Problem 111 - Chemistry Matter and Change

Question

Predict which of the compounds, methane gas \(\left(\mathrm{CH}_{4}\) or methanal vapor \(\left(\mathrm{CH}_{2} \mathrm{O}\right),\) has the greater molar \right. Write and compare the balanced chemical equations for the two combustion reactions.)

Step-by-Step Solution

Answer

Methane, \( \text{CH}_4 \), has greater molar energy release due to needing more oxygen for combustion.

Step 1: Write the Combustion Equation for Methane

The combustion of methane gas, \( \text{CH}_4 \), involves reacting it with oxygen. The balanced chemical equation is: \[ \text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(g) \] This equation shows that 1 mole of \( \text{CH}_4 \) burns with 2 moles of \( \text{O}_2 \) to produce 1 mole of \( \text{CO}_2 \) and 2 moles of \( \text{H}_2\text{O} \).

Step 2: Write the Combustion Equation for Methanal

The combustion of methanal vapor, \( \text{CH}_2\text{O} \), requires a different amount of oxygen. The balanced equation is: \[ \text{CH}_2\text{O}(g) + \text{O}_2(g) \rightarrow \text{CO}_2(g) + \text{H}_2\text{O}(g) \] Here, 1 mole of \( \text{CH}_2\text{O} \) combines with 1 mole of \( \text{O}_2 \) to form 1 mole of \( \text{CO}_2 \) and 1 mole of \( \text{H}_2\text{O} \).

Step 3: Compare the Balanced Equations

Comparing the balanced equations, methane requires more oxygen than methanal for complete combustion. Methane burns with 2 moles of oxygen per mole of fuel, while methanal needs only 1 mole of oxygen per mole of fuel. This suggests that methane releases more energy, assuming complete combustion and that the differences in enthalpies of formation are consistent with typical hydrocarbon combustion.

Key Concepts

Balanced Chemical Equations

In chemistry, a balanced chemical equation is a representation of a chemical reaction that accurately reflects the conservation of mass. The total number of each type of atom on both the reactant and product sides of the equation must be the same. This is crucial because it abides by the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction.
For example, when writing the combustion reaction of methane, you start with the unbalanced equation:

• Reactants: - Methane: \( \text{CH}_4 \)- Oxygen: \( \text{O}_2 \)
• Products: - Carbon dioxide: \( \text{CO}_2 \)- Water: \( \text{H}_2\text{O} \)

After balancing, it becomes:\[ \text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(g) \] This balanced equation ensures that there are equal numbers of carbon, hydrogen, and oxygen atoms on each side.

Moles of Oxygen

Moles of oxygen are a consideration when balancing chemical equations, especially in combustion reactions. A mole is a unit that measures the amount of a substance, based on the number of atoms in exactly 12 grams of carbon-12, equivalent to Avogadro's number, \( 6.022 \times 10^{23} \) entities.
When a substance combusts, it reacts with oxygen. The mole ratio of the reactants, often oxygen, is crucial for determining how much of a substance will react or is needed. For methane's combustion, 2 moles of oxygen are needed per mole of methane:\[ 1~\text{CH}_4(g) + 2~\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2~\text{H}_2\text{O}(g)\]For methanal, however, only 1 mole of oxygen is required per mole of methanal:\[ 1~\text{CH}_2\text{O}(g) + 1~\text{O}_2(g) \rightarrow \text{CO}_2(g) + \text{H}_2\text{O}(g)\]This difference highlights the importance of moles in reacting proportions.

Molar Comparison

Molar comparison involves comparing the amount of different gases involved in reactions, like oxygen, to predict reaction behaviors or outcomes. It is important to analyze these comparisons accurately, as each mole of a substance in a balanced chemical equation corresponds to particles reacting in fixed ratios.
In the combustion reactions highlighted, methane requires 2 moles of \( \text{O}_2 \) to burn completely, whereas methanal only needs 1 mole. This directly affects the energy output of the reactions, as greater oxygen demand usually correlates with greater energy release.
Given balanced equations, calculating reactants and products in terms of mole ratios helps in determining not just quantities for reactions, but also the outcomes like potential energy output or completeness of combustion.

Energy Release in Combustion

Energy release in combustion is the energy given off when a substance reacts with oxygen to form products. This energy, usually in the form of heat, is a key indicator of a fuel's efficiency and is related to the enthalpy change of the reaction. The balanced equations help determine the energy released based on the amount of fuel and oxygen consumed. Methane's combustion reaction, outlined as:\[\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(g)\] suggests a higher energy release when compared with methanal's reaction:\[\text{CH}_2\text{O}(g) + \text{O}_2(g) \rightarrow \text{CO}_2(g) + \text{H}_2\text{O}(g)\]Due to the higher requirement for oxygen, methane often generates more heat per mole, making it a more potent fuel.
Understanding energy in combustion can aid in predicting fuel efficiency, environmental impacts, and the suitability of a substance as a fuel source.