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Chapter 6: Problem 18

What is meant by the reference form of an element? What is the standard enthalpy of formation of an element in its reference form?

Short Answer

Expert verified
The reference form is an element's most stable form at standard conditions, and its standard enthalpy of formation is zero.

Step by step solution

01

Understanding "Reference Form"

The reference form of an element refers to the most stable physical form of that element at standard conditions, which is at 1 bar of pressure and 25掳C (298 K). For example, the reference form for Oxygen is O鈧� gas, not atomic oxygen, due to its diatomic nature.
02

Defining "Standard Enthalpy of Formation"

Standard enthalpy of formation is the change in enthalpy when one mole of a substance is formed from its elements in their reference forms, at standard conditions. This is usually denoted as 螖贬冲蹿镑掳.
03

Standard Enthalpy of Formation for Elements

For any element in its reference form, the standard enthalpy of formation is defined as zero. This is because no energy is required to form an element from itself under standard conditions.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Reference Form of an Element
In the context of chemistry, the reference form of an element is a key concept when considering chemical reactions and energy changes. It refers to the most stable form of an element under standard conditions, which are typically 1 bar of pressure and 25掳C (298 K). The stability of elements can vary based on several factors, but the reference form is determined by how an element is most commonly found in nature. For example:
  • Carbon's reference form is graphite, as this is the most stable allotrope of carbon at standard conditions.
  • For oxygen, it's the diatomic molecule O鈧�, not atomic or ozone forms.
  • Phosphorus's reference form is white phosphorus.

Identifying the reference form of an element is crucial because it sets the baseline for calculating various thermodynamic properties, including the standard enthalpy of formation.
Standard Conditions
When we talk about standard conditions in chemistry, we're setting a common benchmark for measuring and comparing various reactions and processes. Standard conditions are defined as a pressure of 1 bar and a temperature of 25掳C (298 K). These conditions are sometimes referred to as standard ambient temperature and pressure (SATP).
  • Standard conditions allow scientists to compare results from different experiments under consistent criteria.
  • They simplify the presentation of thermodynamic data by ensuring that all measurements are taken under the same conditions.
  • This uniformity is essential for calculating thermodynamic properties like enthalpy, entropy, and Gibbs free energy.

Using these conditions helps in understanding how substances behave and react, providing a clearer picture of their inherent properties. It is under these conditions that the reference form of an element is considered stable.
螖贬冲蹿镑掳
The symbol \( 螖贬冲蹿镑掳 \), pronounced as "Delta H f naught," represents the standard enthalpy of formation. This thermodynamic quantity refers to the change in enthalpy when one mole of a compound is formed from its constituent elements in their reference forms at standard conditions.
螖贬冲蹿镑掳 is expressed in kilojoules per mole (kJ/mol) and provides valuable insight into the energy changes during chemical reactions. Key points include:
  • The standard enthalpy of formation for any element in its reference form is zero. This is because forming an element from its own reference form doesn't involve any energy change.
  • Values of \( 螖贬冲蹿镑掳 \) for compounds give us an idea about the stability of the product. Exothermic reactions, where \( 螖贬冲蹿镑掳 \) is negative, release energy, indicating stable products.
  • On the other hand, a positive \( 螖贬冲蹿镑掳 \) indicates an endothermic process, requiring energy input, often resulting in less stable products.

Understanding \( 螖贬冲蹿镑掳 \) is vital in fields like thermochemistry, where it aids in predicting reaction behavior and helps design energy-efficient chemical processes.

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Most popular questions from this chapter

When 2 mol of potassium chlorate crystals decompose to potassium chloride crystals and oxygen gas at constant temperature and pressure, \(78.0 \mathrm{~kJ}\) of heat is given off. Write a thermochemical equation for this reaction.

Is the following reaction the appropriate one to use in determining the enthalpy of formation of methane, \(\mathrm{CH}_{4}(g) ?\) Why or why not? $$ \mathrm{C}(g)+4 \mathrm{H}(g) \longrightarrow \mathrm{CH}_{4}(g) $$

Define an exothermic reaction and an endothermic reaction. Give an example of each.

You wish to heat water to make coffee. How much heat (in joules) must be used to raise the temperature of \(0.180 \mathrm{~kg}\) of tap water (enough for one cup of coffee) from \(19^{\circ} \mathrm{C}\) to \(96^{\circ} \mathrm{C}\) (near the ideal brewing temperature)? Assume the specific heat is that of pure water, \(4.18 \mathrm{~J} /\left(\mathrm{g} \cdot{ }^{\circ} \mathrm{C}\right)\).

Suppose heat flows into a vessel containing a gas. As the heat flows into the gas, what happens to the gas molecules? What happens to the internal energy of the gas?
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