Problem 68 - Chemistry and Chemical Reactivity

Question

What does the term standard state mean? What are the standard states of the following substances at \(298 \mathrm{K}: \mathrm{H}_{2} \mathrm{O}, \mathrm{NaCl}, \mathrm{Hg}, \mathrm{CH}_{4} ?\)

Step-by-Step Solution

Answer

Standard states at 298 K are: H2O (liquid), NaCl (solid), Hg (liquid), CH4 (gas).

Step 1: Define 'Standard State'

The standard state of a substance is a reference point used to calculate its properties under specified conditions. It typically refers to the state of a material at a pressure of 1 bar (about 1 atmosphere) and at a specified temperature, usually 298 K (25°C), but for gases at their pressure of 1 bar."

Step 2: Determine Standard State for Water ( H2O )

At 298 K, the standard state of water is the liquid phase. This is because water is a liquid at room temperature (about 298 K), which aligns with the definition of its standard state.

Step 3: Determine Standard State for Sodium Chloride ( NaCl )

Sodium chloride is typically found in the solid state at 298 K, so its standard state is solid. This is also true because, at room temperature, NaCl forms a crystalline solid known as table salt.

Step 4: Determine Standard State for Mercury ( Hg )

Mercury is unique among metals as it remains in the liquid state at 298 K. Thus, its standard state at this temperature is a liquid.

Step 5: Determine Standard State for Methane ( CH4 )

Methane is commonly found in its gaseous form at 298 K. Therefore, the standard state for methane is as a gas due to its low boiling point compared to room temperature.

Key Concepts

Properties of Substances at Standard State

Understanding the properties of substances in their standard state is crucial for both theoretical and practical chemistry. The standard state serves as a baseline to compare different chemical substances and reactions. This helps chemists understand how substances behave under a standardized set of conditions.
When we talk about a substance's properties in its standard state, we often refer to:

• Physical State: Is the substance a solid, liquid, or gas at 298 K?
• Standard Enthalpy: The heat content of a substance in its standard state.
• Standard Entropy: The disorder or randomness in a substance's standard state.
• Standard Gibbs Free Energy: Determines the spontaneity of a substance's reaction under standard conditions.

These properties allow scientists to predict and calculate how reactions will proceed and how substances will interact, providing a consistent framework for experiments and industrial applications.

Standard Conditions for Chemical Substances

In chemistry, 'standard conditions' refer to a well-defined set of environmental parameters. These conditions provide a consistent basis to measure and report the properties of chemical substances. The standard conditions are considered as:

• Pressure: 1 bar (approximately 1 atmosphere). This is used for measuring gas pressure and affects gas properties and reactions.
• Temperature: 298 K (25°C). This temperature is commonly adopted because it represents room temperature in many academic and laboratory settings.

By having these constant conditions, it enables chemists to have a universal frame of reference for studying and comparing the properties and behaviors of substances worldwide. It ensures consistency across different reports, studies, and experiments.

Phase of Substances at 298 K

The phase of a substance at 298 K plays a significant role in determining its behavior and interaction in chemical reactions. At this temperature:

• Water ( H_2O ): Exists as a liquid. Water remains in its liquid state due to the relatively high boiling point compared to 298 K.
• Sodium Chloride ( NaCl ): Is solid. NaCl forms a characteristic crystalline structure at this temperature, known as table salt.
• Mercury ( Hg ): Is liquid. Unique among metals, mercury retains its liquid form largely due to its low melting point.
• Methane ( CH_4 ): Is gaseous. CH₄ exists in a gaseous state at 298 K as its boiling point is significantly lower than this temperature.

The phase of these substances at 298 K indicates their normal state of existence within a given environmental condition, essential for accurate scientific calculations and practical industrial applications.