Which Of The Following Is Not A Phase Change?

Chapter 10. Solids and Liquids

Phase Transitions: Melting, Boiling, and Subliming

Draw what happens during a phase modify.
Calculate the energy change needed for a phase change.

Substances can change stage — oftentimes because of a temperature alter. At depression temperatures, nigh substances are solid; as the temperature increases, they get liquid; at higher temperatures even so, they become gaseous.

The process of a solid becoming a liquid is called melting (an older term that you may encounter sometimes is fusion). The opposite process, a liquid condign a solid, is called solidification. For whatever pure substance, the temperature at which melting occurs — known as the melting point — is a characteristic of that substance. It requires energy for a solid to melt into a liquid. Every pure substance has a certain corporeality of energy information technology needs to alter from a solid to a liquid. This amount is called the enthalpy of fusion (or estrus of fusion) of the substance, represented as ΔH _fus. Some ΔH _fus values are listed in Table x.ii "Enthalpies of Fusion for Diverse Substances"; it is causeless that these values are for the melting point of the substance. Note that the unit of ΔH _fus is kilojoules per mole, so we need to know the quantity of cloth to know how much energy is involved. The ΔH _fus is always tabulated every bit a positive number. All the same, information technology tin be used for both the melting and the solidification processes as long as you lot proceed in mind that melting is always endothermic (so ΔH will be positive), while solidification is always exothermic (so ΔH will be negative).

Table ten.2 Enthalpies of Fusion for Various Substances
Substance (Melting Signal)	ΔH _fus (kJ/mol)
H2o (0°C)	6.01
Aluminum (660°C)	10.seven
Benzene (v.5°C)	9.95
Ethanol (−114.three°C)	5.02
Mercury (−38.8°C)	2.29

What is the energy change when 45.seven g of H₂O cook at 0°C?

Solution

The ΔH _fus of H₂O is 6.01 kJ/mol. Yet, 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 nosotros tin apply ΔH _fus every bit a conversion factor. Because the substance is melting, the procedure is endothermic, and then the energy alter will have a positive sign.

$45.7\text{ g }\ce{H2O}\times \left(\dfrac{1\text{ mol }\ce{H2O}}{18.0\text{ g }\ce{H2O}}\right)\times \left(\dfrac{6.01\text{ kJ}}{1\text{ mol }\ce{H2O}}\right)=15.3\text{ kJ}$

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

Test Yourself

What is the energy change when 108 g of C₆H₆ freeze at 5.v°C?

Answer

−thirteen.eight kJ

During melting, free energy goes exclusively to irresolute the phase of a substance; it does not go into irresolute the temperature of a substance. Hence melting is an isothermal process because a substance stays at the same 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, individual particles are stuck in place considering the intermolecular forces cannot be overcome by the energy of the particles. When more energy is supplied (e.thou., past raising the temperature), at that place comes a indicate at which the particles have enough energy to movement effectually but non enough energy to carve up. This is the liquid phase: particles are still in contact but are able to move around each other. This explains why liquids can assume the shape of their containers: the particles move effectually and, under the influence of gravity, fill up the everyman volume possible (unless the liquid is in a cipher-gravity environment — come across Effigy ten.16 "Liquids and Gravity").

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

The stage change 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 have enough energy to become a gas. The process of a liquid condign a gas is chosen humid (or vapourization), while the procedure of a gas becoming a liquid is called condensation. Nevertheless, unlike the solid/liquid conversion process, the liquid/gas conversion process is noticeably affected by the surrounding pressure on the liquid considering gases are strongly afflicted by pressure. This ways that the temperature at which a liquid becomes a gas, the boiling point, can change with surrounding pressure. Therefore, we define the normal boiling point as the temperature at which a liquid changes to a gas when the surrounding pressure level is exactly i atm, or 760 torr. Unless otherwise specified, it is assumed that a boiling point is for ane atm of force per unit area.

Like the solid/liquid phase alter, the liquid/gas stage change involves energy. The corporeality of energy required to convert a liquid to a gas is called the enthalpy of vaporization (or heat of vaporization), represented as ΔH _vap. Some ΔH _vap values are listed in Table 10.3 "Enthalpies of Vaporization for Various Substances"; it is assumed that these values are for the normal boiling point temperature of the substance, which is likewise given in the table. The unit for ΔH _vap is also kilojoules per mole, so we need to know the quantity of cloth to know how much energy is involved. The ΔH _vap is also always tabulated as a positive number. It tin can be used for both the boiling and the condensation processes as long as y'all keep in listen that boiling is e'er endothermic (so ΔH volition exist positive), while condensation is always exothermic (and so ΔH will be negative).

Tabular array x.three Enthalpies of Vaporization for Various Substances
Substance (Normal Humid Betoken)	ΔH _vap (kJ/mol)
Water (100°C)	xl.68
Bromine (59.5°C)	15.4
Benzene (80.i°C)	30.8
Ethanol (78.three°C)	38.vi
Mercury (357°C)	59.23

What is the energy modify when 66.7 grand of Br₂(g) condense to a liquid at 59.v°C?

Solution

The ΔH _vap of Br_two is 15.4 kJ/mol. Even though this is a condensation process, we can still use the numerical value of ΔH _vap equally long every bit we realize that we must take energy out, then the ΔH value will exist negative. To determine the magnitude of the energy modify, we must beginning convert the amount of Br₂ to moles. Then we can use ΔH _vap as a conversion gene.

$66.7\text{ g }\ce{Br2}\times \left(\dfrac{1\text{ mol }\ce{Br2}}{159.8\text{ g }\ce{Br2}}\right)\times \left(\dfrac{15.4\text{ kJ}}{1\text{ mol }\ce{Br2}}\right)=6.43\text{ kJ}$

Because the process is exothermic, the actual value volition exist negative: ΔH = −half dozen.43 kJ.

Test Yourself

What is the free energy change when 822 g of C₂H₅OH(ℓ) boil at its normal boiling point of 78.3°C?

Respond

689 kJ

Every bit with melting, the free energy in boiling goes exclusively to irresolute the stage of a substance; it does not become into changing the temperature of a substance. And then boiling is also an isothermal process. Only when all of a substance has boiled does any additional free energy go to changing its temperature.

What happens when a liquid becomes a gas? Nosotros have already established that a liquid is equanimous of particles in contact with each other. When a liquid becomes a gas, the particles separate from each other, with each particle going its own mode in infinite. This is how gases tend to fill up their containers. Indeed, in the gas phase most of the book is empty space; only most one ane-thousandth of the book is actually taken up by matter (come across Figure 10.17 "Liquids and Gases"). Information technology is this property of gases that explains why they can exist compressed, a fact that is considered in Chapter six "Gases".

Figure 10.17 Liquids and Gases. In (a), the particles are a liquid; the particles are in contact but are also able to movement effectually each other. In (b), the particles are a gas, and well-nigh of the volume is actually empty space. The particles are not to scale; in reality, the dots representing the particles would exist nearly i i-thousandth the size as depicted.

Under some circumstances, the solid phase can transition straight to the gas phase without going through a liquid phase, and a gas can directly 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. At that place is a measurable energy change during sublimation; this energy alter is called the enthalpy of sublimation, represented equally 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 non always tabulated considering it can exist merely calculated from ΔH _fus and ΔH _vap.

There are several common examples of sublimation. A well-known product — dry ice — is actually solid CO_two. Dry water ice is dry considering information technology sublimes, with the solid bypassing the liquid phase and going straight to the gas phase. The sublimation occurs at temperature of −77°C, so it must be handled with circumspection. If you lot have ever noticed that ice cubes in a freezer tend to go smaller over time, information technology is considering the solid water is very slowly subliming. "Freezer burn" isn't really a burn; it occurs when certain foods, such every bit meats, slowly lose solid water content because of sublimation. The nutrient is still good but looks unappetizing. Reducing the temperature of a freezer will slow the sublimation of solid h2o.

Chemical equations can be used to represent a stage change. In such cases, it is crucial to use phase labels on the substances. For example, the chemical equation for the melting of ice to make liquid water is as follows:

H₂O(due south) → H₂O(ℓ)

No chemic alter is taking identify; however, a concrete change is taking place.

Heating Curves

A plot of the temperature versus the corporeality of heat added is known as a heating curve (run into Figure 10.18). These are ordinarily used to visually show the relationship between phase changes and enthalpy for a given substance.

Figure ten.xviii "Generic heating curve diagram."

In Figure 10.18^[ane], the solid gains kinetic free energy and consequently rises in temperature as heat is added. At the melting point, the heat added is used to pause 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 over again, the oestrus added goes to increasing the kinetic energy (and temperature) of the liquid molecules until the humid point. At the boiling bespeak, again, the heat added is used to pause the attractive intermolecular forces instead of supplying kinetic free energy, and the temperature remains abiding until all liquid has been turned to gas.

Phase changes can occur between whatsoever two phases of thing.
All phase changes occur with a simultaneous alter in energy.
All phase changes are isothermal.

Questions

What is the deviation between melting and solidification?
What is the difference between humid and condensation?
Describe the molecular changes when a solid becomes a liquid.
Describe the molecular changes when a liquid becomes a gas.
What is the energy change when 78.0 k of Hg melt at −38.8°C?
What is the energy change when 30.8 thousand of Al solidify at 660°C?
What is the energy change when 111 grand of Br₂ boil at 59.v°C?
What is the free energy change when 98.6 one thousand of H₂O condense at 100°C?
Each of the post-obit statements is incorrect. Rewrite them then they are correct.
1. Temperature changes during a phase alter.
2. The process of a liquid becoming a gas is called sublimation.
Each of the following statements is incorrect. Rewrite them and then they are correct.
1. The book of a gas contains merely about 10% matter, with the residual being empty space.
2. ΔH _sub is equal to ΔH _vap.
Write the chemic equation for the melting of elemental sodium.
Write the chemical equation for the solidification of benzene (C₆H₆).
Write the chemical equation for the sublimation of CO₂.
Write the chemical equation for the boiling of propanol (C₃H_viiOH).
What is the ΔH _sub of H₂O? (Hint: encounter Table 10.2 "Enthalpies of Fusion for Various Substances" and Table ten.3 "Enthalpies of Vaporization for Various Substances".)
The ΔH _sub of I_two is 60.46 kJ/mol, while its ΔH _vap is 41.71 kJ/mol. What is the ΔH _fus of I₂?

Answers

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

The molecules have enough energy to motility about each other simply not enough to completely split from each other.

890 J

x.seven kJ

1. Temperature does not change during a phase change.
2. The process of a liquid becoming a gas is called boiling; the procedure of a solid condign a gas is called sublimation.

Na(southward) → Na(ℓ)

CO₂(s) → CO₂(g)

46.69 kJ/mol

Media Attributions