Problem 80 - Chemistry The Molecular Nature of Matter

Question

Methanol, \(\mathrm{CH}_{3} \mathrm{OH},\) has been suggested as an alternative to gasoline as an automotive fuel. Write a balanced chemical equation for its complete combustion.

Step-by-Step Solution

Answer

The balanced chemical equation for the complete combustion of methanol is 2CH3OH + 3O2 → 2CO2 + 4H2O.

Step 1: Identify the reactants and products

The combustion of methanol (CH3OH) involves reactants which are methanol and oxygen (O2). The products of complete combustion are carbon dioxide (CO2) and water (H2O).

Step 2: Write the unbalanced chemical equation

Set up the unbalanced chemical equation with the reactants on the left side and the products on the right side. The unbalanced equation is: CH3OH + O2 → CO2 + H2O

Step 3: Balance carbon atoms

Balance the carbon atoms first. There is one carbon atom in methanol and one carbon atom in carbon dioxide, so this part of the equation is already balanced.

Step 4: Balance hydrogen atoms

Balance the hydrogen atoms next. There are four hydrogen atoms in methanol and two in each water molecule, so you need two water molecules to balance the hydrogen atoms: CH3OH + O2 → CO2 + 2H2O

Step 5: Balance oxygen atoms

Finally, balance the oxygen atoms. There are two oxygen atoms in carbon dioxide and four in the two water molecules, totaling six oxygen atoms in the products. There is one oxygen atom in methanol and two required for each of the two water molecules, leaving three oxygen atoms needed from the O2 reactant. Since O2 is a diatomic molecule, one and a half (1.5) molecules of O2 will provide the necessary three oxygen atoms: CH3OH + 1.5O2 → CO2 + 2H2O.However, we normally do not use fractions in chemical equations, so we multiply all coefficients by two to get whole numbers: 2CH3OH + 3O2 → 2CO2 + 4H2O

Key Concepts

Balanced Chemical Equation

Understanding the concept of a balanced chemical equation is fundamental in the study of chemistry. Simply put, a balanced chemical equation ensures that the number of atoms for each element is the same on both sides of the equation, which represents the conservation of mass. In our methanol combustion example, the unbalanced equation begins as \(\mathrm{CH}_3\mathrm{OH} + \mathrm{O}_2 \rightarrow \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O}\). Through a process of iterative adjustments, each sort of atom on the reactants side is matched with the same number on the products side.

This process includes balancing one element at a time and in a specific order, generally starting with the most complex molecule. For methanol combustion, carbon and hydrogen were balanced first, followed by oxygen, due to its presence in all compounds involved. The final balanced equation, as derived in the solution, is \(2\mathrm{CH}_3\mathrm{OH} + 3\mathrm{O}_2 \rightarrow 2\mathrm{CO}_2 + 4\mathrm{H}_2\mathrm{O}\). This equation is balanced, meaning if you count the atoms on both sides, they equal, signifying that mass is conserved during the reaction.

Reactants and Products in Combustion

Combustion is a high-temperature exothermic chemical reaction between a fuel and an oxidant, typically oxygen in the air, which produces energy in the form of light and heat. When we discuss the combustion of methanol, we identify methanol \(\mathrm{CH}_3\mathrm{OH}\) and oxygen \(\mathrm{O}_2\) as the reactants. These substances react to form the products of the combustion, which are carbon dioxide \(\mathrm{CO}_2\) and water \(\mathrm{H}_2\mathrm{O}\).

Each substance plays a specific role in the reaction; methanol is the fuel, oxygen is the oxidant, carbon dioxide is one of the combustion gases, and water is produced as a vapor. Complete combustion occurs when there is sufficient oxygen, ensuring all the fuel reacts, limiting the production of byproducts like carbon monoxide or soot.

Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions. It's a quantitative relationship between the elements in a balanced chemical equation. In our example of methanol combustion, the stoichiometry dictates the exact proportions of methanol and oxygen needed to produce specific amounts of carbon dioxide and water without leaving any excess reactants.

The balanced equation \(2\mathrm{CH}_3\mathrm{OH} + 3\mathrm{O}_2 \rightarrow 2\mathrm{CO}_2 + 4\mathrm{H}_2\mathrm{O}\) indicates that two moles of methanol react with three moles of oxygen to produce two moles of carbon dioxide and four moles of water. These ratios can be used to compute the amounts of reactants required or products formed. For instance, should you need to find out how much oxygen is needed to fully combust a certain amount of methanol, stoichiometry would provide the answer.