Understanding Critical Pressure: Why It Matters in Chemistry
What is the Critical Pressure?
The pressure that corresponds to the critical point of the substance is termed as the critical pressure of that substance. It is defined as the point on the temperature and pressure scale in which a liquid substance can coexist with its vapour. For temperatures that exceed the critical temperature, it cannot be liquified no matter the amount of pressure applied. Hence, we can also define critical temperature as the pressure applied to a substance to liquefy it at its critical temperature. Critical pressure is denoted by 'PC.'
Liquefaction of Gases
Gas enters the liquefaction state when the intermolecular forces of attraction become so high that they bind the gas molecules together, forming the liquid state. By increasing the pressure on the molecules, the intermolecular forces of attraction can be increased. Hence, increasing and decreasing the amount of temperature and pressure in the liquefaction of gases is extremely important. As the temperature of gases increases, so makes the difficulty of it to liquify. Thus, they require more pressure to complete the process.
Triple Point
The matter has three common states, namely solid, liquid, and gas. The physical states of these substances are influenced by two primary factors, temperature, and pressure. Each substance has a phase boundary, and therefore by adjusting the temperature-pressure combinations, we can pinpoint the phase boundary of any substance. Some substances have a triple point at which the substance can exist in all three states of matter.
Critical State
In thermodynamics, the critical state is said to be the endpoint of a phase equilibrium curve of any substance. The critical state is the endpoint of the pressure-temperature curve that determines when liquid and its vapour can coexist. We know that, at higher temperatures, gas cannot be liquefied by pressure alone. Therefore, the critical temperature and a critical pressure pc, the phase boundaries vanish.
Critical Pressures of Some Common Substances
The critical pressure of water is 217.7 atm or 22,060 kiloPascals.
The critical pressure of ammonia (chemical formula: NH3) is approximately 111.3 atm or 11,280 kiloPascals.
The critical pressure of chlorine (symbol: Cl) corresponds to 76 atm or 7,700 kiloPascals.
The critical pressure of Helium (symbol: He) is 2.24 atm or 227 kiloPascals.
The critical pressure of nitrogen corresponds to 33.5 atm or 3390 kiloPascals.
The critical pressure of Methane is 45.79 atm or 4,640 kiloPascal.
The critical pressure of Oxygen corresponds to 49.8 atm or 5,050 kiloPascal
The critical pressure of sulfur corresponds to 207 am or 21,000 kiloPascal.
Gold has a critical pressure of about 5,000 atm or 510,000 kiloPascal.
Mercury's critical pressure is 1,720 atm or 174, 000 kiloPascal.
Critical Pressure of Water
[Image will be Uploaded Soon]
The critical pressure of water is defined as the vapour pressure at its critical temperature, which is above the point at which the distinct gas and liquid phases do not exist. When the temperature of water approaches the critical temperature, then the gaseous and liquid phases' properties become the same. It results in the occurrence of only one phase. It is a well-known fact the critical point of water occurs at a temperature point of 647 Kelvin. It is equal to 374 degrees Celsius and 705 degrees Fahrenheit. The critical pressure is equal to 22.064 MPa, which is approximately equal to 218 atmospheres of pressure.
FAQs on Critical Pressure Explained: Chemistry Essentials
1. What is critical pressure in chemistry?
Critical pressure (Pc) is the minimum pressure that must be applied to a gas at its critical temperature to cause it to liquefy. It represents the pressure at the substance's critical point. If the temperature is above the critical temperature, no amount of pressure can turn the gas into a liquid.
2. How are critical pressure and critical temperature related to each other?
Critical pressure and critical temperature are directly related as they both define a substance's critical point. The critical temperature (Tc) is the highest temperature at which a substance can exist as a liquid. The critical pressure (Pc) is the pressure required for liquefaction at that specific critical temperature. A substance with strong intermolecular forces will have a higher critical temperature, which in turn means a higher critical pressure is needed to liquefy it.
3. What is the importance of knowing a gas's critical pressure and temperature?
Knowing a substance's critical pressure and temperature is vital for the industrial liquefaction and storage of gases. Gases with high critical temperatures (like ammonia and CO₂) can be liquefied easily at room temperature simply by applying pressure. Conversely, gases with very low critical temperatures (like hydrogen and helium) are difficult to liquefy because they must first be cooled below their extremely low Tc before pressure can be effective.
4. How can you calculate critical pressure using van der Waals constants?
The critical pressure (Pc) of a real gas can be calculated from the van der Waals constants 'a' (representing intermolecular forces) and 'b' (representing molecular volume). The formula is: Pc = a / (27b²). This equation highlights that a gas with stronger intermolecular attraction (larger 'a') will have a higher critical pressure, assuming a similar molecular size.
5. How is the critical point represented on a Pressure-Volume (p-V) diagram?
On a p-V diagram for a substance, the critical point is the apex of the dome-shaped curve that represents the liquid-vapour coexistence region. At this point, the specific volumes of the liquid and gas phases become identical, and the distinction between the two phases vanishes. The pressure coordinate of this point corresponds to the critical pressure (Pc), and the temperature of this specific isotherm is the critical temperature (Tc).
6. What is the main difference between a substance's critical point and its triple point?
The primary difference lies in the phases of matter that are in equilibrium.
- The Critical Point is the condition of temperature and pressure where the liquid and gas phases of a substance become indistinguishable, merging into a single phase called a supercritical fluid.
- The Triple Point is the specific condition of temperature and pressure where the solid, liquid, and gas phases of a substance coexist in thermodynamic equilibrium.
7. Why can't a gas be liquefied above its critical temperature, regardless of the pressure applied?
Above the critical temperature, the kinetic energy of the gas molecules is so high that it overcomes the intermolecular forces of attraction. Applying pressure can force the molecules closer together, increasing the gas's density, but it cannot overcome their energetic movement to make them condense into the ordered structure of a liquid. The molecules simply have too much energy to be held together in a liquid state.
8. What are some real-world examples of critical pressure values?
Different substances have vastly different critical pressures, which impacts how they are used. For example:
- Water (H₂O): Has a very high critical pressure of about 218 atmospheres (atm).
- Carbon Dioxide (CO₂): Has a critical pressure of about 73 atm. This relatively accessible value allows it to be used as a supercritical fluid for applications like decaffeinating coffee.
- Helium (He): Has an extremely low critical pressure of only 2.2 atm, which, combined with its low critical temperature, makes it very difficult to liquefy.
