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Enthalpy Change

Key Concepts

  • The heat content of a chemical system is called the enthalpy (symbol: H)

  • The enthalpy change (ΔH) is the amount of heat released or absorbed when a chemical reaction occurs at constant pressure.

  • ΔH = H(products) - H(reactants)

  • ΔH is specified per mole of substance as in the balanced chemical equation for the reaction
    The units are usually given as kJ mol-1 (kJ/mol) or sometimes as kcal mol-1 (kcal/mol)
    1 calorie (1 cal) = 4.184 joules (4.184 J)

  • Energy changes are measured at constant temperature and pressure, often 25oC (298 K) & 101.3 kPa (1 atmosphere)
  Exothermic Reaction Endothermic Reaction
Energy absorbed or released Energy is released.
Energy is a product of the reaction.
Reaction vessel becomes warmer.
Temperature inside reaction vessel increases.
Energy is absorbed.
Energy is a reactant of the reaction.
Reaction vessel becomes cooler.
Temperature inside reaction vessel decreases.
Relative Energy of reactants & products Energy of the reactants is greater
than the energy of the products
H(reactants) > H(products)
Energy of the reactants is less than
the energy of the products
H(reactants) < H(products)
Sign of ΔH ΔH = H(products) - H(reactants)
      = negative (-ve)
ΔH = H(products) - H(reactants)
      = positive (+ve)
Writing the equation N2(g)+ 3H2(g) → 2NH3(g)+ 92.4 kJ
N2(g)+3H2(g) → 2NH3(g) ΔH=-92.4 kJ mol-1

2NH3(g)+ 92.4 kJ → N2(g)+ 3H2(g)
2NH3g → N2(g)+3H2(g) ΔH=+92.4 kJ mol-1

Energy Profile Energy of reactants
(N2 & H2) is greater than the energy of the products (NH3).
Energy is released.
ΔH is negative.
ΔH is measured from the energy of reactants to the energy of products on the Energy Profile diagram.
Energy of reactants (NH3) is less than the energy of the products (N2 & H2).
Energy is absorbed.
ΔH is positive.
ΔH is measured from the energy of reactants to the energy of products on the Energy Profile diagram.


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