Question

As we know, methane burns readily in oxygen in a highly exothermic reaction. Yet a mixture of methane and oxygen gas can be kept indefinitely without any apparent change. Explain.

Short answer

A mixture of methane and oxygen doesn't react because the activation energy is not met. Even though the reaction of methane and oxygen is highly exothermic, it only occurs when the mixture is ignited or provided with an initial energy boost to overcome the activation energy barrier. Without that, the gas mixture can be stored indefinitely without any changes.

Step-by-step solution

Understanding Exothermic Reactions

An exothermic reaction is one that releases energy, usually in the form of heat. The reaction of methane (CH4) and oxygen (O2) is an exothermic reaction. Mathematically it can be represented as: CH4(g) + 2 O2(g) --> CO2(g) + 2 H2O(g) + energy

Understanding the role of Activation Energy

Although the reaction of methane and oxygen is exothermic, it does not occur immediately upon mixing the gases. This is because there's an energy barrier that must be overcome before reaction can occur. This barrier is known as the 'activation energy'. Simply put, activation energy is the minimum energy required to initiate the chemical reaction. Even though the reaction releases energy once it starts, you first need to add in some energy to get the reaction going.

Explanation of the phenomenon

For methane and oxygen, the activation energy isn't met even when they're mixed together. Unless the mixture is ignited (or provided with a certain amount of energy), it won't react even though the reaction is highly exothermic. In simpler terms, though methane will burn in oxygen, when the two gases are just mixed together at room temperature, there's not enough energy to overcome the activation energy for the reaction, so no reaction occurs.

Key concepts

Exothermic Reaction

An exothermic reaction is a fascinating chemical process where energy is released, often as heat, into the surrounding environment. The reaction of methane and oxygen is a perfect example of an exothermic reaction. As methane (\( \text{CH}_4 \)) reacts with oxygen (\( \text{O}_2 \)), it forms carbon dioxide (\( \text{CO}_2 \)) and water vapor (\( \text{H}_2\text{O} \)). During this process, energy is liberated, which is why it is classified as exothermic.

• The chemical equation for this exothermic reaction is: \[ \text{CH}_4 (g) + 2 \text{O}_2 (g) \rightarrow \text{CO}_2 (g) + 2 \text{H}_2\text{O} (g) + \text{energy} \]

This means more energy is released than consumed by the reaction, creating heat, light, or both. This is why flames are visible when methane burns. This characteristic is what makes exothermic reactions integral in everyday occurrences, from heating homes to operating engines.

Activation Energy

To understand why methane and oxygen can sit together without reacting, we need to consider the concept of activation energy. Activation energy is like a hurdle that must be overcome for a chemical reaction to begin. Even if a reaction is exothermic, it won't start without the right amount of initial energy.

• Despite the natural tendency of the exothermic reaction to release energy, it won't occur spontaneously.
• This energy acts like a spark that kick-starts the reaction's progress.

In the case of methane combustion, the activation energy is higher than the energy typically present in the environment at room temperature. This is why methane and oxygen can be mixed without causing an explosive reaction until external energy, like heat or a spark, is introduced. Thus, activation energy is crucial in controlling when and how chemical reactions occur.

Methane Combustion

Methane combustion refers to the chemical reaction where methane (\( \text{CH}_4 \)) reacts with oxygen (\( \text{O}_2 \)) to produce carbon dioxide (\( \text{CO}_2 \)), water (\( \text{H}_2\text{O} \)), and energy. This reaction is central to many applications, such as powering gas stoves and heating systems, due to its highly exothermic nature.

• Methane as a fuel is quite efficient thanks to the large amount of energy it releases upon combustion.
• The complete combustion of methane is crucial for safety and efficiency.
  • In complete combustion, all methane is turned into carbon dioxide and water.
  • Incomplete combustion, due to insufficient oxygen, can produce carbon monoxide, a dangerous gas.

Understanding methane combustion also helps in appreciating why mixing methane with oxygen doesn't spontaneously ignite in ambient conditions. The absence of enough activation energy keeps this potent reaction at bay, allowing practical and controlled use.