What is the fastest chemical reaction

The speed of chemical reactions

Seven chemical reactions in comparison

  1. Dynamite explosion
  2. Inflating an airbag
  3. Neutralization of hydrochloric acid
  4. Baking a cake
  5. Rusting iron
  6. Conversion of hydrochloric acid with magnesium
  7. Implementation of iron with sulfur

When people hear the term "chemical reaction", most of them think of an explosion (reactions 1 and 2) because a lot of energy is released in a very short time. In reality, most chemical reactions are slower (reactions 3, 6, and 7) or even imperceptibly slow (reaction 5).

To make it clear that the speed and energy release of a reaction are not necessarily related, the following figure always helps:

Two exothermic reactions in comparison

Which of the two reactions shown is faster, the left or the right?

In over 90% of the cases you get the answer from students (m / f / d): "Clearly, the right reaction is much faster than the left". If you then ask why that is so, there are always some particularly good people who can even justify it: "The reaction on the right releases a lot more energy than the reaction on the left, that is got to they run faster ".

Unfortunately, this answer is not entirely correct! Both reactions run (at least theoretically) at exactly the same speed. Of course the right reaction is more violent than the one on the left, because the reaction energy is much greater. The reaction speed is the same.

Many students do not understand this; but then it helps to look at the following figure:

Compare two other exothermic reactions

Which of the two reactions shown is faster now, the left or the right?

The diagram on the left could for example stand for the neutralization of hydrochloric acid HCl with sodium hydroxide solution NaOH, the diagram on the right for the reaction of black powder.

The correct answer to the question is: The left reaction is faster. The reaction of black powder is very exothermic, and once the reaction starts it is quite violent. But it takes a few seconds before you see the first sparks when you point the flame of the Bunsen burner at the gunpowder heap. And it takes a few seconds to burn off the gunpowder pile too.

The neutralization of the acid, however, begins right away with pouring the lye into the acid; you don't have to wait a second or two here.

The key term we need to explain this phenomenon is activation energy. In the drawing above, the two red arrows stand for this activation energy.

The speed of a chemical reaction depends on its activation energy. The lower the activation energy, the faster the reaction takes place.

Sometimes one reads in the specialist literature about a "mountain of energy" that has to be overcome. The higher this mountain of energy (i.e. the activation energy), the slower the chemical reaction takes place.

Doesn't the reaction energy or enthalpy have an influence on the speed of the reaction after all?

Actually one should answer this question with a clear "yes, possibly". As we will see on the next page, the speed of a reaction depends, among other things, on the ambient temperature: the higher the temperature, the faster the reaction. If a lot of energy is given off in the form of heat in an exothermic reaction, the temperature naturally rises, and this accelerates the reaction. At first, the reaction may still be very slow because the activation energy is quite high. But the longer the reaction goes, the warmer it gets, and of course that speeds up the reaction.

Now that we have made this important connection, three important questions arise:

  1. What exactly is meant by the term reaction speed?
  2. Of what factors does the reaction speed depend on exactly?
  3. How can you change the speed of a chemical reaction measure as precisely as possible?