In an exothermic reaction the enthalpy change is
- A. negative, because the products have less enthalpy than the reactants
- B. positive, because heat is released
- C. always zero
- D. negative, because the products have more enthalpy than the reactants
Explanation
Energy leaves the system as heat, so the products end up at a lower enthalpy and the difference, products minus reactants, is negative. The sign convention is taken from the point of view of the system, which is why released heat carries a minus sign even though the surroundings get warmer. Combustion and neutralisation are the standard exothermic examples.
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