Enthalpy change is also known as heat of reaction, enthalpy of reaction, change in heat content.

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Enthalpy = total energy of a system

It is impossible to measure the total kinetic and potential energy of a system, so instead we measure enthalpy change (using calorimeters).

Enthalpy Change –  is the energy absorbed from or released to the surroundings when a system changes from reactants to products.  It is given the symbol, DH.

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DH = DHproducts DHreactants

DH is measured at STP or SATP for allowance of comparisons between enthalpy changes.

This idea is consistent with the law of conservation of energy, therefore;

Enthalpy change of system  =  quantity of heat that flows from system to  surroundings or vice versa

In other words;

DH  = ± q (DH= Delta H- Enthalpy Change)

Sign of DH

Endothermic Reaction:

  • System has absorbed heat energy from surroundings
  • Hproducts >  Hreactants
  • DH = Hproducts –  Hreactants

= positive value

  • DH is positive
Standard Enthalpies of Formation, ΔH°f

Exothermic Reaction:

  • System releases heat energy into surroundings
  • Hproducts < Hreactants
  • DH = Hproducts –  Hreactants

= negative value

  • DH is negative

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