The change in total internal energy of a system when the pressure or volume of the system is held constant.

In a chemical reaction, this is more or less equal to the the difference between the energy put into breaking bonds and the energy gained from new bond formation.

From the BioTech Dictionary at For further information see the BioTech homenode.

When people think about chemical reactions and the energy released in them, they are often told the energy is stored in chemical bonds and that during a chemical reaction, this energy is released. This gives the impression that chemical bonds are like treasure chests between the atoms, and that during the reaction these treasure chest are ripped open, releasing all the energy like a fountain. This in turn gives the impression that making bonds uses up energy (as in stashing the ‘treasure chest’ with energy) and breaking bonds releases energy.

The truth is, it is actually the other way around, breaking bonds takes up energy, making bonds releases energy. So the question is; what is actually going on? This will require us to delve into physics as well as chemistry.

Imagine a single hydrogen atom, which is one positively charged proton with a single negatively charged electron orbiting it. They are held together by the electrostatic attraction, which is negatively charged stuff being attracted to positively charged stuff. Now, the scientist wants to remove the electron, but how do they do it. They decide to blast the hydrogen atom with high energy photons. This causes the electron to be buzzing so much with energy that it flies away from the proton to some area of space, thus breaking the electrostatic attraction. Now the scientist stops blasting the proton and electron with photons. After awhile the electron buzzes towards the proton. At which point the electrostatic attraction kicks in again, meaning the electron and the proton must join together again. However, this can’t happen because the electron is buzzing around with so much energy. This is resolved by the electron releasing energy in the form of photons, the electron releasing energy equal to what it absorbed whilst it was being blasted. This means it isn’t buzzing around so much so thus can form an electrostatic attraction with the proton, forming a hydrogen atom.

Now then, with an ionic bond (as an example), first the ions must be formed. So one atom must lose an electron to become positively charged, thus energy must be added so that the electron can fly off (known as first ionization energy). Likewise the other atom must gain an electron to become negatively charged, thus the previous electron must lose energy to form the electrostatic attraction with this other atom (known as first electron affinity). Now, both atoms have to form an electrostatic attraction (ionic bond), but both atoms are also buzzing around. Thus, in order to form the ionic bonds, both the atoms also have to release energy (or at least the electrons orbiting the nucleus). Thus energy is released when the ionic bond forms (known as lattice formation enthalpy). Likewise, for ionic bonds to be broken, energy needs to be added so that the two ions can break the electrostatic attraction and buzz off (known as lattice disassociation enthalpy).

Here’s a real life example of why breaking bonds take up energy, and making bonds release energy.

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