Enthalpy Change and Chemical Reactions
Be definition:
DH = enthalpy (H) of products – enthalpy (H) of reactants
\ when H products > H reactants Þ +ve DH = endothermic rxn
\ when H reactants > H products Þ -ve DH = exothermic rxn
Enthalpy Change (DH) and Bond Energies
During chemical reactions energy is required to break bonds in reactants, and energy is released when new bonds form in products.
Breaking bonds = endothermic Forming bonds = exothermic
Consider the following reactions:
2H2 + O2 Þ 2H2O
Bond energies: H-H = 436kJ/mol
O=O = 498kJ/mol
O-H = 464kJ/mol
Determine total energy required to break all bonds in reactants
Determine total energy released by formation of new bonds in products
Will this reaction be exothermic or endothermic?
Draw an enthalpy diagram for this reaction
N2 + 3H2 Þ 2NH3
Bond Energies: NºN = 940kJ/mol
H-H = 436kJ/mol
N-H = 389kJ/mol
Determine total energy required to break all bonds in reactants
Determine total energy released by formation of new bonds in products
Will this reaction be exothermic or endothermic?
Draw an enthalpy diagram for this reaction
Exothermic Reactions: less energy is required to break bonds in the reactants than is liberated by formation of new bonds in products
\The products of EXOTHERMIC reactions have less stored potential energy than the reactants had (more energetically stable)
Endothermic Reaction: more energy is required to break bonds in the reactants than is liberated by formation of new bonds in products
\The products of ENDOTHERMIC reactions have more stored potential energy than the reactants had (less energetically stable)
Facts About Thermochemical Equations and DH
1) DH is a “state dependent” property, it is affected by temperature and pressure.
Ie; if a reaction produces water in the form of a gas or a liquid, the enthalpy change will be different since the enthalpy of liquid water is lower than the enthalpy of water vapour.
2H2(g) + O2(g) Þ 2H2O(l) DH°= -571.0kJ
2H2(g) + O2(g) Þ 2H2O(g) DH°= -483.6kJ
Scientists will often report results at SATP and use the symbol DH° to indicate that the value is the “standard enthalpy of reaction”.
2) The value of DH depends on the molar amounts of reactants and products involved
Ie; 2H2(g) + O2(g) Þ 2H2O(g) DH°= -483.6kJ
3) Exothermic reactions in one direction, become endothermic reactions in the reverse direction
2H2O(g) Þ 2H2(g) + O2(g) DH°= +483.6kJ