Other Properties of Liquids, Chemistry tutorial

Introduction:

A liquid is a sample of matter which matches to the shape of a container in which it is held, and which attains a defined surface in the presence of gravity. The word liquid is as well employed in reference to the state or condition, of matter having this asset.

The liquid state of matter is an intermediate stage between the solid and gas. Similar to the particles of solid, particles in a liquid are subject to the intermolecular attraction; though, liquid particles have more space between them, therefore they are not fixed in position. The attraction between the particles in a liquid maintains the volume of the liquid constant.

The movement of particles causes the liquid to be variable in shape. The liquids will flow and fill the lowest part of a container, taking on the shape of the container however not changing in volume. The limited amount of space among the particles signifies that liquids have just very limited compressibility.

The molecules or atoms of matter in the liquid state are compressed as tightly as those of matter in the solid state; however the atoms or molecules in a liquid can move freely among one other. Illustrations of liquids are water at room temperature (around 20 ºC or 68 ºF), oil at room temperature and alcohol at room temperature.

Whenever a liquid is heated, the atoms or molecules gain kinetic energy. Whenever the temperature becomes adequately high, the liquid becomes a gas, or it might react by chemicals in the atmosphere. Water is an illustration of a liquid that becomes gaseous if it is heated steadily. Alcohol will combust (that is, combined with oxygen in the atmosphere) whenever heated all of a sudden and dramatically.

If a liquid is cooled, the molecules or atoms lose kinetic energy. If the temperature becomes low adequate, the liquid becomes a solid. Water is an excellent illustration. If cooled down, it freezes to the ice.

Vaporization:

Define: Vaporization is the method of converting a liquid to a gas. It is as well termed as evaporation. As we are familiar that the particles of a gas are moving faster than those of a liquid, an input of energy should be needed for a liquid to become a gas. The most general manner to add energy to a liquid system is via adding heat.

Liquid → Δ → gas

Since, liquid gains energy, the molecules start to move around faster. Whenever a molecule is on the surface of the liquid, and consists of adequate energy, it can break free and become a gas molecule. As with anything in the chemistry, or life for that matter, there are other factors which find out how easily a molecule can break free from the liquid.

The stronger, the intermolecular forces are holding a liquid altogether, the more energy that will be needed to pull them apart. What this implies in practical terms is that a liquid having strong intermolecular forces will have to be heated to a higher temperature prior to it will evaporate.

For illustration: Consider Methane (CH4 molecular weight 16 g/mol) and Water (H2O 18 g/mol). Their molecular weights are very identical; however their Heats of Vaporization (that is, how much heat per mole which has to be added to make them evaporate) are very dissimilar. Water consists of a ΔHvap of around 40.7 kJ/mol and Methane consists of a ΔHvap of 8.2 kJ/mol. Methane is in reality a gas at room temperature due to its low heat of vaporization.

The rate of evaporation of a liquid based on a number of factors. For illustration - more is the surface area, faster will be the evaporation. For faster drying, we enlarge the surface area by spreading the wet clothes. Whenever we supply heat to the liquid, evaporation is faster. The wet clothes dry faster in the sun. The raise in temperature rises the kinetic energy of the molecules of the liquid and the liquid evaporates at a faster rate. We experience cool after the bath. Why do we feel so? This is due to the reason that during evaporation water takes the heat from our body and we feel cold.

Now let us evaluate the rate of evaporation of two liquids, for illustration, water and alcohol. Which of such two liquids evaporates quicker? You should have experienced that alcohol evaporates faster. Why does this occur? The number of molecules escaping from the liquid based on the attractive forces. Whenever these forces are stronger, fewer molecule escapes. In alcohol, such attractive forces are weaker than those in the water. Therefore, alcohol evaporates quicker than water.

Vapor Pressure of a liquid:

Vapor pressure is a measurable quantity which exists whenever a liquid and its vapor are in equilibrium. This is simply possible in the closed systems. However please note that the atmosphere of earth is considered as a closed system. On a much smaller scale this would be if you place any liquid to a sealed container. Molecules at the surface of the liquid would be changing to a gas phase molecules and returning to the liquid till equilibrium was reached. This is what we state a 'dynamic equilibrium'. Since molecules from the liquid move to the gas phase in the container, this increases the pressure above the liquid. The measure of this pressure (minus the normal atmospheric pressure) provides us the vapor pressure of the liquid. The higher this pressure, the more volatile a liquid is stated to be. The vapor pressure is as well based on the temperature of the system. The liquid will still encompass a heat of vaporization to contend with so the equilibrium vapor pressure will rise at higher temperatures.

We are familiar that a liquid put in an open vessel evaporates fully. If, though, the liquid is allowed to evaporate in the closed vessel state in stopper bottle or a bell jar, evaporation takes place, however after sometime the level of liquid doesn't change any further and become constant. Let us comprehend how it happens. In the closed vessel, the molecules evaporating from the liquid surface are restrained to a limited space. Such molecules might collide among themselves or by the molecules of air and some of them might start moving towards the surface of the liquid and enter to it. This is termed as condensation. In the starting, rate of evaporation is more than the rate of condensation. However as more and more molecules mount up in the space above the liquid, the rate of condensation gradually rises. After a period of time, the rate of evaporation becomes equivalent to the rate of condensation and an equilibrium phase is reached.

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Fig: Vapor Pressure of a Liquid

The number of molecules in vapor above the liquid becomes steady. Such molecules apply certain pressure over the surface of the liquid. This pressure is termed as the equilibrium vapor pressure, saturated vapor pressure or just vapor pressure. The vapor pressure of a liquid consists of a characteristic value at a particular temperature. For illustration, vapor pressure of water is around 17.5 torr and that of benzene is 75.00 torr at 20º C. The vapor pressure of a liquid rises with increase in temperature. This is so as at a higher temperature more molecules have adequately high energy to overcome the forces of attraction and escape to form the vapor. A plot of vapor pressure as a function of temperature is termed as vapor pressure curve. Figure shown below shows the vapor pressure curves of several liquids.

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Fig: vapor pressure curve

What would happen if we take away some of the vapors from the closed vessel. Would the vapors pressure of the liquid decrease, increase or remain constant? Vapor pressure of the liquid would remain constant at that temperature. In the starting, the vapor pressure would reduce after the elimination of the vapor, however soon more liquid would evaporate to maintain the equilibrium and the original vapor pressure would be restored. Therefore the vapor pressure of a liquid consists of a definite value at a specific temperature.

Boiling:

The boiling point of a liquid is the temperature at which the liquid will change or transform to a gas. If the atmospheric pressure at this temperature is represented as 1 atmosphere or 760 mm Hg, then this temperature is termed as the normal boiling point of the liquid. Boiling points are thus pressure dependent. (That is, the lower the atmospheric pressure, the lower the boiling point and vice-versa). This is due to the reason that the atmospheric pressure is what is pushing against the surface of the liquid and keeping the liquid down. Whenever the vapor pressure of the liquid is equivalent to or more than the atmospheric pressure, boiling will take place (or in other words vapor will form) and the gas state molecules will escape from the surface of the liquid (we notice this as bubbles).

The boiling point of a liquid based on its nature. A more volatile liquid would boil at a lower temperature than the less volatile liquid. Note that, the diethyl ether boils at a much lower temperature than water, as it is highly volatile liquid. The boiling point of ethanol lies in between those of diethyl ether and water. The vapor pressures or boiling points of liquids provides us an idea of the strength of attractive forces among molecules in liquids. Liquids having lower boiling points encompass weaker attractive forces in comparison to such having higher boiling points.

We can make a liquid boil at temperature other than normal boiling point. How? Simply modify the pressure above the liquid. If we increase this pressure, we can increase the boiling point and if we can reduce this pressure we decrease the boiling point. On the mountains, the atmospheric pressure reduces and thus boiling point of water as well reduces. The people living on hills face problem in cooking their meals. They, thus, make use of pressure cooker. How food is cooked faster in it? The lid of pressure cooker doesn't let water vapors to escape out. On heating the water vapors build up and the pressure inside increases. This makes the water boil at a higher temperature and the food is cooked quicker.

Differences between evaporation and boiling:

Evaporation

Boiling

1) It occurs at all temperatures

It occurs at a definite temperature.

2) Evaporation is a slow process.

Boiling is a fast process.

3) It takes place only at the surface of liquid.

It takes place all through the liquid.

Trouton's Rule:

For most of the non-associated liquids, ratio of the latent heat of vaporization per mole, evaluated in joules, to the boiling point, on the absolute scale of temperature, is approximately equivalent to 88 atmospheric pressure.

A rough rule of thumb that illustrates that the latent heat of vaporization divided by the boiling point (in Kelvin) is around constant for a number of liquids. This is due to the reason that the standard entropy of vaporization is itself approximately constant, being dominated via the large entropy of the gas. Though, the rule notably fails for liquids which have significant hydrogen bonding, like ethanol and water, named after Frederick Thomas Trouton (1863-1922).

Mathematically, it can be represented as:

ΔS‾vap = 10.5 R

Here, 'R' is the gas constant

Trouton's rule is valid for lots of liquids; for example, the entropy of vaporization of toluene is 87.30 J K-1 mol-1, that of benzene is 89.45 J K-1 mol-1, and that of chloroform is 87.92 J K-1 mol-1.  Due to its convenience, the rule is employed to estimate the enthalpy of vaporization of liquids whose boiling points are acknowledged.

The rule, though, consists of some exceptions. For illustration, the entropies of vaporization of ethanol, water, formic acid and hydrogen fluoride are far from the expected values. The entropy of vaporization of XeF6 at its boiling point has unusually high value of 136.9 J K-1 mol-1. The feature of those liquids to which Trouton's rule can't be applied is their special interaction between the molecules like hydrogen bonding. The entropy of vaporization of ethanol water and depicts positive deviance from the rule; this is due to reason that the hydrogen bonding in the liquid phase lessens the entropy of the phase. In contrary, the entropy of vaporization of formic acid consists of negative deviance. This fact points out the existence of an orderly structure in the gas phase; it is well-known that formic acid makes a dimmer structure even in the gas phase. The negative deviance can as well take place as an outcome of a small gas state entropy owing to a low population of excited rotational states in the gas phase, specifically in small molecules like methane - a small moment of inertia 'I' giving mount to a large rotational constant B, with correspondingly broadly separated rotational energy levels and, via Maxwell-Boltzmann distribution, a small population of excited rotational states and therefore a low rotational entropy. Trouton's rule validity can be increased via considering,

ΔS‾vap = 4.5 R + R ln T

Here, if T = 400 K, we determine the original formulation for Trouton's rule.

The other equation is:  Tboiling < 2100 K is ΔHboiling = 87 Tboiling - 0.4 J/K.

Liquid Crystals:

The study of liquid crystals started in the year 1888 whenever an Austrian botanist named Friedrich Reinitzer noticed that a material termed as cholesteryl benzoate had two distinct melting points. In his experiments, Reinitzer raised the temperature of a solid sample and watched the crystal change to a hazy liquid. As he raised the temperature further, the material changed again to a clear, transparent liquid. Due to early work, Reinitzer is frequently credited with discovering a new state of matter - the liquid crystal phase.

The liquid crystal is a thermodynamic stable phase characterized via anisotropy of properties devoid of the existence of a (3-D) three-dimensional crystal lattice, usually laying in the temperature range between the solid and isotropic liquid phase, therefore the term mesophase.

Liquid crystal materials are exclusive in their uses and properties. As research into this field carries on and as new applications are developed, liquid crystals will play a significant role in the modern technology.

What are Liquid Crystals?

Liquid crystal materials usually have some common features. Among these are rodlike molecular structures, rigidness of the long axis, and strong dipole and/or simply polarizable subsituents. A dipole is present whenever we have two equivalent electric or magnetic charges of opposite sign, separated via a small distance. In the electric case, the dipole moment is represented via the product of one charge and the distance of separation applies to the charge and current distributions also. In the electric case, a displacement of charge distribution generates a dipole moment, as in the molecule.

The differentiating feature of the liquid crystalline state is the tendency of the molecules (that is, mesogens) to point all along a common axis, termed as the director (that is, the molecular direction of preferred orientation in liquid crystalline mesophase). This is in contrary to molecules in the liquid state that have no intrinsic order. In the solid state, molecules are highly ordered and encompass little translational freedom. The feature orientation order of the liquid crystal state is between the traditional solid and liquid states and this is the origin of the word mesogenic state, employed synonymously having liquid crystal state. Note the average alignment of the molecules for each state in the given diagram.

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Fig: Liquid crystal

A mesogen is a rigid rod-like or disc-like molecule which is the components of liquid crystalline materials.

It is at times difficult to find out whether a material is in the crystal or liquid crystal state. Crystalline materials show long range periodic order in three dimensions. By definition, an isotropic (that is, having properties that are similar regardless of the direction of measurement. In the isotropic state, all directions are indistinguishable from one other) liquid has no orientation order. Substances which are not as ordered as a solid, yet have certain degree of alignment are properly termed as liquid crystals.

Liquid Crystal Phases:

The liquid crystal phase is a distinct phase of matter observed between the crystalline (solid) and isotropic (liquid) states. There are numerous kinds of liquid crystal states, based on the amount of order in the material.

1) Nematics:

The nematic stage is characterized via long-range orientation order, that is, the long axes of the molecules tend to align all along a preferred direction. The locally preferred direction might differ all through the medium, however in the unstrained nematic it doesn't. Much of the interesting phenomenology of liquid crystals comprises the geometry and dynamics of the preferred axis that is stated through a vector n(r) giving its local orientation. This vector is termed as a director. As its magnitude has no importance, it is taken to be unity.

2) Cholesteric:

The Cholesteric stage is similar to the nematic phase in having long-range orientation order and no long-range order in positions of the centers of mass of molecules. It distinct from the nematic phase in that the director differs in direction all through the medium in a regular manner. The configuration is accurately what one would obtain by twisting concerning the x-axis a nematic initially aligned all along the y-axis. In any plane perpendicular to the twist axis the long axis of the molecules tends to align all along a single preferred direction in this plane, however in a sequence of equidistant parallel planes, the preferred direction rotates via a fixed angle.

The secondary structure of the Cholesteric is featured through the distance measured all along the twist axis over which the director rotates via a full circle. This distance is termed as the pitch of the Cholesteric.

The periodicity length of the Cholesteric is in reality only half this distance since n and - n is indistinguishable.

3) Smectics:

The significant characteristic of the Smectics phase that differentiates it from the nematic is its stratification. The molecules are ordered in layers and show some correlations in their positions in addition to the orientation ordering. A number of different classes of Smectics have been acknowledged. In Smectics phase, the molecules are aligned perpendicular to the layers, without long-range crystalline order in a layer. The layers can slide freely over one other. In the Smectics A phase, the preferred axis is not perpendicular to the layers, in such a way that the phase has biaxial symmetry. In Smectics B phase there is hexagonal crystalline order in the layers.

4) Columnar phases:

The disk-shaped mesogens can orient themselves in a layer-like fashion termed as the discotic nematic state. If the disks pack to stacks, the phase is termed as a discotic columnar. The columns themselves might be organized to rectangular or hexagonal arrays. Chiral discotic phases, identical to the Chiral nematic phase, are as well acknowledged.

Columnar phase is a class of liquid-crystalline phases in which the molecules assemble to cylindrical structures to act as the mesogens. Originally, these types of liquid crystals were termed as discotic liquid crystals as the columnar structures are comprised of flat-shaped discotic molecules stacked one-dimensionally. As recent findings give a number of columnar liquid crystals comprising of non-discoid mesogens, it is more common now to categorize this state of matter and compounds by such properties as a columnar liquid crystals.

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