lesson
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If you squeeze a reusable hand warmer, it crystallizes and gets hot; boil it in water later, and it re-absorbs that heat to reset.
Every reversible chemical reaction works on this two-way street of heat exchange — but how do you know which direction releases heat and which absorbs it?
Decoding Enthalpy Change
The enthalpy change (written as ΔH) is the overall amount of heat energy released or absorbed during a chemical reaction at constant pressure.
An exothermic reaction gives off heat to its surroundings and has a negative value (−ΔH), while an endothermic reaction absorbs heat from its surroundings and has a positive value (+ΔH).
📊A side-by-side interactive comparison diagram of enthalpy change signs. Left side: Exothermic reaction card with title 'Exothermic (-ΔH)', showing reactants at higher energy level stepping down to lower energy products, with orange flame arrows pointing outward labeled 'Heat Released to Surroundings'. Right side: Endothermic reaction card with title 'Endothermic (+ΔH)', showing reactants at lower energy level stepping up to higher energy products, with blue arrows pointing inward labeled 'Heat Absorbed from Surroundings'. Clean styling, crisp typography (#1e2945), rounded 12px corners, responsive layout.
In standard chemical notation, any ΔH written next to a reversible reaction equation always refers to the forward reaction (left to right).
What does this tell us about the reverse reaction travelling in the opposite direction?
The Reverse Sign Rule
In 1840, chemist Germain Hess showed that energy changes in chemical pathways are conserved, establishing that energy cannot simply vanish during a reaction.
Because energy is conserved, the reverse reaction must exchange the exact same quantity of energy as the forward reaction, but in the opposite direction.
📊A visual diagram showing a reversible reaction pathway. In the center: A reversible reaction arrow (left-to-right top arrow labeled 'Forward: -92 kJ/mol (Exothermic)' colored warm orange; right-to-left bottom arrow labeled 'Reverse: +92 kJ/mol (Endothermic)' colored cool blue). Below the arrows, a balance scale showing that magnitude is identical (|ΔH| = 92 kJ/mol) while signs are strictly inverted (+ vs -). Clean modern card UI (#ffffff) with dark blue text (#1e2945) and border (#e6e6e6).
If the forward reaction is exothermic, the reverse reaction must be endothermic by the exact same numerical value.