Question

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

Short answer

Standard state: \(\mathrm{H}_2\mathrm{O}(l), \mathrm{NaCl}(s), \mathrm{Hg}(l), \mathrm{CH}_4(g)\) at 298 K.

Step-by-step solution

Understanding Standard State

The term 'standard state' refers to the standard conditions for measuring substances at a specified pressure and often at a specified temperature, usually 1 atmosphere (101.3 kPa) and 298 K (25°C). For a substance in its standard state, it means it is pure, and at 1 atm pressure. The actual physical state (solid, liquid, or gas) depends on the substance and its natural condition at this temperature.

Identify Standard State for H2O at 298 K

Water (\(\mathrm{H}_2\mathrm{O}\)) at 298 K is in its liquid form. Therefore, the standard state of water at 298 K is liquid (\(\mathrm{l}\)).

Identify Standard State for NaCl at 298 K

Sodium chloride (\(\mathrm{NaCl}\)) is a solid at 298 K. Thus, the standard state of \(\mathrm{NaCl}\) at 298 K is solid (\(\mathrm{s}\)).

Identify Standard State for Hg at 298 K

Mercury (\(\mathrm{Hg}\)) at 298 K is known to be in its liquid state. Hence, the standard state of mercury at 298 K is liquid (\(\mathrm{l}\)).

Identify Standard State for CH4 at 298 K

Methane (\(\mathrm{CH}_4\)) is a gas at 298 K under standard pressure conditions. Consequently, the standard state of methane at 298 K is gas (\(\mathrm{g}\)).

Key concepts

Thermochemistry

Thermochemistry is the branch of chemistry that deals with the heat energy involved in chemical reactions and physical transformations. It focuses on understanding how energy, particularly in the form of heat, is absorbed or released during chemical processes. This is why you often see equations indicating heat as a product or reactant.

Key points to remember include:

• Exothermic reactions release heat, making the surrounding temperature rise.
• Endothermic reactions absorb heat, causing the surrounding temperature to drop.
• Calorimetry is a common method used to measure the heat involved in chemical reactions.

Understanding these energy changes provides insight into the spontaneity and feasibility of reactions. It helps predict how physical states might shift under different conditions.

Phase of Matter

The phase of matter refers to the distinct forms that different phases of matter take on. Common phases include solid, liquid, and gas.

Each substance has a particular phase at specific temperature and pressure conditions.

Here's what defines the phases at standard state:

• Solids: Have a definite shape and volume. Particles are tightly packed in a fixed arrangement.
• Liquids: Have a definite volume but take the shape of their container. Particles are close but can move past one another.
• Gases: Have no definite shape or volume, filling the container they are in. Particles move freely and are well separated.

Each of these phases reacts differently under various conditions, which plays a crucial role in determining their behavior in chemical reactions.

Standard Conditions

Standard conditions refer to a set benchmark used by scientists to compare different chemical reactions and processes. These conditions typically include a temperature of 298 K (25°C) and a pressure of 1 atmosphere (101.3 kPa).

Standard conditions simplify the calculation and comparison of thermodynamic properties. They help define the standard state of a substance, which is the form a substance takes under these conditions.

In practical terms, this means associating:

• Water as a liquid.
• Sodium chloride as a solid.
• Mercury as a liquid.
• Methane as a gas.

Knowing standard states is essential for predicting how substances will react or transform under usual environmental conditions.

Chemical Thermodynamics

Chemical thermodynamics explores the interplay between heat, work, and energy in chemical reactions. It involves principles that govern the direction and extent of chemical processes. The three fundamental laws of thermodynamics offer a framework for understanding these interactions.

Here are a few key concepts:

• First Law: Energy cannot be created or destroyed, only transformed (conservation of energy).
• Second Law: Entropy of an isolated system always increases. This often relates to the spontaneity of reactions.
• Third Law: As temperature approaches absolute zero, the entropy of a perfect crystal approaches a constant minimum.

Using these laws, scientists can predict how energy changes will affect a chemical reaction’s behavior, including the stability of substances under standard conditions.