In this article, we shall study a change in the state of a substance.

Melting (Solid → Liquid):

The process of change of solid substance into its liquid state is called melting or fusion. The constant temperature at which the solid becomes liquid upon absorption of heat at constant pressure is called the melting point of that solid at that pressure.

Generally melting point increases with the increase in pressure. Ice is the exception to this because its melting point decreases with the increase in the pressure. Melting point at standard pressure is a characteristic property of a substance. The melting point decreases with the addition of the impurity. Hence melting point can be considered as criteria for purity.

Melting points of some important substances are ice (0 °C), iron (1535 °C), aluminium (660 °C), gold (1064 °C), silver (961 °C), aluminium (660 °C), tin (232 °C), zinc (419.5 °C), copper (1084°C), etc.

Explanation on the Basis of Kinetic Model: 

When solids are heated the thermal energy of particles increases. Thus the cohesive forces between the particles weaken to such extent that the particles can have relative motion with respect to each other but cannot move out of the bulk.  Thus solid gets converted into liquid (melts).

Applications of Melting:

• Melting is very important in the production of alloys. If a binary alloy is to be produced. The element with a higher melting point is melted in a crucible and an element with a lower melting point is added to the molten metal. The second element also melts forming almost a homogeneous solution called alloy. Alloys have many applications in everyday life. Some examples of alloys are

• Substances with a high melting point are used to make high-temperature devices. For example, tungsten is used in an incandescent bulb.
• Metals are melted and they are cast (moulded) to give the solids required shape.

Factors Affecting Melting Point:

Internal factors:

• Inter-Molecular (Particle) Forces: If the attractive forces between the molecules of solid are weaker and then the solid has a low melting point. The attraction between the molecules of covalent compounds is weaker than that in ionic solids and hence covalent compounds have a lower melting point than that of the ionic compounds.
• The shape of molecules: If the shape of the molecule is such that it can have a closed packing of the molecules, then the substance has a higher melting point.
• Size of the molecule: The smaller size of molecules can have a closed packing (less void space) of the molecules, then the substance has a higher melting point.

External Factors: 

• Impurity: The melting point of a substance decreases with the presence of impurities in it, The phenomenon is called melting point depression. The particles of impurity disrupt the repeating pattern of forces that hold the solid together. Hence less energy is required to melt the part of the solid surrounding the impurity. Salt is spread on the frozen street so that the melting point decreases and the ice melt fast.
• Pressure: For the solids, those expand on heating, the melting point increases with increase in the pressure. It is due to the fact that the pressure opposes the increase in the distance between molecules (expansion). e.g. silver, gold, copper, paraffin wax, etc. For the solids, those contract on heating, the melting point decreases with increase in the pressure. It is due to the fact that the pressure supports the decrease in the distance between molecules (contraction). e.g. ice, cast iron, bismuth, brass, etc.

When two ice cubes are pressed together they form a single block of ice. The phenomenon is called regelation. When the two cubes are pressed against each other. the ice at the interface melts due to lowering of melting point. When the pressure is released the melted ice (water) at the interface solidifies again and a single block of ice is obtained.

Sublimation (Solid ⇔ Gas):

Sublimation is the process by which a heated solid directly changes into its gaseous state i.e. vapour state. These vapours on cooling directly give solid. Such substances are called sublimates. Examples are ammonium chloride, ammonia, naphthalene balls, camphor, etc.

Explanation on the Basis of Kinetic Model: 

Certain solids are heated the thermal energy of molecules increases so that the interparticle forces become negligible and the particles can move freely.  Thus such solids on heating get converted directly into gases. This phenomenon is known as sublimation. The cohesive forces between the particles in such substances are weak.

Freezing (Liquid → Solid):

The process of change of matter from a liquid state to a solid state is called freezing or solidification. The constant temperature at which a liquid changes into solid by giving out heat energy (or cooling) is called the freezing point of the liquid. The freezing point of a liquid is a characteristic property of the liquid. Hence can be considered as criteria of purity.

Freezing points of some important substances are water (0 °C), benzene (5.5 °C), mercury (- 38.87 °C), etc.

Explanation on the Basis of Kinetic Model: 

When liquids are cooled the thermal energy of particles decreases. Thus the cohesive forces between the particles strengthen to such extent that the particles can not have relative motion with each other and they occupy the fixed positions.  Thus liquid gets converted into solid (freezes).

Applications of Freezing:

• It is used for the preparation of ice creams.
• The lowering of the freezing point on the addition of solute to the solution is used to find molecular mass of the solute.

Factors Affecting Freezing Point:

For the same substance, the freezing point of the liquid is equal to the melting point of the solid. Therefore the factors those affect melting point of solid obviously affect the freezing point of the liquid.

Freezing Mixtures: 

a mixture of two or more substances (e.g. ice water and salt, or dry ice and alcohol) which can be used to produce temperatures below the freezing point of water.

A freezing mixture of 3 parts of ice and 1 part of NaCl produces a temperature of – 21 °C. A freezing mixture of 2 parts of ice and 3 parts of K₂CO₃ produces a temperature of  – 46 °C.  A freezing mixture of dry ice and alcohol or ethers can produce a temperature of – 60 °C.

In a freezing mixture, a soluble salt is added. The heat required to dissolve one mole of soluble solute in a solvent is called heat of solvation. This heat required for dissolution of solid is taken from the mixture itself and thus the freezing point decreases in steps.

Freezing mixtures ate used to preserve perishable foodstuff like meat and fishes. They are used for producing sub-zero temperatures in laboratories and industrial units.

Evaporation or Vaporization (Liquid → Gas):

The process of conversion of a substance from the liquid state to its vapour state at any temperature below boiling point is called evaporation or vaporization.

Explanation on the Basis of Kinetic Model: 

Some particles from liquid surface possess kinetic energy sufficient to overcome the attractive forces from remaining particles of the liquid and become completely free and escape out as a gas particle in the surroundings. This phenomenon is called evaporation or vaporization.

The rate of evaporation is directly proportional to the surface area and the temperature of the liquid.

During evaporation, the temperature of liquid falls. To maintain temperature balance the liquid particles absorb heat from the surroundings making the surrounding cooler. We have already seen that the molecules with higher kinetic energy leave the surface of the liquid, thus there is an overall decrease in the kinetic energy of liquid. This is one of the reasons for the decrease in the temperature of the liquid.

To increase the rate of evaporation we should increase the surface area, the temperature and the wind speed and should decrease the humidity.

Characteristics of Evaporation:

• It is a surface phenomenon as it takes place on the surface of the liquid.
• It takes place at all temperatures.
• It is a slow process
• The temperature of liquid falls.

Applications of Evaporation:

• During hot day sweat is formed on the body which evaporates. The necessary heat required for the evaporation of the sweat is taken from the body and thus the body temperature is maintained.
• Common salts are produced in shallow lagoons. The water from creek or sea is collected. Water evaporates leaving common salt behind.
• Water gets cooled in an earthen pot (matka). Water seeps through the porous earthen pot and gets on the surface of the pot. It evaporates and the necessary heat required for the evaporation of the water is taken from the water inside the pot and thus the temperature of the water inside the pot decreases.
• Drying of clothes is due to evaporation of water. We have to spread the clothes (increase in surface area), under the sun (increasing temperature) at a windy place.
• In refrigerator the cooling gas (freon) gets evaporator in tubes surrounding freezer region, The necessary heat required for the evaporation of the water is taken from the freezer region and thus the temperature of the freezer region decreases.

Boiling (Liquid → Gas):

Boiling process of change of a liquid into a vapour at a particular temperature and pressure from all part of the liquid. Boiling is a bulk process and takes place throughout the liquid.

When we supply heat energy to liquid the particles start moving faster. At a certain temperature, a point is reached when the particles have enough energy to break free from the forces of attraction of each other. At this temperature, the liquid starts changing into a gas (vapours). The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point. Pure liquids have fixed boiling points. It can be considered as the criteria of purity.

The constant temperature at which a liquid changes to vapour under normal atmospheric pressure is called the boiling point of the liquid. Boiling points of some important liquids are water (100 °C), Ethyl alcohol (78.3 °C), benzene (80.2 °C), chloroform (62 °C), sulphuric acid (280 °C), diethyl ether (35 °C), etc.

Explanation on the Basis of Kinetic Model: 

During boiling, not only the particles on the surface of the liquid but those near walls of the container also start leaving the liquid. It can be seen that small vapour bubbles are formed inside the liquid on walls of the container. As temperature increases the pressure of vapours in bubble increases. The bubbles start growing in size. A point is reached when the vapour pressure inside the bubble is equal to that of atmospheric pressure. At that instant, the bubble detaches from the walls of the container and rise upward. Reaching the surface it bursts giving vapours to the surroundings. Thus there is continuous agitation of the mass of liquid and we say liquid is boiling.

As the pressure increases the boiling point increases. Soluble impurities increase boiling point.

Characteristics of Boiling:

• It is a bulk phenomenon as it takes place throughout the liquid.
• It takes place at fixed temperatures.
• It is a fast process
• The temperature of liquid remains constant.

Factors Affecting Boiling Point:

• Pressure: As the external (atmospheric) pressure decreases boiling point decreases. Hence at higher altitude water boils below 100 °C. Hence higher altitude, food is not cooked properly. To avoid this problem pressure cooker is used for cooking food.

Working of Pressure Cooker:

The basic principle of a pressure cooker is that the boiling point of water increases with the increase in pressure. A pressure cooker is a steel or aluminum vessel with a lid which is airtight. There is a safety valve to release steam to decrease the excess pressure above certain designated pressure. The steam is formed from water in the pressure cooker which has no escape route gets collected in the vessel which put extra pressure on water, which leads to increase in the boiling point of water above 100 °C. Thus gradually the boiling point of water goes on increasing. When the required pressure is reached, the safety valve lifts due to steam pressure and excess of steam is blown out. The safety wall closes and the process restarts. The pressure of steam is even throughout the vessel and hence the food is cooked fast and evenly. The pressure cooker saves a lot of fuel required for cooking.

• Impurity: When a solid is dissolved in liquid the boiling point increases beyond the normal boiling point. Hence during steaming of food, some salt is added to water, so that the food cooks well.

Liquefaction or Condensation:

Liquefaction is the process in which the gaseous substance changes into a liquid state at a particular temperature.

Explanation on the Basis of Kinetic Model:

On cooling the particles of gas lose their kinetic energy and their speed decreases. The decrease in their speed reduces interparticle space and the particles come so close so that the attractive forces between them increase and the gas gets converted into a liquid.

Science > Chemistry > States of Matter > Change of State of a Substance

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In the last article, we have studied the chemical classification of substances. Most of the available substances in nature are in mixture form. The useful component of the mixture can be obtained by separating individual components of the mixture by a suitable method. The method of separation employed depends upon the nature of components of the mixture. In this article, we shall study different methods of separation of mixtures.

Separation of Solid-Solid Mixtures:

Mechanical Picking:

Property exploited: the physical appearance is different for different substances

In this method, the components are separated on the basis of physical characters like shape, size, and appearance. e.g. Removing stones or husk particles from grains.

Magnetic Separation:

Property exploited: the magnetic property of one component.

Ferromagnetic substances like iron, cobalt, nickel are separated from non-magnetic substances using magnetic separators or magnetic belts. e.g. Separation of iron filings from sulphur using magnet.

Sublimation:

Property exploited: the ability of one component to sublime.

The solids which on heating gets converted into vapours directly (instead of into liquid) are called sublimate or volatile solid and the phenomenon is known as sublimation. This process is used to separate volatile solids from non-volatile solids. In this method, the mixture is heated. The sublimable solid gets converted into vapours living non-volatile substance behind. The vapours of sublimation can be condensed by cooling to get the volatile solid back. e.g. Separation of iodine from sand or separation of ammonium chloride from the sand.

Solvent Extraction:

Property exploited: Solubility of one component in the solvent.

In this process, one of the components is dissolved in a particular solvent in which it is soluble while other components do not dissolve.

e.g. Separation of sulphur and iron. Carbon disulphide is added to the mixture in which sulphur get dissolved and iron remains in solid state. Then the solution is filtered iron particles are filtered out. The filtrate contains carbon disulphide with sulphur dissolved in it. Carbon disulphite evaporates and sulphur remains.

All the compounds of ammonia, sodium, potassium and all the nitrates and nitrites of metal, All chlorides (except chlorides of mercury, silver and lead), all metal sulphates (except sulphates of calcium, lead and barium)n  are soluble in water. All metal oxides, hydroxides, carbonates and sulphides are insoluble in water.Organic substances are not soluble in water but are soluble in organic solvents like chloroform, benzene, alcohol etc. Some important substances and their solvents are as follows.

Gravity Separation:

Property exploited: Difference in densities of components.

The mixture containing two components having different densities but both insoluble in particular solvent can be separated by this method.

A mixture of chalk powder and sand is added to water. Due to more density sand particles settle at the bottom of the vessel while chalk particles being lighter floats on the surface of the water.

Fractional Crystallization:

Property exploited: Difference in a solubility of the components in the same solvent.

A mixture of KNO₃ and NaCl can be separated by this method. Both are soluble in water.  But solubility of KNO₃ is more than NaCl. When the saturated solution containing KNO₃ and NaCl is heated and allowed to cool down, NaCl being less soluble crystalizes first and it is filtered out the filtrate contains KNO₃ which crystallizes on further cooling.

Separation of  Solid-Liquid Mixtures:

Filtration:

Property exploited: Size of the insoluble solid component is more than the size of the grid of the filter paper.

This method is used for separating the insoluble suspended particles from a liquid using filtering materials like filter paper, sand bed etc. The liquid that passes trough the filter is called the filrate and insoluble solid left on filter is called the residue. For colloidal particles parchment membrane is used for separating colloidal particles from the solution.

e.g. filtration of water.

Sedimentation and Decantation: 

Property exploited; The high density of insoluble solid component.

In this method, the liquid containing insoluble suspended particles is allowed to stand still for some time. The suspended particles settle down at the bottom under gravity. The solid substance that settles down is called sediment. The upper liquid can be separated by decantation. In decantation the upper liquid is carefully poured in another beaker without distrurbing sediments.

e.g. sedimentation of muddy water.

Evaporation:

Property exploited: A solid remains unaffected on heating the mixture up to the boiling point of the liquid. Liquid gets converted into vapours.

In this method, the solid soluble component of the mixture is separated from a liquid. The mixture is heated up to the boiling point of a liquid. The liquid evaporates living the solid behind. The vapours of liquid can be condensed to get liquid back.

e.g. Separation of sugar from sugar in water solution.

Distillation:

Property exploited: A solid remains unaffected on heating the mixture up to the boiling point of the liquid. Liquid gets converted into vapours.and vapours of liquid cooled down to obtain pure liquid.

This method is used to separate liquid from soluble solid.  In this method, the mixture is heated up to the boiling point of the liquid, at which the vapours of liquid are formed living the solid behind. The vapours of liquid on cooling condense to give the liquid. The condensed vapours are called distillates.

e.g. Separation of water from salt in water solution.

Note: The difference between the evaporation and distillation is that in case of evaporation, the component evaporated is lost permanently. While in distillation both the components one which is evaporated and one which has no effect are recovered.

Centrifuging:

Property exploited: Density of solid particles is different from that of liquid particles.

In this method, the mixture of solid in a liquid is rotated in a centrifuge. The centrifugal force causes more dense substances to separate out along the radial direction (the bottom of the tube). Whereas the lighter particles will tend to move to the top of the tube.

e.g. Blood plasma can be separated from its solid constituents (RBC, WBC, Platelets) by this method. When milk is stirred very fast the cream particles and milk particles are separated. When rotation or sturring stops cream particles float on the top.

Separation of  Liquid-Liquid Mixtures:

Separating Funnel:

Property exploited: Difference in densities of two immiscible liquid components

In this method, the mixture of two immiscible liquids having different densities is taken in separating funnel. The lighter (lower density) liquid forms the top layer while heavier(higher density)  liquid forms lower level. Now the bottom valve is open and the heavier liquid is separated.

e.g. Separation of kerosene from the water.

Fractional Distillation:

Property exploited: Difference in boiling points of liquids.

This method is used to separate two miscible liquids having close boiling points (Difference of about 10 K). In this method, the mixture is heated. The liquid having lower boiling point gets evaporated first living the liquid having a higher boiling point behind. The vapours of evaporated liquid can be condensed by cooling to get it back in the liquid state.

e.g. Separation of different components from petroleum.

Separation of  Gas-Gas Mixtures:

Diffusion:

Property exploited: Difference in densities of component gases.

The rate of diffusion is inversely proportional to its molar mass. It means the gas with lower molar mass diffuses faster than that with higher molar mass.

e.g. Separation of hydrogen (lower molar mass) from methane (higher molar mass)

Fractional Evaporation:

Property exploited: Difference in boiling points of component gases.

A mixture of liquefied gases is evaporated the respective gases separates out from the liquefied mixture at their boiling points.

e.g. When liquefied air is allowed to evaporate nitrogen having the lowest boiling point boils off the mixture leaving behind oxygen in the liquefied air. on further evaporation, oxygen boils off leaving behind remaining components of air.

Preferential Liquefication:

Property exploited: Difference in liquefication of gases under pressure.

a mixture of gases is liquefied using high pressure. The gas which is easily liquefiable gets liquefied first.

e.g. When a mixture of ammonia and hydrogen are subjected to high pressure, ammonia gets liquefied while hydrogen remains in gaseous state and hence can be separated very easily.

Dissolution in Suitable Solvent:

Property exploited: Difference in the solubility of components in the given solvent.

The mixture of gases is allowed to dissolve in the solvent such that one component is soluble in the solvent. Other component remains in the gaseous state and can be separated easily.

e.g. When a mixture of carbon dioxide and carbon monoxide is dissolved in KOH, carbon dioxide gets dissolved in it while carbon monoxide remains in the gaseous state and thus can be separated.

Separation of  Liquid-Gas Mixtures:

Heating:

Property exploited: As the temperature increases, the solubility of a gas in the solvent decreases.

When a solution of a gas in a liquid is heated slightly below the boiling point of the liquid The gas escapes out leaving behind liquid component. The escaping gas can be collected separately.

e.g. Separation of dissolved oxygen from water.

Lowering of Pressure:

Property exploited: The solubility of a gas in a liquid is directly proportional to the pressure on it.

In soda water bottle carbon dioxide gas is dissolved in water under pressure. When the cap of the bottle is open the solubility of the gas decreases and dissolved carbon dioxide gas escapes out leaving water behind.

Miscellaneous Method:

Chromatography:

Property exploited: The different rates of adsorption of different components.

This method was discovered by Michael Tswett in 1906. This technique is based on preferential adsorption of a substance at the surface of the adsorbent. In this method, the mixture is mixed with the liquid in which it is soluble. This liquid is called eluant. Then this solution is poured into a tube containing adsorbing column. Different components of the mixture would adsorb to different extent forming separate layers (Different colours) on the adsorbing column. Depending upon the fixed and moving phases, chromatography is further classified as a paper chromatography, column chromatography, thin layer chromatography and gas chromatography.

e.g. separation of coloured constituents in a mixture of ink.

In chromatography, a very small quantity present in micrograms can be separated. it is used for quantitative estimation. It is used for the purification of many industrial products.

In the next article, we shall study one of the important topic of the chemistry: Naming of chemical compounds and writing formula from the name of the chemical compound.

Previous Topic: Chemical Classification of Substances

Next Topic: Naming of Chemical Compounds

Science > Chemistry > Introduction to Chemistry > Methods of Separation of Mixtures

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In this article, we shall study change in enthalpy for different chemical processes.

Enthalpy of Formation (Δ_fH° or Δ_formationH°):

The change in enthalpy of a chemical reaction at a given temperature and pressure, when one mole of the substance is formed from its constituent elements in their standard states is called the heat of formation.

Explanation: Consider the following reaction

C_(s) + O_2(g)  → CO_2(g)   ,  Δ_formationH°  =  -395.39 kJ mol^-1

Since one mole of carbon dioxide gas is formed we can say that the heat of formation of carbon dioxide gas is -395.39 kJ.

Notes:

• The standard state of the element is that stable state of the element in which it exists at 1 atm. Pressure and 298 K
• Enthalpies of elements in their standard states are arbitrarily taken as zero.
• Enthalpy of a compound is equal to its heat of formation.
• When solving problems on the heat of formation make sure that the product side of the thermochemical equation has one mole of the substance whose heat of formation is to be calculated.

Enthalpy of Dissociation (Δ_dissociationH°):

Change in enthalpy of a chemical reaction at a given temperature and pressure, when one mole of a substance is dissociated into its constituent elements is called the heat of dissociation.

Explanation: Consider the following reaction

H_2(g) →  2H_(g),    Δ_dissociationH° = + 435.136 kJ mol^-1

Since one mole of hydrogen gas is dissociated the heat of dissociation of hydrogen gas is + 435.136 kJ

Enthalpy of Combustion (Δ_cH° or Δ_combustionH°):

Change in the enthalpy of a chemical reaction at a given temperature and pressure when one mole of a substance is combusted (burn) completely in excess of oxygen is called the heat of combustion.

Explanation: Consider the following reaction

C_(s) + O_2(g)  → CO_2(g)   , ΔH  =  -395.39 kJ mol^-1

Since one mole of carbon is combusted completely the heat of combustion of carbon is – 395.39 kJ.

Enthalpy of Neutralization (Δ_{neutralization}H°):

Change in the enthalpy of a chemical reaction at a given temperature and pressure when one gram equivalent weight of acid is completely neutralized by one gram equivalent weight of the base is called the heat of neutralization.

Explanation: Consider following reaction

HCl_(aq) + NaOH_(aq)  →   NaCl  +    H₂O     Δ_{neutralization}H°  =  -56.9 kJ

The heat of neutralization of HCI by NaOH is -56.9 KJ.

For all strong acids and bases, the heat of neutralization is the same because, in their neutralization reaction, there is a combination of H+ ions of an acid with OH- ions of the base to produce un-dissociated water.

Change of Phase:

Change of phase involves the change in the physical state of matter. During the phase change, the chemical properties of the substance do not change but physical properties change. The following are the types of phase changes.

Fusion: This is the process in which the matter changes from solid-state to liquid state. It is endothermic process.

e.g. melting of ice H₂O_(s) → H₂O_(l)

Vapourization: This is the process in which the matter changes from liquid state to gaseous state. It is an endothermic process.

e.g. boiling of water H₂O_(l) → H₂O_(g)

Sublimation: This is the process in which the matter changes from the solid-state into a gaseous state directly. It is an endothermic process.

e.g. heating of camphor Camphor_(s) → Camphor_(g)

Characteristics of Change of Phase:

• The phase change always takes place at constant pressure and temperature.
• During the phase transition, there is an equilibrium between the two phases. Thus both the phases exist simultaneously.
• The Change in temperature takes place only when completion of phase transition.

Enthalpy of Fusion (Δ_fusH°):

The enthalpy-change that accompanies the fusion of one mole of a solid without the change in temperature at constant pressure is called its enthalpy of fusion.

Explanation: Consider the following reaction

H₂O_(s) → H₂O_(l),               Δ_fusionH  = +6.01  kJ mol^-1

Thus, the equation indicates that when one mole of ice melts at 0° C at 1 atm pressure, the enthalpy-change is 6.01 kJ. i.e. 6.01 kJ of energy is absorbed.

Enthalpy of Freezing (Δ_freezeH°):

The enthalpy change that accompanies the freezing of one mole of a liquid without a change in temperature at constant pressure is called its enthalpy of freezing.

Explanation: Consider the following reaction

H₂O_(l) → H₂O_(s),               Δ_freezeH  = +6.01  kJ mol^-1

Thus, the equation indicates that when one mole of water freezes at 0° C at 1 atm pressure, the change in enthalpy is -6.01 kJ. i.e. 6.01 kJ of energy is released.

Enthalpy of Vaporization (Δ_vaporizationH°):

The enthalpy change that accompanies the vaporization of one mole of a liquid without a change in temperature at constant pressure is called its enthalpy of vaporization.

Explanation: Consider the following reaction

H₂O_(l) → H₂O_(g),   Δ_{vapourization}H  = + 40.7  kJ mol^-1

Thus, the equation indicates that when one mole of water vaporizes at 100° C at 1 atm pressure, the change in enthalpy is + 40.7 kJ. i.e. 40.7 kJ of energy is absorbed.

Enthalpy of Condensation (Δ_condensationH°):

The enthalpy change that accompanies the condensation of one mole of a liquid without a change in temperature at constant pressure is called its enthalpy of condensation.

Explanation: Consider the following reaction

H₂O_(l) → H₂O_(g),   Δ_condensationH  = – 40.7  kJ mol^-1

Thus, the equation indicates that when one mole of water vapours condense at 100° C at 1 atm pressure, the enthalpy-change is – 40.7 kJ. i.e. 40.7 kJ of energy is released.

Enthalpy of Sublimation (Δ_sublimationH°):

The direct conversion of solid to vapour without going through the liquid state is called sublimation. The enthalpy-change that accompanies the condensation of one mole of a solid directly into vapours at a constant temperature at constant pressure is called its enthalpy of sublimation.

Explanation: Consider the following reaction

H₂O_(s) → H₂O_(g),       Δ_sublimationH  = +51.08  kJ mol^-1

Thus, the equation indicates that when one mole of ice sublimes at O° C at 1 atm pressure, the enthalpy change is + 51.08 kJ. i.e. 51.8 kJ of energy is absorbed.

It is to be noted that Δ_sublimationH = Δ_fusionH + Δ_{vapourization}H

Atomic or Molecular Changes:

Enthalpy of Ionization (Δ_ionizationH°):

The enthalpy change that accompanies the removal of an electron from each atom or ion in one mole of gaseous atoms or ions is called its enthalpy of ionization.

Explanation: Consider the following reaction

Na_(g)   →   Na⁺_(g)+ e^– ,    Δ_ionizationH  = 494  kJ  mol^-1

Thus, the equation indicates that when one mole of gaseous sodium atom ionizes to Na⁺_(g) ion, the change in enthalpy is + 494 kJ. i.e. 494 kJ of energy is absorbed.

Enthalpy of Atomization (Δ_atomizationH°):

The enthalpy change that accompanies the dissociation of all the molecules in one mole of gas-phase substance into gaseous atoms is called its enthalpy of atomization.

Explanation: Consider the following reaction

Cl_2(g) → Cl_(g)+ Cl_(g),          Δ_atomizationH  = 242  kJ mol^-1

Thus, the equation indicates that when one mole of gaseous chlorine molecule dissociates completely into its atomic form in the gaseous state then the change in enthalpy is + 242 kJ. i.e. 242 kJ of energy is absorbed.

Enthalpy of Solution (Δ_solutionH°):

Change in enthalpy of chemical reaction at a given temperature and pressure, when one mole of a solution is dissolved in a specified quantity of solvent so as to form a solution of particular concentration is called as enthalpy of a solution.

Explanation: Consider the following reaction

KOH_(s)   +   H₂O_(l) → KOH_(aq)        Δ_solutionH   = -58.57 KJ mol^-1

Thus, the equation indicates that when one mole of potassium hydroxide (solute) dissolves in one mole of water (solvent) to form one mole of potassium hydroxide solution in water then the change in enthalpy is -58.57 kJ. i.e. 58.57 kJ of energy is released

Bond Enthalpy (Bond Energy):

Chemical reactions involve the breaking and making of chemical bonds. Energy is required to break a bond and energy is released when a bond is formed. It is possible to relate the heat of reaction to changes in energy associated with the breaking and making of chemical bonds. With reference to the enthalpy changes associated with chemical bonds, two different terms are used in thermodynamics. (i) Bond dissociation enthalpy (ii) Mean bond enthalpy. Let us discuss these terms with reference to diatomic and polyatomic molecules.

Diatomic Molecules:

Consider the following process in which the bonds in one mole of dihydrogen gas (H2) are broken:

H_2(g) →  2H_(g) ;   ΔH–HHO = 435.0 kJ mol^-1

The enthalpy change involved in this process is the bond dissociation enthalpy of H–H bond.

The bond dissociation enthalpy is the change in enthalpy when one mole of covalent bonds of a gaseous covalent compound is broken to form products in the gas phase. Note that it is the same as the enthalpy of atomization of dihydrogen. This is true for all diatomic molecules.

Polyatomic Molecule:

In the case of polyatomic molecules, bond dissociation enthalpy is different for different bonds within the same molecule. Let us consider a polyatomic molecule like methane, CH4.  The overall thermochemical equation for its atomization reaction is given below:

CH_4(g) →  C_(g) + 4H_(g) , ΔH = +1665 KJ.

In methane, all the four C – H bonds are identical in bond length and energy. However, the energies required to break the individual C – H bonds in each successive step differ. In such cases, we use mean bond enthalpy

CH_4(g) →  CH_3(g) + H_(g) , ΔH = +427 KJ.

CH_3(g) → CH_2(g) + H_(g), ΔH = +439 KJ

CH_2(g) → CH_(g) + H_(g), ΔH = +452 KJ

CH_(g) → C (g) + H_(g),   ΔH = +347 KJ

Adding above reactions we get

CH_4(g) →  C_(g) + 4H_(g) , ΔH = +1665 KJ

We find that mean C–H bond enthalpy in methane as 1664/4 = 416 kJ/mol. Using Hess’s law, bond enthalpies can be calculated.

Enthalpy of Reaction from Bond Enthalpy:

The reaction enthalpies are very important quantities as these arise from the changes that accompany the breaking of old bonds and the formation of the new bonds. We can predict the enthalpy of a reaction in the gas phase if we know different bond enthalpies. The standard enthalpy of reaction is related to bond enthalpies of the reactants and products in gas phase reactions as:

ΔH = ∑ Bond enthalpies _reactants  –     ∑ Bond enthalpies _products

Remember that this relationship is approximate and is valid when all substances (reactants and products) in the reaction are in a gaseous state.

The values of given bond enthalpy can be used to calculate bond enthalpies of specific bonds in the molecule.

Crystal Lattice Energy (Δ_LatticeH°):

Crystal lattice energy is defined as the enthalpy change or energy released accompanying formation of 1 mole of crystalline solid from its constituent ions in the gaseous state at a constant temperature.

Explanation:

M⁺_(g)  + X^–_(g) → M⁺X^–_{(s) ,}  ΔH = – x KJ  mol^-1

Thus, the equation indicates that when one mole of ionic compound M+X-(s) is formed from its constituent ions the change in enthalpy is – x kJ i.e. x kJ of energy is evolved. Crystal lattice energy is always negative.

The sequence of actions involved in the formation of 1 mole of an ionic compound in its standard state from its constituent elements in their states at constant temperature and pressure is called Born-Haber cycle.

Factors Affecting Crystal Lattice Energy:

Crystal lattice energy depends on the interionic distance in the crystalline solid. As the distance decreases, the crystal lattice energy increases. Crystal lattice energy depends on the charge of constituent cations and anions.

Enthalpy of Hydration (Δ_HydrationH°):

It is defined as the heat evolved when one mole of gaseous ions dissolve in water by hydration to give infinitely dilute solution at constant temperature and pressure.

Explanation: Consider the following reaction

Na⁺_(g) + aq → Na⁺_(ag) ,     ΔH = – 390 KJ mol^-1

Thus, the equation indicates that when one mole of sodium ion in the gaseous state is dissolved in water the change in enthalpy is – 390 kJ. i.e. 390 kJ of energy is released.

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