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(a) Enthalpy change and standard conditions
Enthalpy (H) is the heat content of a system.  In a reaction this may increase or decrease and produce an enthalpy change (/\H).
/\H = H₂ - H₁
where   H₁ = enthalpy of reactants and H₂ = enthalpy of products
The enthalpy change is affected by pressure and temperature so standard conditions are used for measurements.  These conditions are 1 atmosphere pressure and a temperature of 25^oC or 298K.
for 2H₂(g) + O₂(g) -----> 2H₂O(l) standard enthalpy of reaction /\H^o₂₉₈ = -571.6 kJmol^-1
This enthalpy change is per mole of equation.

(b) Enthalpy of formation and combustion
Enthalpy of formation (/\H^o_f)is the enthalpy change when 1 mole of a substance is formed from its constituent elements in their standard states at 298K and 1 atm.
C(s) + 2S(s) -----> CS₂(l)  /\H^o_f = + 88kJmol^-1
The positive sign means that for each mole of CS₂ formed 88kJ  are absorbed.
Enthalpy of combustion (/\H^o_c) is the enthalpy change when 1 mole of a substance  is burnt completely in oxygen with measurements made at 298K and at 1 atm.
C₃H_6(g) + 4 1/2 O_2(g) ----> 3CO_{2 (g)} + 3H₂ O _(l)   /\H^o_c = -2219.7 kJ per mole
The negative sign means that each mole of propane burnt releases 2219.7 kJ.

(c) Enthalpy level diagrams
Exothermic reaction enthalpy level diagram         Endothermic reaction enthalpy level diagram

reactants                                                               products
                   |                                                                                                       |
              |   negative enthalpy  change                                       |    positive enthalpy change
                 |                                                                               |
products \/                                                            reactants  \/
 

(d) Thermodynamic and kinetic stability
One system is thermodynamically stable with respect to a second one if the first one is lower than the second on an enthalpy level diagram.
e.g. Oxygen is energetically stable with respect to ozone.
ozone       (unstable)
               |
               |
oxygen    \/  (stable)
Even if a system is thermodynamically unstable and is expected to react to form a stable one the system may not react.  The system will not react if it is kinetically stable. This means that the reaction proceeds too slowly for any reaction to be seen. If kinetically unstable, a reaction is fast and observations can be made.  When a system is thermodynamically unstable but kinetically stable, the reaction is likely to be seen but only under favourable conditions.  Sugar and oxygen is a system like this with respect to carbon dioxide and water.

(e) Enthalpy and the direction of spontaneous change
Exothermic reactions with a -ve enthalpy change are often proceed spontaneously.  This is in the direction of greatest stability.  Endothermic reactions with +ve enthalpy changes are often not spontaneous.  The sign of enthalpy change does not always indicate the direction of spontaneous change because other factors affect events e.g. kinetic stability.

(f) Hess's Law
This states that the total enthalpy change for a chemical reaction is the same regardless of the route taken for the reaction. It is also a consequence of a more general physical law- the Law of Conservation of Energy which states that energy can not be created nor destroyed.

route A                   reactants     ----/\H1---->     products
                                             |                                       /\
                                /\H3                                  |
                                             |                                       |
route B                        \/                                      |
                              intermediates  -----/\H2--------->
/\H1 + /\H2 = /\H3

(g) Bond enthalpies
Bond dissociation enthalpy is the enthalpy change when one mole of bonds of a particular type in a particular environment are broken.
The Bond Enthalpy Term or E is an average value of bond dissociation enthalpies for a particular bond.  The bond dissociation enthalpies for the O-H bonds in water differ slightly, the bond enthalpy term is the average.
so E(O-H) = Average bond dissociation enthalpy  = (494 + 430)/2 kJmol^-1
so E(O-H) = +462 kJmol^-1

(h) Enthalpy changes and industrial processes
The efficient use of energy in industrial processes is vital for their economic success. The more the are reactants subjected to external heat the more money it costs. Waste heat from burning products should be recycled, fuels such as liquids can be burnt to provide external heat.  Industrial plant producing its own fuel reduces fuel transportation costs. The need to make the reaction to proceed as fast as possible conflicts with the need to maximise yield and with economical and environmental issues, but because of this need, much external heat usage can be justified. The effect of temperature on reaction rate and the equilibrium position must be carefully balanced.
 

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© Copyright junxiang21@yahoo.com 1998-2000.
Newly revised: January 17, 2000.