Problem 72 - Chemistry: An Atoms-Focused Approach

Question

A reaction vessel contains equal masses of magnesium metal and oxygen gas. The mixture is ignited, forming MgO. After the reaction has gone to completion, the mass of the \(\mathrm{MgO}\) is less than the mass of the reactants. Is this result a violation of the law of conservation of mass? Explain your answer.

Step-by-Step Solution

Answer

Why or why not? Answer: The result does not violate the law of conservation of mass. The apparent discrepancy can be attributed to the loss of reactants or products during the reaction process, such as magnesium or oxygen escaping from the reaction vessel or the vaporization of some MgO produced. These factors would lead to a lower mass of MgO formed than expected, but they do not mean that mass has been created or destroyed during the reaction.

Step 1: Write down the balanced chemical equation for the reaction

The chemical reaction between magnesium metal (Mg) and oxygen gas (O2) can be written as:$$ 2 \mathrm{Mg}(s) + \mathrm{O}_2(g) \rightarrow 2 \mathrm{MgO}(s) $$This is the balanced chemical equation for the reaction, as the atoms of each element are equal on both sides of the equation.

Step 2: Understand the law of conservation of mass

The law of conservation of mass states that the total mass of the reactants in a chemical reaction is equal to the total mass of the products. In other words, mass cannot be created or destroyed during a chemical reaction.

Step 3: Analyze the given information

We are given that the mass of the MgO formed is less than the mass of the reactants, which consist of equal masses of magnesium metal and oxygen gas. The vessel is ignited, allowing the reaction to proceed until completion. This implies that not all of the initial mass of reactants has been converted into MgO.

Step 4: Determine if this is a violation of the law of conservation of mass

The law of conservation of mass states that the total mass of reactants should equal the total mass of products. In this case, the mass of the final product (MgO) is less than the mass of the reactants, which appears to contradict the law of conservation of mass. However, it is important to consider the possibility that some of the reactants are lost during the reaction process. For example, some magnesium or oxygen may escape from the reaction vessel when the mixture is ignited, thus not reacting to form MgO and not being included in the measured mass of the products. Additionally, some of the MgO produced might be vaporized in the process, also not being included in the measured mass of the products. If one or both of these scenarios occur, the loss of reactants or products would lead to a lower mass of MgO formed than expected, and the mass of the products would not equal the mass of the reactants. In this case, the result does not violate the law of conservation of mass as the missing mass can be attributed to the loss of reactants or products during the reaction process.

Key Concepts

Chemical Reactions

Chemical reactions are processes where substances, known as reactants, transform into new substances, called products. These transformations involve the making or breaking of chemical bonds. During reactions, atoms of the reactants rearrange themselves to create new substances with different properties from the original materials.
For example, in the reaction between magnesium and oxygen described in the problem, magnesium (Mg) and oxygen gas ( O_2 ) react to form magnesium oxide ( MgO ).
The process can be represented as:

• Reactants: Magnesium and Oxygen
• Products: Magnesium Oxide

It's important to understand that even though the substances involved in the reaction change, the number of atoms remains the same, which is crucial in ensuring that matter is neither created nor destroyed. This characteristic is essential to explaining why the law of conservation of mass holds true.

Balanced Chemical Equation

To accurately describe a chemical reaction, we use a balanced chemical equation. A balanced equation reflects the law of conservation of mass by having the same number of each type of atom on both the reactant and product sides.
Here's how we balanced the equation for the reaction discussed in the exercise:\[ 2 \text{Mg}(s) + \text{O}_2(g) \rightarrow 2 \text{MgO}(s) \]This equation shows:

• Two magnesium atoms reacting with one oxygen molecule (comprising two oxygen atoms).
• Yielding two magnesium oxide units.

Balancing ensures that every atom is accounted for and stays in the system throughout the reaction. It satisfies the principle that atoms, and thus mass, are conserved. Understanding balanced chemical equations as representations of real-world reactions is key to comprehending how and why we observe the conservation of mass.

Reactants and Products

Reactants are the starting substances in a chemical reaction, while products are the substances formed as a result. In any given chemical reaction, reactants undergo transformations to become products.
For the magnesium-oxygen reaction, the reactants are magnesium metal and oxygen gas, and the product formed is magnesium oxide:
- Reactants: - Magnesium (Mg) - Oxygen (\(O_2\))
- Product: - Magnesium Oxide (\(MgO\))
The number and type of atoms are conserved from reactants to products. However, the disappearance of reactants and formation of products are observed as a transformation process.
This conservation is important in explaining situations like the one given in the exercise. While it seems mass is lost, it's possible that some reactants didn't transform (due maybe to experimental conditions like escape of gases). Therefore, understanding reactants and products helps clarify that no mass is truly lost, just transformed.