What is the enthalpy of vaporization, and what are its alternative names?

In thermodynamics, the enthalpy of vaporization, also known as the latent heat of vaporization or heat of evaporation, is the amount of energy in the form of enthalpy that must be supplied to a liquid substance to convert a specific quantity of it into a gas. This transformation process is referred to as vaporization or evaporation.

What factors influence the enthalpy of vaporization?

The enthalpy of vaporization is dependent on the pressure and temperature at which the phase transition from liquid to gas occurs. It is not a constant value for a substance but varies with these conditions.

Under what conditions is the enthalpy of vaporization typically reported?

The enthalpy of vaporization is commonly reported for the normal boiling temperature of a substance. While tabulated values are often adjusted to a standard temperature of 298 K, this correction is frequently smaller than the inherent uncertainty in the measured value.

How does temperature affect the heat of vaporization?

The heat of vaporization is temperature-dependent. It can be assumed to be constant over small temperature ranges or when the reduced temperature is significantly less than 1. However, the heat of vaporization decreases as temperature increases and completely disappears at the critical temperature, where the liquid and vapor phases become indistinguishable.

What is the critical temperature in the context of vaporization?

The critical temperature is the specific point at which the heat of vaporization of a substance becomes zero. Above this temperature, the distinction between the liquid and vapor phases ceases to exist, and the substance exists as a supercritical fluid.

What are the common units used to express the enthalpy of vaporization?

Values for enthalpy of vaporization are typically expressed in units of joules per mole (J/mol) or kilojoules per mole (kJ/mol) for molar enthalpy. Specific heat of vaporization is often given in kilojoules per kilogram (kJ/kg) or joules per gram (J/g). Older units like kilocalories per mole (kcal/mol), calories per gram (cal/g), and British thermal units per pound (Btu/lb) are also sometimes encountered.

How does the enthalpy of condensation relate to the enthalpy of vaporization?

The enthalpy of condensation is defined as being equal in magnitude but opposite in sign to the enthalpy of vaporization. While vaporization requires energy input (positive enthalpy change), condensation releases energy (negative enthalpy change).

What is the thermodynamic definition of the enthalpy of vaporization?

Thermodynamically, the enthalpy of vaporization (ΔHvap) is expressed as the sum of the increase in internal energy during vaporization (ΔUvap) and the work done against the ambient pressure (pΔV). This can be represented by the equation ΔHvap = ΔUvap + pΔV.

What does the internal energy component of vaporization represent?

The increase in internal energy during vaporization represents the energy needed to overcome the intermolecular forces that hold the molecules together in the liquid (or solid, in the case of sublimation) phase.

Why does helium have a notably low enthalpy of vaporization?

Helium exhibits a particularly low enthalpy of vaporization because the intermolecular forces between its atoms, specifically the van der Waals forces, are exceptionally weak. This results in a low energy requirement for the transition from liquid to gas.

How does water's enthalpy of vaporization compare to the energy required for its sensible heating?

Water's enthalpy of vaporization is 40.65 kJ/mol, which is more than five times the energy needed to heat the same amount of water from 0°C to 100°C. This significant difference is attributed to the relatively strong hydrogen bonds holding water molecules together in the liquid state.

What caution should be exercised when using enthalpy of vaporization to assess intermolecular forces?

Care must be taken when using enthalpy of vaporization to measure the strength of intermolecular forces, as these forces may persist to some degree in the gas phase. For instance, in hydrogen fluoride, these forces remain, leading to a calculated bond strength that is lower than the actual value. For metals, which often form covalently bonded molecules in the gas phase, the enthalpy of atomization should be used instead to determine true bond energy.

What is the relationship between entropy, enthalpy, and boiling point?

At the boiling point (Tb), the liquid and gas phases are in equilibrium, meaning the change in Gibbs free energy (ΔG) is zero. This leads to the relationship where the change in entropy of vaporization (ΔvS) equals the enthalpy of vaporization divided by the boiling point (ΔvS = ΔvH / Tb). This indicates that the increased entropy of the gas phase overcomes the intermolecular forces at the boiling point.

How does Gibbs free energy change with temperature during vaporization?

According to the Gibbs free energy equation (ΔG = ΔH - TΔS), the Gibbs free energy change decreases as temperature increases. This thermodynamic principle explains why gases are favored at higher temperatures, consistent with practical observations.

What does the table of enthalpies of vaporization for elements represent?

The table displays the enthalpies of vaporization for various chemical elements, measured in kilojoules per mole (kJ/mol) at their respective normal boiling points. This data is crucial for applications like metal vapor synthesis.

What do the different background colors in the elements table signify?

The background colors in the elements table categorize the enthalpies of vaporization into ranges: light pink for 0-10 kJ/mol, light green for 10-100 kJ/mol, light blue for 100-300 kJ/mol, and light yellow for values greater than 300 kJ/mol.

Which element in the table has the lowest enthalpy of vaporization?

Helium (He), located in Period 1, Group 18, has the lowest enthalpy of vaporization listed in the table, with a value of 0.08 kJ/mol.

What is the enthalpy of vaporization for zinc (Zn)?

The enthalpy of vaporization for zinc (Zn), as listed in the table of elements, is 115 kJ/mol.

What does a dashed border around an element in the table indicate?

A dashed border around an element in the table, such as for elements in Period 7 and some in Period 5 and 6, typically indicates that the enthalpy of vaporization value is less reliably determined, estimated, or may not be applicable in the same standard context as elements with solid borders. For radioactive elements, it often signifies that the value is an estimate or not readily measured.

What does 'n/a' signify in the elements table for certain elements?

The notation 'n/a' in the elements table, particularly for elements in Period 7 like Lawrencium (Lr) and beyond, indicates that the enthalpy of vaporization is not available or has not been determined, often due to the element's radioactivity and short half-life, making standard measurements impractical.

What is the enthalpy of vaporization for water listed in the 'Other common substances' table?

For water, the enthalpy of vaporization is listed as 40.66 kJ/mol at its standard boiling point of 373.15 K (100 °C).

Compare the enthalpy of vaporization of water to that of ethanol.

Water has an enthalpy of vaporization of 40.66 kJ/mol, while ethanol has a higher value of 38.6 kJ/mol. This comparison highlights the different strengths of intermolecular forces present in these two substances.

What is the enthalpy of vaporization for hydrogen (parahydrogen)?

The enthalpy of vaporization for parahydrogen is 0.8992 kJ/mol, measured at its boiling point of 20.271 K (-252.879 °C).

What is the enthalpy of vaporization for methane (CH4)?

Methane has an enthalpy of vaporization of 8.170 kJ/mol, measured at its boiling point of 112 K (-161 °C).

What is the enthalpy of vaporization for acetone?

Acetone has an enthalpy of vaporization of 31.300 kJ/mol, measured at its boiling point of 329 K (56 °C).

What is the enthalpy of vaporization for aluminum (Al)?

Aluminum has a high enthalpy of vaporization of 294.0 kJ/mol, measured at its very high boiling point of 2792 K (2519 °C).

What does the first image caption describe?

The first image caption describes a visual representation showing the temperature dependency of the heats of vaporization for four specific substances: water, methanol, benzene, and acetone.

What information is provided in the caption for the image of zinc's molar enthalpy?

The caption for the image detailing the molar enthalpy of zinc explains that it is shown above 298.15 K at 1 atm pressure. It highlights discontinuities at the melting and boiling points, specifying the enthalpy of melting (ΔH°m) as 7323 J/mol and the enthalpy of vaporization (ΔH°v) as 115330 J/mol.

What related thermodynamic concepts are listed in the 'See also' section?

The 'See also' section lists related concepts such as the Clausius-Clapeyron relation, the Shimansky equation (which describes temperature dependence of heat of vaporization), enthalpy of fusion, enthalpy of sublimation, the Joback method for estimating heat of vaporization, and the general concept of latent heat.

What thermodynamic models can be used for estimating the enthalpy of vaporization of electrolyte solutions?

The Pitzer model and the TCPC model are mentioned as chemical thermodynamic models that can be used for estimating the enthalpy of vaporization of electrolyte solutions.

What is the relationship between the enthalpy of vaporization and the entropy change at the boiling point?

At the boiling point, where the liquid and gas phases are in equilibrium, the change in Gibbs free energy is zero. This leads to the relationship where the entropy of vaporization is equal to the enthalpy of vaporization divided by the boiling temperature (ΔvS = ΔvH / Tb).

What does the Navbox titled 'States of matter' list?

The Navbox titled 'States of matter' provides a categorized list of various states of matter, including common states like solid, liquid, gas, and plasma, as well as states associated with low energy (like Bose-Einstein condensates), high energy (like quark-gluon plasma), and other or exotic states. It also includes related phase transitions and thermodynamic quantities.

What does the Navbox section 'Quantities' include?

The 'Quantities' section within the 'States of matter' Navbox lists key thermodynamic quantities related to phase transitions, such as enthalpy of fusion, enthalpy of sublimation, enthalpy of vaporization, latent heat, latent internal energy, and Trouton's rule.

What does the Authority Control section indicate for the topic of Enthalpy of Vaporization?

The Authority Control section provides links to various databases that catalog information about the enthalpy of vaporization. These include international databases like GND (German National Library), FAST (Freely Accessible Substances Thesaurus), and national databases such as the US Library of Congress, the French National Library (BnF), and the National Library of Israel.

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Definition

The Core Concept

In thermodynamics, the enthalpy of vaporization (symbol $\u2206Hvap$ ), also known as the latent heat of vaporization or heat of evaporation, quantifies the energy required to convert a specific amount of a liquid substance into a gas at a given pressure and temperature. This transformation, known as vaporization or evaporation, involves overcoming the intermolecular forces present in the liquid phase.

Dependence on Conditions

The enthalpy of vaporization is not a fixed value; it is intrinsically dependent on the specific pressure and temperature at which the phase change occurs. While often quoted at the substance's normal boiling point, tabulated values are frequently adjusted to standard conditions like 298 K (25 °C). The value diminishes as temperature increases, ultimately reaching zero at the substance's critical temperature, beyond which the liquid and gas phases become indistinguishable.

Intermolecular Forces

A significant portion of the enthalpy of vaporization is attributed to the energy needed to overcome the attractive forces between molecules in the liquid state. Substances with strong intermolecular forces, such as water with its extensive hydrogen bonding, exhibit high enthalpies of vaporization. Conversely, substances with weak forces, like helium, have very low values.

Units of Measurement

Standard Units

The enthalpy of vaporization is commonly expressed in units that reflect energy per amount of substance. The most frequent units are:

Joules per mole (J/mol)
Kilojoules per mole (kJ/mol): This is the standard molar enthalpy of vaporization.

Alternative Units

Depending on the context and field of study, other units may be encountered:

Joules per gram (J/g) or Kilojoules per kilogram (kJ/kg): These represent the specific heat of vaporization.
Older or specialized units like kilocalories per mole (kcal/mol), calories per gram (cal/g), and British thermal units per pound (Btu/lb) are sometimes still used.

Thermodynamic Principles

Energy Balance

The enthalpy of vaporization ( $\u2206Hvap$ ) can be understood through the first law of thermodynamics. It equals the change in internal energy ( $\u2206Uvap$ ) plus the work done by the system against the surrounding pressure ( $p$ ) as the volume expands ( $\u2206V$ ). Mathematically:

ΔH_vap = ΔU_vap + p * ΔV

This equation highlights that vaporization requires energy not only to increase the internal energy (breaking molecular bonds/interactions) but also to perform mechanical work by expanding against external pressure.

Entropy and Equilibrium

At the boiling point ( $T b$ ), the liquid and gas phases are in equilibrium ( $\u2206vG = 0$ ). This relationship connects enthalpy and entropy changes:

Δ_vS = S_gas - S_liquid = Δ_vH / T_b

This indicates that the increase in entropy during vaporization is directly proportional to the heat absorbed and inversely proportional to the boiling temperature. Since entropy generally increases with temperature, the gas phase is favored at higher temperatures.

Electrolyte Solutions

Estimating the enthalpy of vaporization for electrolyte solutions requires specialized thermodynamic models. Advanced approaches like the Pitzer model or the TCPC model are employed to accurately calculate these values, considering the complex interactions between ions and solvent molecules.

Enthalpies of Vaporization: Elements

The table below presents the molar enthalpies of vaporization (in kJ/mol) for various chemical elements, measured at their respective normal boiling points. Note the significant variations reflecting differences in metallic bonding strength and atomic interactions.

Enthalpies of vaporization of the elements (kJ/mol)
	1	2		3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
Group →
↓ Period
1	H 0.90																		He 0.08
2	Li 136	Be 292												B 508	C 715	N 5.57	O 6.82	F 6.62	Ne 1.71
3	Na 97.4	Mg 128												Al 284	Si 359	P 12.4	S 45	Cl 20.4	Ar 6.53
4	K 76.9	Ca 155		Sc 333	Ti 425	V 444	Cr 339	Mn 221	Fe 340	Co 377	Ni 379	Cu 300	Zn 115	Ga 256	Ge 334	As 32.4	Se 95.5	Br 30.0	Kr 9.08
5	Rb 75.8	Sr 141		Y 390	Zr 573	Nb 690	Mo 617	Tc 585	Ru 619	Rh 494	Pd 358	Ag 254	Cd 99.9	In 232	Sn 296	Sb 193	Te 114	I 41.6	Xe 12.6
6	Cs 63.9	Ba 140	*	Lu 414	Hf 648	Ta 733	W 807	Re 704	Os 678	Ir 564	Pt 510	Au 342	Hg 59.1	Tl 165	Pb 179	Bi 179	Po 103	At 54.4	Rn 18.1
7	Fr 65	Ra 113	**	Lr n/a	Rf n/a	Db n/a	Sg n/a	Bh n/a	Hs n/a	Mt n/a	Ds n/a	Rg n/a	Cn n/a	Nh n/a	Fl n/a	Mc n/a	Lv n/a	Ts n/a	Og n/a

*				La 400	Ce 398	Pr 331	Nd 289	Pm 289	Sm 172	Eu 176	Gd 301	Tb 391	Dy 280	Ho 251	Er 280	Tm 191	Yb 129
**				Ac 400	Th 514	Pa 481	U 417	Np 336	Pu 333	Am n/a	Cm n/a	Bk n/a	Cf n/a	Es n/a	Fm n/a	Md n/a	No n/a
Enthalpy in kJ/mol, measured at their respective normal boiling points. Dashed/dotted borders indicate estimated or radioactive elements.

Enthalpies of Vaporization: Common Substances

A comparative look at the enthalpy of vaporization for several common compounds, illustrating the wide range of energy requirements for phase transition.

Compound	Boiling point, at normal pressure			Heat of vaporization
Compound	(K)	(°C)	(°F)	(kJ/mol)	(J/g)
Acetone	329	56	133	31.300	538.9
Aluminium	2792	2519	4566	294.0	10500
Ammonia	240	−33.34	−28	23.35	1371
Butane	272–274	−1	30–34	21.0	320
Diethyl ether	307.8	34.6	94.3	26.17	353.1
Ethanol	352	78.37	173	38.6	841
Hydrogen (parahydrogen)	20.271	−252.879	−423.182	0.8992	446.1
Iron	3134	2862	5182	340	6090
Isopropyl alcohol	356	82.6	181	44	732.2
Methane	112	−161	−259	8.170	480.6
Methanol	338	64.7	148	35.2	1104
Propane	231	−42	−44	15.7	356
Phosphine	185	−87.7	−126	14.6	429.4
Water	373.15	100	212	40.66	2257

Related Concepts

Further Study

Understanding the enthalpy of vaporization often involves exploring related thermodynamic principles and phenomena. Key areas include:

Clausius–Clapeyron relation: Describes the temperature dependence of vapor pressure.
Shimansky equation: Models the temperature dependence of the heat of vaporization.
Enthalpy of fusion: Energy required for melting (solid to liquid).
Enthalpy of sublimation: Energy required for sublimation (solid to gas).
Joback method: Estimation of thermodynamic properties from molecular structure.
Latent heat: General term for heat absorbed or released during a phase change.

Broader Context

The concept is also linked to the broader study of the states of matter and their transitions. Understanding these relationships is fundamental in fields ranging from chemical engineering to materials science.

States of Matter & Transitions
States Transitions Boiling Point Critical Point

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References

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The Energetics of Transformation

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Thermodynamic Principles 🧮

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⚖️ Entropy and Equilibrium

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Enthalpies of Vaporization: Elements 💎

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Related Concepts 🔗

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Dive in with Flashcard Learning!

Definition

The Core Concept

Dependence on Conditions

Intermolecular Forces

Units of Measurement

Standard Units

Alternative Units

Thermodynamic Principles

Energy Balance

Entropy and Equilibrium

Electrolyte Solutions

Enthalpies of Vaporization: Elements

Enthalpies of Vaporization: Common Substances

Related Concepts

Further Study

Broader Context

Teacher's Corner

True or False?

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Discover other topics to study!

References

Feedback & Support

Disclaimer

Important Notice