Question

The burning of methane in oxygen is a highly exothermic reaction. Yet a mixture of methane and oxygen gas can be kept indefinitely without any apparent change. Explain.

Short answer

The methane and oxygen mixture is stable due to high activation energy preventing spontaneous combustion.

Step-by-step solution

Understand the Reaction

The chemical reaction for the burning of methane in oxygen is given by the balanced equation: \[ \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} \] This is a combustion reaction where methane (CH₄) reacts with oxygen (O₂) to form carbon dioxide (CO₂) and water (H₂O), releasing a large amount of energy as heat.

Analyze Conditions for the Reaction

For a chemical reaction to occur, the reactant molecules must collide with sufficient energy to overcome the activation energy barrier. The activation energy is the minimum energy required for a reaction to proceed.

Identify the Role of Activation Energy

Despite being an exothermic reaction, the activation energy required to initiate the burning of methane in oxygen is relatively high. Without an external energy source to provide this energy, the methane and oxygen molecules do not have enough energy to collide effectively and react.

Conclusion of Reaction Stability

In the absence of a catalyst, spark, or another form of energy to provide the necessary activation energy, the mixture of methane and oxygen remains stable indefinitely, meaning it will not spontaneously react without sufficient energy input.

Key concepts

Understanding Activation Energy

The concept of activation energy is crucial in understanding why certain reactions, like the combustion of methane, require an initial push to get started. Activation energy is defined as the minimum amount of energy necessary for reactants to transform into products. It acts like a barrier that reactant molecules need to overcome to start a chemical reaction.

In the case of burning methane, despite it being a powerful reaction, methane and oxygen molecules are quite stable under normal conditions. They need an external energy source, such as a spark, to provide the activation energy. Once this initial hurdle is overcome, the reaction can proceed, releasing more energy than was required to start it.

In short, if the activation energy is not provided, the reactants just coexist without engaging in a reaction. This explains why methane and oxygen can be stored together without reacting indefinitely unless a source of energy like a flame or spark is introduced.

Characteristics of an Exothermic Reaction

Exothermic reactions are chemical reactions that release energy to their surroundings, usually in the form of heat. This is a characteristic feature of the reaction between methane and oxygen.

During the combustion of methane, energy is released because the bonds formed in the products (carbon dioxide and water) are more stable and lower in energy compared to the bonds in the reactants (methane and oxygen). The excess energy is emitted as heat, making the surroundings warmer. This heat release is what we exploit when burning fuels like methane for energy.

Key attributes of exothermic reactions include:

• Heat release as a byproduct
• Often observable with a rise in temperature
• Products have lower energy than reactants

The energy given off is usually greater than the energy required to initiate these reactions, contributing to their rapid and often vigorous progression once started.

Exploring Chemical Stability

Chemical stability refers to the ability of a chemical substance to remain unchanged without decomposing or reacting with other substances. It is this quality that allows methane and oxygen to exist together without spontaneously reacting.

The stability of a substance is influenced by various factors, including bond strength, molecular structure, and the surrounding environmental conditions like temperature and pressure.

In the case of methane and oxygen, although they can combust together in an exothermic reaction, they are stable because:

• The energy required to break the initial bonds (activation energy) is high
• Without that energy input, no new reactions occur
• The molecules can coexist without interaction due to the high energy barrier

Chemical stability ensures that mixtures don’t cause unexpected reactions under normal conditions, which is vital for safe storage and handling of reactive gases like methane.