Question

If even a tiny spark is introduced into a mixture of \(\mathrm{H}_{2}(\mathrm{g})\) and \(\mathrm{O}_{2}(\mathrm{g}),\) a highly exothermic explosive reaction occurs. Without the spark, the mixture remains unreacted indefinitely. (a) Explain this difference in behavior. (b) Why is the nature of the reaction independent of the size of the spark?

Short answer

The mixture of hydrogen and oxygen gas doesn't explode on its own due to the absence of the required activation energy needed to overcome the energy barrier and break the molecular bonds. The introduction of a spark provides this necessary activation energy, leading to an explosive chain reaction. Regardless of the spark's size, as long as it provides the minimum activation energy, the nature of the reaction remains the same because the spark only initiates the reaction, which then becomes self-sustaining.

Step-by-step solution

Understanding the concept of activation energy and its role in the reaction

Activation energy can be defined as the least amount of energy needed to initiate a chemical reaction. In the case of the mixture of hydrogen gas (\(\mathrm{H}_{2}(\mathrm{g})\)) and oxygen gas (\(\mathrm{O}_{2}(\mathrm{g})\)), the activation energy barrier is relatively high. In the absence of the spark, the system lacks the energy needed to break the bonds of the reactant molecules and hence, is unable to reach the transition state, leading to no reaction.

Explaining the impact of the spark on the reaction

When a spark is introduced to the gaseous mixture, it provides the required activation energy needed to overcome the energy barrier. It helps to break the bonds in the hydrogen and oxygen molecules and creates free radicals which react with more hydrogen and oxygen molecules, leading to a chain reaction. This chain reaction results in the explosive reaction observed.

Discussing the influence of the size of the spark

The size or intensity of the spark does not affect the nature of the reaction because all it does is provide the initial activation energy needed to cross the energy barrier. As long as this minimum energy requirement is met, irrespective of how much bigger the spark is, the reaction will proceed the same way. The spark's role is to initiate the reaction, and thereafter, the reaction becomes self-sustaining due to the production of free radicals.

Key concepts

Exothermic Reaction

In the context of chemistry, an exothermic reaction releases energy in the form of heat, making the surroundings warmer.
Such reactions usually occur spontaneously once initiated, as they continually produce energy that sustains the reaction.
A common example of this is the reaction between hydrogen and oxygen gases. Once a spark introduces the activation energy necessary for breaking bonds, the reaction proceeds rapidly.
The energy released from forming new bonds creates a cascade of heat, propagating the reaction.
Features of an Exothermic Reaction:

• Releases energy, typically as heat.
• Makes the environment warmer.
• Occurs spontaneously after an initial energy input.

These reactions are crucial in both natural processes and industrial applications because they help in energy production and are relatively easy to maintain once started.

Chain Reaction

A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place.
Once initiated, they can continue without further external influence, provided that the necessary conditions are met.
In the case of hydrogen and oxygen, once an initial reaction is started by a spark, the reaction propagates itself through a series of subsequent reactions.
How It Happens:

• The initial spark breaks molecular bonds.
• Free radicals formed in the initial step continue reacting with other molecules.
• These reactions release further energy, sustaining the cycle.

This concept is critical in understanding how certain reactions can start small but quickly become large-scale due to this self-propelling nature.

Free Radicals

Free radicals play a significant role in chemical reactions, especially in chain reactions.
They are atoms or molecules possessing unpaired electrons, making them highly reactive.
In the example of a hydrogen and oxygen reaction, the initial spark generates free radicals which are the key players in propagating the chain reaction.
Key Characteristics of Free Radicals:

• Have unpaired electrons and are highly reactive.
• Often initiate further reactions.
• Can create a snowball effect in reactions like explosions.

Understanding free radicals is essential as they are not only fundamental in explosive reactions but also in many biological and environmental processes.