Flash, bang, wallop

Hydrogen and fluorine react explosively on meeting. The reaction is instantaneous. According to school textbooks this happens even when the gases are mixed at very low temperatures in the dark.

H2 + F2 –> 2HF

Hydrogen will also react explosively with oxygen, but this reaction is very unlikely to occur spontaneously. To get these two gases to react some energy needs to be supplied, typically in the form of a spark or a small flame, as in the hydrogen ‘pop’ test. Once started however, hydrogen and oxygen react just as rapidly as do hydrogen and fluorine.

2H2 + O2 -> 2H2O

Hydrogen and oxygen combustion

Hydrogen and oxygen explosion
The yellow colour is from the detergent used to trap the bubbles of gas

That hydrogen and fluorine react spontaneously on mixing, whilst hydrogen and oxygen do not, perhaps suggests something in particular about the reactivity of fluorine.

The reaction of hydrogen and fluorine using Lewis diagrams

The reaction between hydrogen and fluorine illustrated using Lewis diagrams

A chemical reaction can be seen as a rearrangement of the order in which the atoms are bonded together in the reactants to form products.

For a reaction to occur the molecules involved in the reaction must first meet; they must collide. This is quite easy to imagine for gases. Gases mix immediately and in all proportions. Their molecules are moving around at random and at relatively high speeds. Many collisions take place. But not all collisions between molecules lead to chemical reactions, for a collision to be successful and result in a reaction the molecules must collide with enough energy and in the correct orientation (that is, in the right way).

The sequence of events by which certain bonds in the reactants are broken and new bonds in the products are formed is described as the reaction mechanism. And without knowing what the actual chain of events for the reaction between hydrogen and fluorine is, by comparing the relative strengths of the bonds in the reactants and products, we can still get some idea as to why this reaction is both spontaneous and explosive.

Table of some bond energies

F-F 159 kJ mol1
H-H 435
O=O 498
H-F 569
O-H 465

The covalent bond in fluorine has a lower value than the other bond energies in the table. This means a smaller amount of energy needs to be put in to break the fluorine fluorine bond, than the bonds in hydrogen or oxygen. Indeed fluorine is a very reactive gas that reacts with just about everything it comes into contact with and cannot be used in schools.

When molecules of fluorine collide with molecules of hydrogen less energy is needed to initiate a reaction during the collision, compared to the energy required for molecules of hydrogen and oxygen to react. It would appear that the energy needed to start the reaction between hydrogen and fluorine, the so-called activation energy, is particularly low.

Part of the answer to the question I asked last time as to why hydrogen and fluorine react so readily, is because the covalent bond in fluorine is so weak and easy to break. The reaction between hydrogen and fluorine is explosive and gives out energy. It is exothermic. It gives out energy because the product molecules (hydrogen fluoride) are more stable than the reactant molecules (hydrogen and fluorine). Can you calculate a value for the energy change for this reaction (the enthalpy change of reaction) using the bond energies in the table?

And why does a small spark or flame provide enough energy to initiate an explosive reaction between hydrogen and oxygen?

Another spontaneous reaction that is commonly encountered at school is the reaction between ammonia and hydrogen chloride. Even though this reaction is not explosive it does provide some interesting insights into the nature of chemical reactions and the way we describe them as chemists.

Ammonia and hydrogen chloride diffusion experiment

Ammonia and hydrogen chloride diffusion experiment

What is the product formed when ammonia reacts with hydrogen chloride?


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