Problem 28 - Chemistry and Chemical Reactivity

Question

28\. Which observations below describe chemical properties? (a) Sodium metal reacts violently with water. (b) The combustion of octane (a compound in gasoline) gives \(\mathrm{CO}_{2}\) and \(\mathrm{H}_{2} \mathrm{O}\) (c) Chlorine is a green gas.

Step-by-Step Solution

Answer

(a) and (b) describe chemical properties.

Step 1: Understanding Chemical Properties

Chemical properties describe a substance's ability to undergo chemical changes or reactions. These properties can only be observed when a substance is undergoing a chemical change.

Step 1: Analyzing Option (a)

Option (a) states, "Sodium metal reacts violently with water." This describes sodium's ability to react with water, which is a chemical change. Thus, it is describing a chemical property.

Step 2: Analyzing Option (b)

Option (b) states, "The combustion of octane (a compound in gasoline) gives \(\mathrm{CO}_{2}\) and \(\mathrm{H}_{2} \mathrm{O}\)." This describes the chemical reaction of octane burning to produce carbon dioxide and water, which involves a chemical change, indicating a chemical property.

Step 3: Analyzing Option (c)

Option (c) states, "Chlorine is a green gas." This describes the color of chlorine, a physical property, not involving a chemical change. Thus, it is not a chemical property.

Key Concepts

Chemical Change

Chemical changes are transformations that result in the formation of new substances. These changes also frequently involve the release or absorption of energy. Unlike physical changes, where the substance's form might change, such as from a solid to a liquid, chemical changes alter the substance at a molecular level.
How can we identify a chemical change? Here are some typical signs:

• Color change that indicates a new substance has formed
• Formation of gas or bubbles (other than boiling)
• Emission or absorption of odor and energy (heat, light)
• Formation of a precipitate (solid) when two liquids react

Remember, chemical changes are permanent and typically cannot be easily reversed. When sodium reacts with water or octane combusts, these are clear-cut examples of chemical changes.

Sodium Reaction with Water

Sodium’s reaction with water is one of the most well-known chemical reactions, often used to demonstrate the concept of chemical properties in classrooms. Sodium is an alkali metal, known for its high reactivity, especially with water.
When sodium is introduced to water, it reacts vigorously, producing hydrogen gas and a great amount of heat. The equation for the reaction is:\[ 2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g) \]

• The sodium metal floats on the water's surface, reacting swiftly.
• Hydrogen gas is released, sometimes producing a popping sound due to combustion from the heat generated.
• The solution becomes basic due to the formation of sodium hydroxide \((NaOH)\).

This reaction demonstrates sodium's chemical property--its innovative and vigorous ability to react chemically with water.

Combustion of Octane

The combustion of octane is a fundamental chemical reaction, often discussed in the context of burning fuels. Octane, a primary component of gasoline, reacts with oxygen from the air to produce carbon dioxide and water.The balanced chemical equation for this process is:\[ 2C_8H_{18}(l) + 25O_2(g) \rightarrow 16CO_2(g) + 18H_2O(g) \]Several key points about this reaction include:

• It's an exothermic reaction, releasing large amounts of energy as heat.
• Oxidation occurs as octane is combined with oxygen.
• This energy powers internal combustion engines in vehicles.

Combustion is a vital chemical change, producing energy essential for everyday activities, though it also highlights environmental impacts, like greenhouse gas emissions.