic?
Endothermic. Ice absorbs heat from its surroundings to break the attractions holding the solid together, which is why it cools your drink.
Does a negative ΔH mean a reaction happens by itself?
Not always. Whether a reaction proceeds also depends on rate and on entropy, which you meet later in thermodynamics.
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