Thermodynamics in a nutshell

Firstly, what is thermodynamics? Well it’s all about heat energy changes and how energy is transferred through chemical systems. For example, in exothermic reactions the overall potential energy of the products in less than that of the reactants so more energy is released than is required to break the necessary bonds. And vice versa for endothermic reactions.

So, let’s look at the fundamental laws of thermodynamics. The first is one most of us will have heard at some point: energy cannot be created nor destroyed but merely transferred from one form to another. The second is that energy will always flow from being concentrated to being dispersed and that no process can have 100% efficiency for the conversion of heat energy into “work” which is essentially the energy transfer into our desired product. This is where it starts to get all confusing, when we consider entropy. We’ve all seen water evaporate from a pan and seen salt dissolve in water, both of these happen because particles have a tendency to spread out and disorder. This disordering or randomness, known as entropy can be calculated using the entropy change of both the surroundings and the system so that we can determine whether a reaction is feasible or not feasible. For this we use the equation: ΔG = ΔH –TΔS where ΔH is the enthalpy change, ΔS in the entropy change, T is the temperature of the reaction and ΔG is the Gibbs free energy. When ΔG is negative the reaction is said to be feasible and when positive, not feasible. This feasibility of a reaction is different at different temperatures. The third law states that as a system approaches 0K the entropy change also approaches 0JK^-1, theoretically though because reaching 0K is impossible.  This happens only in perfect crystals because in non-pure ones energy is required for the imperfect alignment of the bonds within the crystal. 

Author: Grace Ronnie

Resonance structures

Resonance is a way of defining delocalised structures, such as benzene, of which a single Lewis structure is not sufficient. Numerous resonance structures can be used to show the structure, but no single one is correct. What this means is that an approximate intermediate between the resonance structures is in fact the real structure, often referred to as the resonance hybrid. If we take benzene as the example, we know that the Kekulé structure, devised by Freidrich August Kekulé in 1972, is incorrect and benzene, with its delocalised ring is the correct structure. It does not have three double bonds but six bonds that are half way between single and double carbon-carbon bonds and a delocalised pi cloud above and below it, bearing in mind it is a planar molecule. The idea behind the Kekulé structures (see image below) was that it shifted between the two structures so fast that it is never one or the other in one moment. We now know that is doesn’t shift but is a midway state.

A common analogy to explain these structures is that a rhinoceros is a resonance hybrid between a unicorn and a dragon, neither is actually real but both share characteristics of a rhinoceros. A rhinoceros does not shift between a unicorn and a dragon; it is just simply a rhino.

You hopefully have expected that benzene is not the only compound that resonance structures are used for, there are others such as the nitrile group (-NO₂), sulphur dioxide (SO₂), the carbonate ion (CO₃^2-) and more.

Above shows the two resonance structures for the nitrile group

Above shows the two resonance structures for sulphur dioxide

Above shows the three resonance structures used to show the carbonate ion

Note that not one of these contain double bonds but all do contain a delocalised structure that is half way between the length of a single and double bond.

Author: Grace Ronnie.