Question

Consider the combustion reaction of propane, used in barbecue grills: $$ \mathrm{C}_{3} \mathrm{H}_{5}(g)+5 \mathrm{O}_{2}(g) \longrightarrow 3 \mathrm{CO}_{2}(g)+4 \mathrm{H}_{2} \mathrm{O}_{(g)} $$ Which of the following is conserved in this reaction? (a) moles of molecules (b) moles of atoms (c) atoms (d) mass (e) Which of your answers to (a) through (d) are true for any reaction?

Short answer

The correct answer is: (b) Moles of atoms, (c) Atoms, and (d) Mass are conserved in the combustion reaction of propane. Additionally, (b) Moles of atoms, (c) Atoms, and (d) Mass are conserved in any chemical reaction according to the laws of conservation.

Step-by-step solution

Analyzing option (a)

Option (a) states that the number of molecules (moles of molecules) is conserved. Looking at the reaction, the number of reactant molecules (namely, one propane molecule and five oxygen molecules) is not equal to the number of product molecules (namely, three carbon dioxide molecules and four water molecules). Hence, the number of molecules is not conserved in this reaction.

Analyzing option (b)

Option (b) suggests that the number of atoms (moles of atoms) is conserved. Analyzing the reaction, it becomes evident that the numbers of each type of atom (carbon, hydrogen, and oxygen) are the same on both sides of the reaction. Hence, the number of atoms (moles of atoms) is indeed conserved in this reaction.

Analyzing option (c)

Option (c) suggests that atoms are conserved. This is essentially similar to option (b). As explained in step 2, the number of each type of atom before and after the reaction remains the same. The atoms merely rearrange to form different molecules through chemical bonding. Thus, atoms are conserved in this reaction.

Analyzing option (d)

Option (d) posits that mass is conserved. According to the law of conservation of mass, the total mass of the reactants should equal the total mass of the products. Since the number of each type of atom remains the same in a chemical reaction, and as all atoms have a fixed mass, the total mass remains constant during the reaction. Therefore, mass is conserved in this reaction.

Analyzing option (e)

Option (e) asks which of the above statements are valid for any reaction. Let's evaluate. (a) The number of molecules is not always conserved in reactions as seen in our examined reaction. (b and c) The number of atoms and identity of atoms are always conserved in any reaction. This is the fundamental principle of chemical reactions, known as the Law of Conservation of Matter. (d) According to the Law of Conservation of Mass, the total mass before and after a reaction always remains the same. Hence, options (b), (c), and (d) are universally correct for all chemical reactions.

Key concepts

Chemical Reactions

Chemical reactions are processes during which substances, called reactants, are transformed into different substances, called products. This change happens through the breaking and forming of chemical bonds between atoms. Let's take the combustion of propane as an example, used commonly in BBQ grills:\[ \text{C}_3\text{H}_8(g) + 5\text{O}_2(g) \rightarrow 3\text{CO}_2(g) + 4\text{H}_2\text{O}(g) \]In this reaction, propane (\( \text{C}_3\text{H}_8 \)) and oxygen (\( \text{O}_2 \)) join together in a high-energy process to form carbon dioxide (\( \text{CO}_2 \)) and water (\( \text{H}_2\text{O} \)).Some key things to know about chemical reactions include:

• Reactants are consumed; products are produced.
• Reactions can release or absorb energy, such as heat or light.
• The types and numbers of atoms remain constant, they merely rearrange.

Understanding chemical reactions helps in predicting the products and energy changes involved.

Law of Conservation of Matter

The Law of Conservation of Matter states that matter cannot be created or destroyed in a closed system. This principle is a cornerstone of chemistry and holds true for all chemical reactions. In our example with propane combustion, the total number of atoms on the reactants side (left side of the equation) equals the total number on the products side (right side of the equation). This law leads to the following important insights:

• Atoms are preserved; they don't just disappear or appear out of nowhere.
• The identity and amount of each type of atom are maintained throughout the reaction.
• Matter's transformation in a reaction involves a simple rearrangement of atoms.

Thus, in any chemical process, the mass of reactants equals the mass of products.

Combustion Reaction

Combustion reactions are a specific type of chemical reaction where a substance combines with oxygen, often producing heat and light. These reactions are familiar as they are involved in burning fuels like propane.Key aspects of combustion reactions:

• Always involve oxygen as one of the reactants.
• Commonly produce carbon dioxide and water as products when hydrocarbons like propane or gasoline combust.
• Typically release a significant amount of energy, explaining why they are used in engines and grills.

In propane combustion specifically:\[ \text{C}_3\text{H}_8(g) + 5\text{O}_2(g) \rightarrow 3\text{CO}_2(g) + 4\text{H}_2\text{O}(g) \] The propane is the hydrocarbon fuel, and the reaction proceeds with oxygen to produce carbon dioxide, water, and energy. Understanding combustion reactions is crucial for industries focusing on energy production and environmental protection.