Critical Temperature

The critical temperature is a measure of the strength of the intermolecular forces of attraction between molecules of a substance. It is particularly essential since it determines the amount of liquefaction that occurs in solid materials. Liquidizing a gas will be more difficult if the intermolecular force is weaker than it is currently.

Graphical Representation : 

The graph is displayed with pressure on the Y-axis and temperature on the X-axis, as can be seen. As a result, the critical temperature can be calculated using the critical point’s X-axis value. The critical pressure of the material is the corresponding Y-axis value of the critical point, which is the pressure required to liquefy a substance at its critical temperature.

In the year 1822, French physicist Charles Cagniard de la Tour identified the critical point of a liquid. He discovered that carbon dioxide could be liquified at 31oC when 73 atm of pressure was applied, but not at higher temperatures, even when pressures greater than 3000 atm were applied. Dmitri Mendeleev dubbed this maximum temperature at which chemicals might exist in the liquid phase “Critical Temperature” in the year 1860.

Explanation on Critical Temperature :

Pressure and temperature cannot be independent variables in the two-phase zone where liquid and vapour coexist, according to Gibb’s Law of Equilibrium. Phase boundaries or pressure-temperature combinations that allow two phases to coexist separate the solid, liquid, and vapour phases. The liquid-vapor barrier, on the other hand, comes to a halt at a critical temperature and pressure.

Dew Points are detected at temperatures above the critical temperature. At the bubble point pressure, the mix will separate into two phases at temperatures below the critical temperature. Reservoirs are classified using the bubble point, dewpoint, and single-phase zones.

Heating above the Critical Temperature :

As we continue to raise the temperature of a substance, its molecules begin to move and collide at a rapid rate. At this moment, the density of the substance in its liquefied state decreases, while the density of the substance in its vapourized or gaseous state increases. The vapour pressure rises to the point where the density of the vapour equals the density of the liquid at a specific temperature. As a result, the substance’s vapourized and liquified phases become nearly identical or indistinguishable. This temperature is referred to as the critical temperature.

The density and numerous other properties of the liquid and the vapour become the same at this crucial temperature. At this stage, the molecular forces are so strong that no amount of critical pressure will be enough to condense the substance into a calmer, liquified state.

Formula :

The critical temperature is represented by TC

And the formula for calculating this is 

TC=8a / 27bR

Where a and b are Van der Waals constants and R is the gas constant 

The critical constants formula (temperature and pressure) can be used to calculate the Van der Waals constant for actual gases, and the use of volume in the expression is omitted due to the difficulty of determining it. Van der Waals constants are derived from critical constants such as temperature, pressure, and volume. 

b=VC/3

a=27R2TC2 / 64 PC

Critical Temperature and Pressure for few Components :

Conclusion :

The critical temperature of a substance is the temperature at or above which the substance’s vapour cannot be liquified, regardless of the amount of pressure applied. By compressing a gas at a proper temperature, it can be converted to a liquid. However, because the kinetic energy of the particles that make up the gas grows as the temperature rises, liquefying the gas becomes more difficult.