Why Do Chemical Reactions Occur?
A couple of qualifiersThere may be a substantial activation energy that stops a thermodynamically feasible reaction from occurring.
Many reaction processes appear to violate the ΔG < 0 rule for various reasons.
Consider crystallisation. The process is exothermic and the crystal is more ordered than the melt/solution and so surely, ΔG is positive? However, this analysis does not describe the thermodynamic system. Crucially heat must be lost to the surroundings and overall ΔS is positive. Such violations to the the ΔG < 0 rule are always a "local". Overall here is always an increase in entropy when the surroundings are included as part of the system.
That said, it is possible to design and build real world chemical reaction systems using glass & metal that remove heat and/or products. The effect is to separate and isolate the local system from the global system.
If these issues are not pointed out, much of the following discussion could be highly ambiguous.
The Gibbs function, ΔG = ΔH TΔS, tells us that the entropy term, ΔS, is multiplied by the thermodynamic temperature, T, and so the energy dispersion contribution becomes progressively more important as the temperature is increases.
The enthalpy of formation and entropy data for liquid water and steam are both and can be looked in tables. The data on both sides of the reaction is summed, and the left-hand-side (reactant) data is subtracted from the right-hand-side (product) data. For this reaction process this gives, ΔHreaction = +44.0 kJ/mol and ΔSreaction = +0.119 kJ/Kmol.
Choose a temperature, say 50°C (323 Kelvin), plug the numbers into the Gibbs function and calculate the free energy of the system:
At 50°C (323 Kelvin), the Gibbs free energy is +5.6 kJ/mol, so the reaction will NOT proceed, in fact it will go in the reverse direction. As is well known, at 50°C and 1.0 atm steam condenses to water.
At equilibrium ΔG = 0. In this state the Gibbs relationship can be rearranged to T = ΔH/ΔS, an equation that tells us the temperature at which, in this case, water will boil. Even this rather simplistic version of the Gibbs equation predicts 370K or 97°C... really quite close to 100°C... and this is not a full thermochemical calculation.
Consider the classic and industrially important Haber Process for the synthesis of ammonia.
3 H2(g) + N2(g) 2 NH3(g)
Interesting factoid: 1% of the world's energy supply is consumed in the manufacture of ammonia derived fertiliser and other chemicals, Science 297(1654), Sep 2002.
The process operating conditions are a balance between:
Overcoming the activation energy of the reaction
Having a sufficient rate of reaction rate
Moving the equilibrium position to the production of ammonia
The cost of the industrial plant
The Gibbs function allows us to model and understand numerous thermal reactions and processes, it tells us a great deal about the entropy the dispersion of energy ΔS and its relationship with temperature T.
The Gibbs function explains how we can manipulate reaction systems so that they produce chemical substances that are not in thermodynamic equilibrium with their surroundings.
Consider the production of quick lime (calcium oxide, CaO) from limestone, one of the world's oldest and in terms of scale one of the largest, chemical processes:
CaCO3(s) CaO(s) + CO2(g)
The production of quicklime illustrates the general principle of: heat, separate, cool quickly.
Heat the reactant substance(s) so that there is a change in phase space. High temperature means that the change occurs quickly. When calcium carbonate, CaCO3(s), is heated to high temperature the equilibrium position changes to carbon dioxide, CO2(g), plus calcium oxide, CaO(s).
Separate the phases. In the quick lime process, the hot carbon dioxide is gas is easily removed from the local system by venting to atmosphere.
Quickly cool the system, so trapping the various components in what is now an out-of-thermodynamic-equilibrium state.
In the quick lime case, the calcium oxide, CaO, is removed and placed in gas & water proof containers. At room temperature if calcium oxide comes into contact with carbon dioxide it will slowly react to reform calcium carbonate and if it comes into contact with water it forms slaked lime (calcium hydroxide), Ca(OH)2.
This type of process relies on the general principle that the time scale for thermodynamic equilibration is longer than the time it takes to physically separate the phases. This is achieved through manipulation of the reaction system using good design and operation procedures.
The Gibbs function tells us about the dispersion of phases at various temperatures: how the entropy, ΔS, of the system is influenced by temperature, but it says nothing about the nature of the ΔH bonding term. It does not in any way explain why the atoms choose to react in the various ways that they do.
So, what is in it for the atoms?
Taking Occam's Razor to The Set of Chemical Reactions
William of Occam (1285-1349) emphasised that it is important to get to point, to strip away all of the unnecessary detail and to deal with the core of the issue at hand. Or, as William eloquently put it:
To find the answer to the question why do chemical reactions occur?, it is necessary to take Occam's razor to the set of chemical reactions and to look for the few, simple, illuminating ideas and examples amongst the morass of interesting but complicating detail.
In his book Modern Inorganic Chemistry, William Jolly states that:
"The most stable arrangement of [polar] covalent bonds connecting a group of atoms is that arrangement in which the atom with the highest electronegativity be bonded to the atom with the lowest electronegativity."
Jolly, Modern Inorganic Chemistry, McGraw-Hill (1985) pp 61-62
Jolly gives four examples to illustrate this general principle. In each case there is a net increase in electronegativity difference:
Here are the reactions with the electronegativity calculations that illustrate this:
This author's choice for the reaction that most clearly illuminates what's in it for the atom is the disproportionation of difluoromethane, CH2F2, to methane, CH4, and tetrafluoromethane, CF4.
This reaction reinforces the maximisation of electronegativity difference, but it shows something else as well:
This reaction can be considered can be considered thermochemically:
Download the Excel spreadsheet here.
The reaction system is particularly simple because:
Another example: On heating formaldehyde (methanal) disproportionates to methane and carbon dioxide:
For organic chemists, this is the 'parent example' of the Cannizzaro reaction, a reaction that involves the disproportionation of an aldehyde into a primary alcohol and a carboxylic acid.
A third example, but this time a ligand exchange rather than a disproportionation reaction. When heated together, trifluoroiodomethane reacts with fluoromethane to give tetrafluoromethane and methyl iodide.
In 1967 Ralph Pearson cited these three reactions in support of his (then new) HSAB principle, saying:
"The symbiotic principle states that there is an extra stabilisation if several soft bases (ligands) or several hard bases cluster about a single acidic atom."
Pearson & Songstad, JACS, 89, 1827 (1967). [Read more about Pearson's HSAB principle look elsewhere in this web book.]
In this authors opinion, these reactions show no such thing. What they do show is general and far more interesting: if an atomic centre has multiple ligands/bonding partners, then the atomic centre prefers to have similar ligands rather than mixed bonding partners.
Atomic centres behave so as to maximise their spherical symmetry.
Spherical symmetry can be defined (from Symmetry: A Unifying Concept by István & Magdolna Hargittai):
"Everything is the same in all directions, as if on the surface of a sphere."
The pollen of hollyhock exhibits good spherical symmetry:
A CSIRO scientist examines a 'perfect' silicon sphere, similar to one that will be used to determine the exact atomic weight of a kilogram, from The Age:
Consider again the species involved in the 2 CH2F2 CH4 + CF4 disproportionation reaction:
Methane, CH4, and tetrafluoromethane, CF4, are perfect tetrahedral octapoles: they have zero dipole moment and belong to Td the symmetry point group.
Difluoromethane, CH2F2, has a dipole moment of 2.29 debyes (calculated) and the molecule belongs to the C2v symmetry point group.
The HOMO and LUMO frontier molecular orbitals are clearly more spherically symmetric in CH4 and CF4 compared with CH2F2:
Compare the HOMO of CH2F2 with the HOMOs of CH4 and CF4. The CH4 and CH4 HOMOs are clearly have more spherical symmetry than the CH2F2 HOMO. It is the same with the LUMOs. Diagrams obtained using Spartan.
The observation that atoms strive to achieve spherical symmetry should not be unexpected because it can be seen with the simplest chemical systems, including isolated atoms.
Indeed, an atom's quest for spherical symmetry is a dominant theme in chemistry.
Consider the electronic structure of atoms. Electrons add to atoms according to a few simple rules:
Nitrogen as 1s2 2s2 2px1 2py1 2pz1 is more spherically symmetric than 1s2 2s2 2px2 2py1 2pz0
Copper as [Ar] 3d10 4s1 is more spherically symmetric than [Ar] 3d9 4s2
Consider the valence shell electron pair repulsion, VSEPR method.
In this technique, bonding and non-bonding ("lone-pairs") of electrons are arranged about the atomic centre so as to maximise the spherical symmetry. Interestingly, nature adopts the same strategy, the maximisation of spherically. Thus, the VSEPR method can be used to predict molecular and ionic shape rather well.
Read more about VSEPR elsewhere in this web book.
Atom centres with multiple ligands adopt the most spherically symmetric conformation, as predicted by VSEPR.
Likewise, atomic centres with mixed ligands undergo exchange reactions to maximise spherical symmetry.
Thus, on heating difluoromethane, CH2F2, disproportionates to methane, CH4, and tetrafluoromethane, CF4 because methane, CH4, and tetrafluoromethane, CF4, are more spherically symmetric than difluoromethane, CH2F2.
The ΔH enthalpy term of the Gibbs function, or what's in it for the atom, concerns the maximisation of atomic spherical symmetry.
Atoms in molecules and materials co-operate and compete so as to maximise the net atomic spherical symmetry. In a reaction there may be winners as losers, although the winners will always win more than the losers lose.
Aluminium chloride, AlCl3, reacts with alkyl chlorides, R-Cl, to give tetrachloroaluminate ion, [AlCl4], and a carbenium ion (carbocation), [R-CH2]+. The reaction is used in the first step of the Friedel-Crafts alkylation reaction.
In this reaction the trigonal planar aluminium chloride Lewis acid, AlCl3, gains more spherical symmetry on going to the tetrahedral tetrachloroaluminate ion, [AlCl4], than the alkyl chloride, R-CH2Cl, looses on forming a carbenium ion, [R-CH2]+.
The crystal structure of an ionic compound can be predicted using a set of empirical rules that act to maximise spherical symmetry about as many centres as possible:
Crystal structures are usually named after a definitive crystal structure, such as: zinc sulfide (structure), sodium chloride, cesium chloride, calcium fluoride (fluorite), rutile, diamond, etc.
Consider again the 2 CH2F2 CH4 + CF4 disproportionation reaction, and ask the question: Are there any possible competing reactions?
There is one: the carbon forming, or coking reaction, producing of hydrogen fluoride and carbon.
CH2F2 C + 2HF
Thermochemistry calculations show this pathway is energetically preferred over disproportionation:
At 300° ΔG for disproportionation is -76 kJ/mol and for carbon + HF formation is -150 kJ/mol.
Download the spreadsheet.
Yet the 2 CH2F2 CH4 + CF4 disproportionation reaction occurs.
So, the question is:
Why does CH2F2 disproportionate to CH4 + CF4 and not to C + 2HF?
The answer is subtle.
At a higher temperature the globally most stable reaction will happen, and 'coking' will occur:
The arguments given above have only concerned thermal equilibria: heat, pressure & concentration.
There also exist other types of chemical reactivity which exhibit subtle selectivities.
In photochemical reactions a photon of specific energy is adsorbed by a specific chemical entity creating a population inversion (in thermochemical terms) of excited species. Interesting & complex chemistry invariably ensues...
A classic example is the formation of ozone, O3:
O2 + UV light → 2 O•
O• + O2 → O3
Ozone is meta-stable, and so is thermodynamically unstable with respect to normal oxygen, triplet O2:
ΔfH O3 = +142.3 kJ·mol-1
3O3 → 3O2
Is exothermic and dispersion increases. This reaction is spontaneous/feasible at all temperatures.
Thus, O3 cannot be formed thermally from O2, but it can be formed photochemically.
The Gibbs equation can predict whether a particular reaction is thermodynamically feasible and it can be used to compare reactions, but it tells us nothing about other possible possible reactions or the activation energies of those reactions.
The topology of this complex space must be determined by experiment.
A physical geography example illustrates this crucial point:
And so it is with chemistry:
Chemical reactivity is dominated by the mechanistic topology of the local reaction chemistry space.
Thus, this Chemogenesis web book is primarily concerned with the nature of mechanism and complexity.
|Pericyclic Reaction Chemistry||
© Mark R. Leach 1999-
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