Reaction steps and their energy

Energy of chemical bonds
If atoms are connected in a chemical way (a bond is made), there is always delivery of energy (exothermic). The system loses energy. The value of ΔH has a negative sign.
The same amount of energy is needed to break such a bond, just to remove those atoms from each other. Then ΔH will get a positive sign.
There are tables with bonding energies. You may read such a table in two ways:
  1. If bondings are made, when atoms come togheter, then you give the amount a minus sign.
  2. If bondings are broken, when atoms leave each other, then you give the same amount a plus sign.

Every step of a reaction comes with energy changes, because every reaction step will change something in the particles and/or the bondings.

General rule:

Breaking bonds, separate particles, costs energy, is endothermic.

Making bonds, join particles, creates energy, is exothermic.


Separation costs energy and joining gives energy.

[So if you want to economise energy, stay toghether!]

The fundamental steps in a chemical reaction are: breaking of bonds and making new ones.
To understand what happens in a chemical reaction, always try to imagine what the particles are doing.

Mixing two gases, ammonia and hydrogencloride, a white smoke appears of the solid (salt) ammonium chloride.

Barious reaction steps are part of the total reaction:

NH3(g) + HCl(g) NH4Cl(s)

An advantage is that the reactants already are gaseous, so, there is no need to separate the gas particles (the molecules NH3 and HCl) Een voordeel hier is dat de reagentia al gasvormig zijn, dus, het is hier niet nodig die gasdeeltjes eerst uit elkaar te halen. (Meestal, bij vaste stoffen en vloeistoffen dus, is dat wel zo en dat alleen al kost alweer energie).
Of each HCl molecule, first a H—Cl bonding must be broken, an endothermic process, costing energy.
Immediately after that, the new bonds N—H are made (result: NH4+) and that will deliver energyt, in an exothermic process.

The table with bondings energies indicates (tabel VI):

to break 1 mol H—Cl bondings costs 432 kJ

to make 1 mol N—H bondings gives 391 kJ

Only counting with these data, your conclusion could be: the total reaction must be endothermic (it costs more than it delivers).

But: we are not ready yet:
Ions are formed (1 mol ions NH4+ and 1 mol ions Cl-).
Those ions attrackt and join each other in an ionic lattice. This joining delivers a good amount of energy:
Making 1 mol of this ionic lattice of NH4Cl(s) delivers 400 kJ.
The total process turns out to be exothermic with a reaction energy of: 391 + 400 - 432 kJ = 359 kJ per mol ammoniumchloride product.

If the elements Sodium and Clorine react with each other, a chemical reaction occurs with salt as a product (NaCl).
Based on a well balanced reaction equation, and with help of the energy table (table VI) you can calculate how much energy will be freed or needed at the production of one mol of salt out of the elements.
Other data are:

The production of water out of element is a very strong exothermic process (explosion).
The product, initially made in gaseous form (vapor), becomes liquid at cooling.
H2(g) + ½O2(g) H2O(s)      ∆H < 0
one can find all steps of this process until the liquid water; every step with its energy.

In general, but certainly in the carbon chemistry, we may divide reaction steps in:
  1. the start, or initiation of the reaction
  2. the main reaction, the production of the product
  3. end reactions, or termination
Steps 2 and 3 can cause side products.