Question

The equation for the combustion of ethane \(\left(\mathrm{C}_{2} \mathrm{H}_{6}\right)\) is: $$ 2 \mathrm{C}_{2} \mathrm{H}_{6}(g)+7 \mathrm{O}_{2}(g) \longrightarrow 4 \mathrm{CO}_{2}(g)+6 \mathrm{H}_{2} \mathrm{O}(l) $$ Explain why it is unlikely that this equation also represents the elementary step for the reaction.

Short answer

The equation involves too many molecules to be an elementary step.

Step-by-step solution

Understanding the Chemical Equation

The given chemical equation represents the combustion of ethane, showing the reactants turning into products. Ethane ( C_2H_6 ) reacts with oxygen ( O_2 ) to form carbon dioxide ( CO_2 ) and water ( H_2O ). The stoichiometric coefficients indicate that 2 moles of ethane react with 7 moles of oxygen to produce 4 moles of carbon dioxide and 6 moles of water.

Defining the Elementary Step

An elementary step in a reaction is a single step that involves a small number of molecules reacting in a single collision. These elementary steps are the simplest units in a reaction mechanism and usually involve a limited number of molecules, typically two or three.

Analyzing the Reaction Complexity

The given equation involves 9 molecules (2 C_2H_6 and 7 O_2 ) colliding simultaneously to form 10 molecules (4 CO_2 and 6 H_2O ). This large number of molecules colliding at once is highly improbable because the likelihood of 9 molecules meeting in the correct orientation and energy for a single collision is extremely low in the gas phase.

Conclusion About the Elementary Step

An elementary step is expected to involve no more than three molecules, due to the probability and complexity of such collisions. Since this equation involves the simultaneous collision of 9 molecules, it is unlikely to represent a single elementary step in the mechanism of the reaction.

Key concepts

Elementary Step

In chemical reactions, an "elementary step" is a fundamental concept that describes a single event where molecules collide and interact at a molecular level. Each elementary step involves just one collision event leading to changes in the molecules involved.
Think of it as the smallest building block in a reaction mechanism. Elementary steps are simple and usually involve only two to three molecules colliding.

• Example: The combination of a hydrogen atom and a chlorine molecule to form hydrogen chloride ( HCl ) is an elementary step.
• Most real-world reactions involve multiple such steps forming a complex chain.

In our exercise, the idea of an elementary step is crucial for understanding why the given combustion equation is unlikely to represent a single stepping event.

Reaction Mechanism

The "reaction mechanism" refers to the sequence of elementary steps that describe how a chemical reaction proceeds. Unlike a simple overall equation, a reaction mechanism shows each step from reactants to products, providing deeper insight into the process.
Understanding a reaction mechanism helps us:

• Predict the rate of reaction and which steps might be the slowest (called rate-determining steps).
• Visualize how reactants are transformed at a molecular level.

Each step in the mechanism corresponds to an elementary step where certain bonds are broken, and new ones are formed. In the ethane combustion example, although the overall reaction involves many molecules, the mechanism would break this down into simpler, more realistic steps involving fewer molecules per step.

Chemical Equation Analysis

"Chemical equation analysis" is about understanding what a chemical equation tells us about a reaction and its limitations. It's essential to distinguish between the overall equation and the mechanism.
A balanced chemical equation gives us the following insights:

• Reactants and products involved in the reaction.
• The stoichiometry or the ratio of molecules involved.
• The states of reactants and products (solid, liquid, gas).

However, it doesn't show how the reaction proceeds at the molecular level or the sequence of steps. In the ethane combustion reaction, while the overall equation shows 2 C_2H_6 reacting with 7 O_2 to produce 4 CO_2 and 6 H_2O , it doesn’t reflect the highly improbable simultaneous collision of all these molecules. This is why understanding both the equation and its corresponding mechanism is crucial.