Problem 18 - Chemistry: The Central Science

Question

(a) When a compound containing \(\mathrm{C}, \mathrm{H}\), and \(\mathrm{O}\) is completely combusted in air, what reactant besides the hydrocarbon is involved in the reaction? (b) What products form in this reaction? (c) What is the sum of the coefficients in the balanced chemical equation for the combustion of acetone, \(\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}(l)\), in air?

Step-by-Step Solution

Answer

(a) The reactant besides the hydrocarbon involved in the combustion reaction is oxygen (O2). (b) The products formed in complete combustion reactions are carbon dioxide (CO2) and water (H2O). (c) The sum of the coefficients in the balanced chemical equation for the combustion of acetone, \(C_3H_6O(l) + 4O_2(g) \rightarrow 3CO_2(g) + 3H_2O(l)\), is 11.

Step 1: Part (a): Identifying the reactant involved in the reaction besides the hydrocarbon

In a combustion reaction, the compound containing carbon, hydrogen, and oxygen (in our case, the hydrocarbon) reacts with another substance. This other substance is oxygen (O2) from the air.

Step 2: Part (b): Determining the products formed in the reaction

In complete combustion reactions, the products formed are always carbon dioxide (CO2) and water (H2O).

Step 3: Part (c): Balancing the chemical equation for the combustion of acetone

We will first write down the unbalanced chemical equation for the combustion of acetone (C3H6O). C3H6O(l) + O2(g) → CO2(g) + H2O(l) Now, we will balance the chemical equation step by step: 1. Balance the carbon atoms: C3H6O + O2 → 3CO2 + H2O 2. Balance the hydrogen atoms: C3H6O + O2 → 3CO2 + 3H2O 3. Balance the oxygen atoms: C3H6O + 4O2 → 3CO2 + 3H2O The balanced chemical equation for the combustion of acetone is: C3H6O(l) + 4O2(g) → 3CO2(g) + 3H2O(l)

Step 4: Part (c): Finding the sum of coefficients in the balanced chemical equation for the combustion of acetone

In the balanced chemical equation: C3H6O(l) + 4O2(g) → 3CO2(g) + 3H2O(l) The coefficients are 1 (for C3H6O), 4 (for O2), 3 (for CO2), and 3 (for H2O). The sum of the coefficients is: 1 + 4 + 3 + 3 = 11 The sum of the coefficients in the balanced chemical equation for the combustion of acetone is 11.

Key Concepts

Reactants in Combustion

Understanding the reactants involved in a combustion reaction is key to grasping how these energetic transformations work. Combustion, a high-temperature exothermic chemical reaction between a fuel and an oxidant, typically involves hydrocarbons, substances made up of carbon (C) and hydrogen (H).

The magic ingredient that drives the combustion process is oxygen (O2) from the air. Without oxygen, a fire won't start or continue, as it's essential for the chemical process that releases energy from the fuel. In the exercise, acetone, which contains not only carbon and hydrogen but also oxygen, serves as the fuel while oxygen from the air acts as the oxidant.

It's important to realize that even though the fuel might contain oxygen itself, additional oxygen is almost always required to sustain a combustion reaction. This concept highlights the importance of carefully examining the reactants to predict the outcome of the reaction.

Products of Combustion

The products that result from a complete combustion reaction give us important clues about the chemistry taking place. In a perfectly efficient combustion process, two products always emerge: carbon dioxide (CO2) and water (H2O).

Carbon dioxide is the product of carbon atoms from the fuel combining with oxygen, and water forms when hydrogen atoms bond with oxygen. Understanding these products is crucial not only for grasping the basics of combustion but also for environmental and health reasons, as any deviations can lead to harmful byproducts. For example, incomplete combustion can produce carbon monoxide (CO), a dangerous pollutant.

In our example, the combustion of acetone yields carbon dioxide and water as byproducts. Remembering that CO2 and H2O are the expected products of complete combustion simplifies the process of writing and balancing chemical equations for these types of reactions.

Balancing Chemical Equations

The law of conservation of mass mandates that the mass of reactants and products in a chemical reaction must be equal. Balancing chemical equations is the act of ensuring this law is upheld by making sure that the number of atoms for each element is the same on both sides of the equation.

In the combustion of acetone, the equation starts unbalanced. To balance it, one must methodically adjust the coefficients—the numbers placed before reactants and products—until equilibrium is achieved. This process involves balancing, typically, the number of atoms of carbon first, then hydrogen, and finally oxygen. It's a cornerstone concept in chemistry because it embodies the principle that matter cannot be created or destroyed.

Correctly balancing equations requires practice and attention to detail, often involving trial and error. It's important to proceed systematically to avoid getting lost in the complexity of the reactions, especially when multiple elements are involved.

Stoichiometry

Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction. It allows chemists to predict the amounts of substances consumed and products formed in a reaction. The balanced equation for the combustion of acetone revealed a stoichiometry of 1 part acetone to 4 parts oxygen creating a sum of coefficients equal to 11.

The stoichiometric coefficients tell the story of the reaction in terms of moles—the basic unit in chemistry used to express the amount of a substance. By using these coefficients, one can calculate how much fuel is needed to react completely with a given amount of oxygen or vice versa. This has practical applications in industrial settings, allowing for efficient resource management and minimizing waste.

For students, understanding stoichiometry is vital for solving problems not only in their homework but also in real-world scenarios where precise chemical calculations are crucial.