OXYGEN

OXYGEN.

INTRO

Let us study oxygen. It is a very different element if we compare it with the rest of its group, sulphur, selenium of tellurium. Intermolecular interactions are very weak so ebullioscopy temperature is very low. The solubility in water is also very low.

Oxygen molecule is paramagnetic and the bond coefficient will be two.

The electronic configuration is:

1s² 2s² 2p⁴

So the molecular orbital diagram is:

OZONE

If there is only atomic oxygen, there are two possibilities:

O  +  O  =  O₂             ΔH₁

O  +  O₂  =  O₃           ΔH₂

ΔH₁  <  ΔH₂.

Attending to a thermodynamic point of view, the first chemical reaction is produced, but this is not true because the probability of the second reaction is higher because the kinetic of the first reaction is very low.

 But finally, ozone transforms in oxygen.

2 O₃  =  3O₂.

 The structure of ozone is angular. Attending to the electronic configuration of oxygen atom, the molecular diagram orbital could be:

IONS OF OXYGEN.

The only element whose electro negativity is higher than oxygen is fluorine. Ionization energy is very high, so we can think that O₂¹⁺ does not exist, but we can see that oxygen can lose one electron, increasing its bond coefficient to two point five.

PtF₆  +  O₂  ->  O₂⁺[PtF₆]^1-.

                 t = 600ºC

O^2- is formed in two steps:

a.-   O  +  e^-  =  O^1-;      ΔH < 0

b.-   O^1-  +  e^-  =  O^2-;      ΔH > 0

Let us analyze the endothermic process. We could think O^1- is more stable than O^2-, but a compensation with the reticular energy of the compound formed is produced in O^2-, so finally this ion is more stable. It forms part of many ionic compounds, and its polarizing power is low, so the contribution of covalent bond in the compound is very low; we can say they are ionic compounds.

 O^2- in water is a Broensted base.

O^2-  +  H₂O  ->  2HO^1-

The molecule of the ion O₂^2- is similar to the molecule of F₂. The bond coefficient is 1, and it is a diamagnetic molecule.

CHEMICAL PROPERTIES.

Let us study direct chemical reactions. Oxygen forms covalent compounds with no metals. Oxygen forms compounds with a high percentage of ionic bond. It will depend the oxidation state of the metal; if the oxidation state is high (more than three), the polarization power of the metal will be high, so the percentage of covalent bond will increase. We can see Cr and Mn, that can form anions like:

CrO₄^2-, MnO₄^1-.

It is curious that reactivity of oxygen with carbon, sulphure, hydrogen, is very high, but in environmental temperature, oxygen does not react with this compounds. But if we give energy to start the process, the reaction will be exothermic and the kinetic of the reaction will be high. They can be violent reactions.

 Let us study reactions in water. Oxygen is a very good oxidant agent. It makes higher the oxidation state of the metals.

Fe²⁺  -  e^-  ->  Fe³⁺.

Sometimes, we can find exothermic reactions where the metal loses one electron, but the activation energy required is very big so finally the chemical reaction is not possible.

 Silver, loses one electron with the light.

Ag¹⁺  ->  Ag²⁺

AgO is formed in AgNO₃ solutions (with the light).

HOW TO OBTAIN OXYGEN.

 Distillation can be used to separate air into oxygen and nitrogen. We liquefy the air cooling it to -100ºC. We heat the liquid air, and when temperature is -96ºC. it will stop despite of keeping on heating, because nitrogen will change from liquid to gas. When temperature started to climb, only oxygen and another compounds will be in liquid state (in a proportion of 1%). When temperature is at -73ºC, temperature will stop again and oxygen will change from liquid to gas, so we can save it in cylinders to high pressure.

 We can separate oxygen and hydrogen from water through electrolysis.

WE CAN USE OXYGEN FOR:

-To prepare SO₃ to synthesize sulphuric acid. The compound will reacts with oxygen will be SO₂.

-To synthesize NO using NH₃ to prepare HNO₃.

-To medical uses.