# Do thermodynamically favourable chemical reactions always proceed?

I wanted to know some common parameters which decide the feasibility of any chemical reaction. And can we make a non favourable reaction favourable.

Because a reaction is thermodynamically favoured does not mean that it will be a fast reaction. So 'yes' they always proceed but possibly infinitesimally slowly. Temperature is the most common way of quickening a reaction. Of course a catalyst also does so but this generally by changing the way the reaction occurs, i.e. the mechanism is different using a catalyst but the product is the same. Enzymes are natures catalysts, in synthetic chemistry (and in you car exhaust) metal based compounds are often used.

All reactions have an activation energy $E_a$ between reactants and products and a small increase in the size of this can slow a reaction exponentially. Experimentally, the rate constant is generally found be of the form $k=k_0 exp(-E_a/RT)$, which is the Arrhenius equation, with R the gas constant and T the temperature in Kelvin.

The Boltzmann thermal distribution explains why this type of equation applies. This shows that as temperature increases molecules have more energy and so surmount the activation energy more frequently and so reaction rate increases,

The answer is NO in practice.

While porphyrin is correct to say what he says about the speed of some reactions being really slow despite being thermodynamically favourable, in practical terms this means many reactions just don't happen. It would be a dull world indeed if they did.

Take some simple examples. Graphite not diamond in the stabler form of carbon at normal pressures. But nobody is suing the advertisers for claiming "diamonds are forever". Perhaps more significantly the thermodynamically favourable oxidation of carbon in air doesn't lead to diamonds or lumps of coal disappearing (unless you add a great deal of heat to them). In practical terms this reactions just don't happen at normal conditions.

The reason is that the thermodynamics of a reaction is only part of the picture. Two other interrelated things are required for a reaction to occur in practice. You need a feasible mechanism and you need enough energy to overcome the activation energy implied by the mechanism. There is no feasible way to rearrange the bonding in diamond to give graphite even though doing so would release energy: most of the strong bonds in the crystal would have to break. In reality we can convert carbon between diamond and graphite only by dissolving it in molten metal which, effectively, breaks all the bonds allowing the conditions to dictate which allotrope crystallises.

The problem with diamond-graphite interconversion is the lack of any mechanism to cause the reaction under normal conditions. Other reactions have mechanisms but not enough energy to kick them off at normal temperatures. Oxidation of carbon is a good example. Oxygen can react directly with carbon to generate CO and CO2. But in solid carbon there just isn't enough energy to drive that mechanism without some significant input of heat. Oxygen molecules will bang into the surface of a diamond or a lump of coal but, at room temperature, none will have enough energy to react with a surface carbon. Take a blowtorch to a diamond, however, and there is plenty enough energy to bang oxygens and surface carbons together to cause the reaction to happen.

So, just because something is thermodynamically favourable doesn't mean it will happen. You also need a mechanism and enough energy to overcome the barriers to the mechanism (called activation energy).

• I like your addition of a mechanism, but we have to assume that a reaction is possible in the normal chemical way. We don't expect C+H+O+Mg + etc to make chlorophyll. However, I think that your 'no in practice' is wrong, because you give a counter examples, heating diamond , or carbon plus oxygen etc under common conditions such as in a flame. So it boils down to what we consider 'normal conditions' I suppose :) – porphyrin Jul 9 '16 at 12:32