What is the term used to describe the phase change of a liquid to a gas?

Chapter ten. Solids and Liquids

Stage Transitions: Melting, Boiling, and Subliming

1. Draw what happens during a phase change.
2. Calculate the free energy change needed for a stage alter.

Substances tin can change phase — oftentimes considering of a temperature change. At depression temperatures, nearly substances are solid; as the temperature increases, they become liquid; at higher temperatures however, they become gaseous.

The process of a solid becoming a liquid is called melting (an older term that you lot may run into sometimes is fusion). The reverse process, a liquid becoming a solid, is chosen solidification. For whatever pure substance, the temperature at which melting occurs — known as the melting bespeak — is a characteristic of that substance. It requires energy for a solid to melt into a liquid. Every pure substance has a certain amount of energy information technology needs to change from a solid to a liquid. This amount is chosen the enthalpy of fusion (or heat of fusion) of the substance, represented as ΔH _fus. Some ΔH _fus values are listed in Tabular array 10.2 "Enthalpies of Fusion for Diverse Substances"; it is causeless that these values are for the melting bespeak of the substance. Note that the unit of measurement of ΔH _fus is kilojoules per mole, so nosotros need to know the quantity of material to know how much energy is involved. The ΔH _fus is always tabulated as a positive number. However, information technology can exist used for both the melting and the solidification processes every bit long as y'all keep in mind that melting is always endothermic (and so ΔH volition be positive), while solidification is e'er exothermic (and then ΔH will be negative).

Table 10.2 Enthalpies of Fusion for Various Substances

Substance (Melting Point)	ΔH fus (kJ/mol)
H2o (0°C)	6.01
Aluminum (660°C)	x.7
Benzene (5.v°C)	nine.95
Ethanol (−114.iii°C)	5.02
Mercury (−38.viii°C)	2.29

What is the energy change when 45.7 g of H₂O melt at 0°C?

Solution

The ΔH _fus of H₂O is half dozen.01 kJ/mol. Even so, our quantity is given in units of grams, not moles, so the first step is to convert grams to moles using the molar mass of H₂O, which is 18.0 m/mol. Then we tin can use ΔH _fus as a conversion factor. Because the substance is melting, the process is endothermic, so the free energy change will take a positive sign.

Without a sign, the number is assumed to be positive.

Examination Yourself

What is the energy change when 108 yard of C_sixH_six freeze at 5.5°C?

Reply

−thirteen.8 kJ

During melting, energy goes exclusively to irresolute the phase of a substance; it does not go into changing the temperature of a substance. Hence melting is an isothermal process considering a substance stays at the aforementioned temperature. Only when all of a substance is melted does any additional energy go to irresolute its temperature.

What happens when a solid becomes a liquid? In a solid, private particles are stuck in identify because the intermolecular forces cannot exist overcome by the free energy of the particles. When more than energy is supplied (e.grand., by raising the temperature), at that place comes a point at which the particles have enough free energy to move effectually but not enough energy to separate. This is the liquid phase: particles are all the same in contact but are able to move around each other. This explains why liquids tin presume the shape of their containers: the particles move around and, under the influence of gravity, fill the lowest volume possible (unless the liquid is in a zero-gravity environment — run across Figure 10.sixteen "Liquids and Gravity").

Figure 10.16 "Liquids and Gravity." (a) A liquid fills the bottom of its container as it is drawn downward by gravity and the particles slide over each other. (b) A liquid floats in a zero- gravity environment. The particles all the same slide over each other because they are in the liquid phase, but now there is no gravity to pull them downwardly.

The phase modify between a liquid and a gas has some similarities to the phase change between a solid and a liquid. At a certain temperature, the particles in a liquid accept plenty energy to become a gas. The process of a liquid becoming a gas is called boiling (or vapourization), while the procedure of a gas becoming a liquid is called condensation. However, unlike the solid/liquid conversion process, the liquid/gas conversion process is noticeably affected past the surrounding pressure on the liquid because gases are strongly affected by pressure. This means that the temperature at which a liquid becomes a gas, the humid point, can change with surrounding pressure level. Therefore, we ascertain the normal humid point equally the temperature at which a liquid changes to a gas when the surrounding pressure is exactly one atm, or 760 torr. Unless otherwise specified, it is assumed that a boiling bespeak is for 1 atm of pressure.

Like the solid/liquid stage change, the liquid/gas stage modify involves energy. The amount of energy required to convert a liquid to a gas is called the enthalpy of vaporization (or estrus of vaporization), represented as ΔH _vap. Some ΔH _vap values are listed in Table 10.3 "Enthalpies of Vaporization for Diverse Substances"; it is assumed that these values are for the normal boiling signal temperature of the substance, which is too given in the tabular array. The unit for ΔH _vap is besides kilojoules per mole, so we demand to know the quantity of material to know how much energy is involved. The ΔH _vap is also always tabulated as a positive number. It can exist used for both the boiling and the condensation processes as long as you keep in mind that boiling is always endothermic (then ΔH will be positive), while condensation is always exothermic (then ΔH will exist negative).

Table 10.three Enthalpies of Vaporization for Various Substances

Substance (Normal Boiling Point)	ΔH vap (kJ/mol)
Water (100°C)	40.68
Bromine (59.5°C)	fifteen.four
Benzene (80.1°C)	30.8
Ethanol (78.iii°C)	38.six
Mercury (357°C)	59.23

What is the free energy change when 66.7 k of Br₂(g) condense to a liquid at 59.5°C?

Solution

The ΔH _vap of Br_two is 15.4 kJ/mol. Fifty-fifty though this is a condensation process, we tin still utilise the numerical value of ΔH _vap as long as we realize that nosotros must have energy out, and then the ΔH value will be negative. To make up one's mind the magnitude of the energy change, we must first convert the corporeality of Br₂ to moles. Then we tin use ΔH _vap equally a conversion factor.

Because the process is exothermic, the actual value will be negative: ΔH = −vi.43 kJ.

Test Yourself

What is the energy modify when 822 g of C₂H₅OH(ℓ) boil at its normal boiling bespeak of 78.iii°C?

Answer

689 kJ

As with melting, the energy in boiling goes exclusively to changing the phase of a substance; information technology does not go into changing the temperature of a substance. So boiling is likewise an isothermal process. Merely when all of a substance has boiled does any additional energy become to changing its temperature.

What happens when a liquid becomes a gas? We accept already established that a liquid is equanimous of particles in contact with each other. When a liquid becomes a gas, the particles split up from each other, with each particle going its ain way in infinite. This is how gases tend to make full their containers. Indeed, in the gas phase near of the volume is empty space; merely about i one-thousandth of the book is actually taken upwardly by affair (see Figure 10.17 "Liquids and Gases"). It is this property of gases that explains why they can be compressed, a fact that is considered in Chapter 6 "Gases".

Figure ten.17 Liquids and Gases. In (a), the particles are a liquid; the particles are in contact simply are also able to move effectually each other. In (b), the particles are a gas, and about of the volume is really empty space. The particles are non to calibration; in reality, the dots representing the particles would be about one one-thousandth the size as depicted.

Under some circumstances, the solid stage tin transition directly to the gas phase without going through a liquid stage, and a gas can straight become a solid. The solid-to-gas change is called sublimation, while the reverse process is called deposition. Sublimation is isothermal, like the other phase changes. There is a measurable energy change during sublimation; this free energy change is called the enthalpy of sublimation, represented as ΔH _sub. The relationship between the ΔH _sub and the other enthalpy changes is as follows:

ΔH _sub = ΔH _fus + ΔH _vap

As such, ΔH _sub is not e'er tabulated because it can exist just calculated from ΔH _fus and ΔH _vap.

There are several common examples of sublimation. A well-known product — dry ice — is really solid CO₂. Dry water ice is dry because it sublimes, with the solid bypassing the liquid phase and going direct to the gas phase. The sublimation occurs at temperature of −77°C, then it must exist handled with caution. If you lot have ever noticed that water ice cubes in a freezer tend to become smaller over time, it is considering the solid h2o is very slowly subliming. "Freezer burn" isn't actually a burn; it occurs when certain foods, such as meats, slowly lose solid water content considering of sublimation. The food is all the same good just looks unappetizing. Reducing the temperature of a freezer volition boring the sublimation of solid water.

Chemic equations can exist used to represent a phase change. In such cases, information technology is crucial to use phase labels on the substances. For instance, the chemical equation for the melting of ice to brand liquid water is as follows:

H_twoO(s) → H_iiO(ℓ)

No chemical change is taking place; however, a physical change is taking place.

Heating Curves

A plot of the temperature versus the amount of heat added is known as a heating curve (see Effigy ten.18). These are commonly used to visually show the relationship between phase changes and enthalpy for a given substance.

Figure ten.xviii "Generic heating curve diagram."

In Effigy x.xviii^[1], the solid gains kinetic energy and consequently rises in temperature equally heat is added. At the melting point, the rut added is used to intermission the attractive intermolecular forces of the solid instead of increasing kinetic energy, and therefore the temperature remains constant. After all the solid has melted, once once more, the heat added goes to increasing the kinetic free energy (and temperature) of the liquid molecules until the humid point. At the boiling point, over again, the heat added is used to break the attractive intermolecular forces instead of supplying kinetic energy, and the temperature remains constant until all liquid has been turned to gas.

• Phase changes can occur betwixt whatsoever ii phases of matter.
• All phase changes occur with a simultaneous change in energy.
• All stage changes are isothermal.

Questions

1. What is the deviation between melting and solidification?
2. What is the difference betwixt boiling and condensation?
3. Describe the molecular changes when a solid becomes a liquid.
4. Describe the molecular changes when a liquid becomes a gas.
5. What is the energy change when 78.0 g of Hg cook at −38.8°C?
6. What is the energy change when thirty.8 g of Al solidify at 660°C?
7. What is the energy change when 111 g of Br_two boil at 59.5°C?
8. What is the free energy change when 98.6 grand of H₂O condense at 100°C?
9. Each of the following statements is wrong. Rewrite them and so they are right.
   1. Temperature changes during a stage change.
   2. The process of a liquid becoming a gas is called sublimation.
10. Each of the following statements is incorrect. Rewrite them so they are right.
    1. The volume of a gas contains only about 10% thing, with the rest existence empty space.
    2. ΔH _sub is equal to ΔH _vap.
11. Write the chemical equation for the melting of elemental sodium.
12. Write the chemical equation for the solidification of benzene (C₆H_six).
13. Write the chemical equation for the sublimation of CO₂.
14. Write the chemical equation for the boiling of propanol (C₃H₇OH).
15. What is the ΔH _sub of H₂O? (Hint: come across Tabular array 10.2 "Enthalpies of Fusion for Diverse Substances" and Table x.3 "Enthalpies of Vaporization for Various Substances".)
16. The ΔH _sub of I_ii is threescore.46 kJ/mol, while its ΔH _vap is 41.71 kJ/mol. What is the ΔH _fus of I_ii?

Answers

1. Melting is the phase alter from a solid to a liquid, whereas solidification is the phase change from a liquid to a solid.

3. The molecules have plenty free energy to move about each other but not enough to completely separate from each other.

5. 890 J

7. x.vii kJ

9. 1. Temperature does not alter during a phase change.
   2. The process of a liquid becoming a gas is chosen humid; the procedure of a solid becoming a gas is called sublimation.

11. Na(south) → Na(ℓ)

13. CO_two(south) → CO₂(grand)

15. 46.69 kJ/mol

Media Attributions

• "Drinking glass of Water" © 2005 by Derek Jensen is licensed under a Public Domain license
• "Clayton Anderson goose egg g" © 2010 by NASA is licensed under a Public Domain license

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Source: https://opentextbc.ca/introductorychemistry/chapter/phase-transitions-melting-boiling-and-subliming/

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