Question

Predict which of the two compounds, methane gas \(({CH}_{4})\) or methanal vapor \(({CH}_{2} {O})\) , has the greater molar enthalpy of combustion. Explain your answer. (Hint: Write and compare the balanced chemical equations for the two combustion reactions.)

Short answer

The combustion of methane gas (\(CH_4\)) likely has a greater molar enthalpy of combustion than the combustion of methanal vapor (\(CH_2O\)), due to the higher number of moles of oxygen consumed in the combustion reaction of methane.

Step-by-step solution

Combustion of Methane Gas (\(CH_4\))

\(CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)\)

Combustion of Methanal Vapor (\(CH_2O\))

\(CH_2O(g) + O_2(g) \rightarrow CO_2(g) + H_2O(l)\) Next, we need to compare the molar enthalpies of the two combustion reactions:

Molar Enthalpy of Combustion of Methane Gas (\(CH_4\))

The combustion of 1 mole of \(CH_4\) produces 1 mole of \(CO_2\) and 2 moles of \(H_2O\), consuming 2 moles of \(O_2\). In the process, the reaction releases a certain amount of heat (energy) per mole of methane, which is its molar enthalpy of combustion.

Molar Enthalpy of Combustion of Methanal Vapor (\(CH_2O\))

The combustion of 1 mole of \(CH_2O\) produces 1 mole of \(CO_2\) and 1 mole of \(H_2O\), consuming 1 mole of \(O_2\). In this reaction, the molar enthalpy of combustion represents the heat (energy) released per mole of methanal. In general, the greater the number of moles of oxygen consumed in a combustion reaction, the greater the amount of heat (energy) that is released in the process. As seen in the balanced chemical equations above, the combustion of methane gas consumes more oxygen per mole of the compound burned than the combustion of methanal vapor.

Conclusion

It can be concluded that the combustion of methane gas (\(CH_4\)) likely has a greater molar enthalpy of combustion than the combustion of methanal vapor (\(CH_2O\)), due to the higher number of moles of oxygen consumed in the combustion reaction of methane.

Key concepts

Chemical Equations

Grasping the idea behind chemical equations is crucial for students delving into the realms of chemistry. These equations are the shorthand notation for depicting a chemical reaction. In essence, they convey which substances interact (reactants) and what gets produced (products), just like a recipe outlines ingredients and the final dish.

As an analogy, consider a simple baking recipe: the ingredients (flour, sugar, eggs) are analogous to reactants, while the cake is the product. Similarly, a chemical equation lists reactants on the left, products on the right, and an arrow indicating the direction of transformation. A balanced chemical equation respects the law of conservation of mass, ensuring that the number of atoms of each element is the same on both sides of the equation. This step is tantamount to making sure a recipe has the correct proportions.

The exercise demonstrated this concept through the combustion reactions of methane and methanal. By writing and balancing these equations, students compared the amount of reactants converting to products, which is key to understanding related concepts such as molar enthalpy.

Combustion Reactions

Let's light a spark on the topic of combustion reactions, which are a type of chemical reaction where a substance (usually a hydrocarbon) burns in the presence of oxygen to produce energy in the form of heat or light. In layperson's terms, it's like the reaction in a fireplace when wood, the fuel, burns with oxygen from the air, creating a cozy, warm fire.

These exothermic reactions always involve oxygen as a reactant and typically yield carbon dioxide and water as products. The intriguing exercise posed earlier provides excellent examples: methane (\(CH_{4}\)) and methanal (\(CH_{2}O\)) react with oxygen to form carbon dioxide and water. These combustion reactions are fundamental in industry and daily life since they're the core of power generation in engines and power plants. By comparing the combustion of different substances, students uncover the intricacies of energy release, which is directly related to molar enthalpy, another cornerstone of chemistry.

Enthalpy in Chemistry

Diving into the warmth of enthalpy in chemistry is like getting cozy under a blanket of energy content within a substance. Enthalpy, denoted by the letter 'H', refers to the total heat content of a system at constant pressure and is analogous to the thermal energy. It's a thermodynamic potential that helps in understanding heat transfer during chemical reactions without the need to consider the kinetic energy changes.

In the context of our exercise, 'molar enthalpy of combustion' refers to the energy released as heat when one mole of a substance burns in oxygen. Think of it as a price tag that tells you how much heat you get out of burning a specific amount of fuel. Since chemical bonds vary in strength, different substances require and release different amounts of energy during chemical reactions. The comparison of methane and methanal's combustion reveals the concept of molar enthalpy: methane releases more energy per mole than methanal because it consumes more oxygen, illustrating the direct impact of chemical bonding on energy changes. This understanding is fundamental for students who need to predict and control the energy aspects of reactions in practical applications.