Problem 45 - General Chemistry

Question

A sample of mercury(II) oxide was heated to produce mercury metal and oxygen gas. Then the liquid mercury was cooled to \(-40^{\circ} \mathrm{C},\) where it solidified. A glowing wood splint was thrust into the oxygen, and the splint burst into flame. Identify each physical change and each chemical change.

Step-by-Step Solution

Answer

The decomposition and combustion are chemical changes; the solidification is a physical change.

Step 1: Understand Chemical and Physical Changes

Begin by understanding the difference between chemical changes and physical changes. A chemical change results in the formation of new substances, while a physical change affects the form of a chemical substance but not its chemical composition.

Step 2: Identify the Reaction of Mercury(II) Oxide

When mercury(II) oxide is heated, it decomposes into mercury metal and oxygen gas, represented by the chemical reaction: \[2 ext{HgO} (s) \rightarrow 2 ext{Hg} (l) + ext{O}_2 (g)\]This is a chemical change because new substances (mercury and oxygen) are formed from mercury(II) oxide.

Step 3: Mercury Solidifying

Cooling mercury from liquid to solid at \(-40^{\circ} \text{C}\) is a physical change. Here, mercury changes its state from liquid to solid, but its chemical composition remains mercury (Hg).

Step 4: Oxygen Reaction with Wood Splint

When the glowing wood splint is thrust into oxygen and bursts into flame, a chemical change occurs. This is combustion, a chemical reaction between the oxygen and the wood resulting in new substances such as carbon dioxide and water vapor.

Step 5: Summarize Changes

Identify the changes: - The decomposition of mercury(II) oxide is a chemical change. - The solidification of mercury is a physical change. - The combustion of the wood splint in oxygen is a chemical change.

Key Concepts

Decomposition Reaction

A decomposition reaction is a type of chemical change where a single compound breaks down into two or more simpler substances. This kind of reaction involves the breakdown of chemical bonds, leading to the formation of new substances.
When studying decomposition reactions, it's crucial to recognize the breakdown of complex substances, which usually require energy input such as heat. Decomposition reactions are represented by the general equation: \[ AB \rightarrow A + B \] Here, \(AB\) is the original compound, and \(A\) and \(B\) are the products. In the case of mercury(II) oxide, this compound decomposes when heated, producing mercury and oxygen. This process is a textbook example of a decomposition reaction.
Key aspects include:

• Breakdown of compounds
• Energy requirement (often heat)
• Formation of simpler substances from a complex one

State Changes in Matter

State changes in matter involve physical changes where a substance changes its state without altering its chemical composition. Common state changes include transitions between solid, liquid, and gas phases. These processes are driven by changes in temperature or pressure.
In the context of mercury, when it's cooled from liquid to solid, this transformation is a state change — specifically freezing or solidification. Despite the change in form, mercury's chemical identity as \(Hg\) remains unchanged.
Key points about state changes include:

• No new substances formed
• Only physical form changes
• Reversible processes

Combustion Reaction

A combustion reaction is a chemical change that occurs when a substance reacts quickly with oxygen, producing heat and often light. This type of reaction is commonly associated with burning and is characterized by the release of energy.
Combustion reactions often involve hydrocarbons and result in the formation of carbon dioxide and water. In the example of the glowing wood splint thrust into oxygen, the combustion reaction produces carbon dioxide and water vapor, igniting the wood and causing it to burn bright.
Essential aspects to remember about combustion reactions are:

• Requires oxygen
• Releases energy (heat and light)
• Formation of new substances like \(CO_2\) and \(H_2O\)

Mercury(II) Oxide Decomposition

Mercury(II) oxide decomposition is a classic demonstration of a decomposition reaction where heat is applied to break down mercury(II) oxide into mercury and oxygen gas. The specific reaction is: \[ 2 \text{HgO} (s) \rightarrow 2 \text{Hg} (l) + \text{O}_2 (g) \] Here, solid mercury(II) oxide \((HgO)\) decomposes into liquid mercury \((Hg)\) and oxygen gas \((O_2)\).
This reaction highlights the formation of new substances, marking it as a chemical change. It is often performed in laboratories to teach about decomposition reactions as well as the concepts of chemical change.
Key details about this chemical reaction include:

• Involves heat input
• Results in formation of a metal and a gas
• Illustrates a basic type of chemical reaction