well-known reaction is the synthesis of cellulose in plants from carbon dioxide and water using the energy of sunlight. Simply, one can say that the input of external energy into carbon dioxide and water causes the formation of new chemical bonds, resulting in the formation of a new compound, cellulose. The energy of sunlight is transformed into the newly formed chemical bonds in the cellulose molecules and to a lesser extent into physical interaction forces that stabilise their molecular arrangement. One can regard chemical bonds and physical interaction forces as a ‘warehouse’ for the energy input that was necessary for the formation of molecules and structures (Snyder 1995). Cellulose may be retransformed to its original reactants, carbon dioxide and water, by combustion in the presence of oxygen. By this reverse reaction that destroys cellulose, the energy stored in its chemical bonds is released as heat through bond separation. Energy is measured in joules (J) = 1 kg×m2/s2. The energy that is needed to break one chemical bond of average strength is of the order of 105 J, i.e. several hundred thousand joules. It is therefore usually expressed in kilojoules (kJ) (Table 1.2).
| type of bond or interaction force | example | length nm |
dissociation energy kJ/mol at 20°C |
|
|---|---|---|---|---|
| chemical bond (intramolecular bond) |
ionic bond | Na+•••Cl− | 0.240 | 412 |
| covalent bond | ❳C — H | 0.074 | 345 | |
| ❳C — C❴ | 0.154 | 346 | ||
| ❳C — O — C❴ glucoside |
0.144 | 250 | ||
| H — O — H water |
0.096 | 500 | ||
| physical bond (intermolecular bond) |
hydrogen bond liquid water |
—O•••H—O | 0.136 – 0.376 |
20 |
| hydrogen bond ice |
—O•••H—O | 0.276* | 6 | |
| hydrogen bond cellulose |
—H•••OH—Cell | 0.170 | 19 | |
| van der Waals forces | Ne•••Ne | ~0.330 | 3.9 | |
*total length between the oxygens of two water molecules held together by a hydrogen bond
In physical chemistry, thermodynamic laws allow one to predict whether a mixture of reactants will undergo a reaction. They further make it possible to predict the final state that a chemical process will reach when all of the involved opposing forces have reached a state of