Physicochemical properties of gases
The gaseous state is characterized by the fact that the average kinetic energy of molecules exceeds the energy of their mutual attraction. As a result, particles move chaotically, and the distances between them are many times greater than their own sizes.
The structural units of most gases (e.g., oxygen, nitrogen, methane) are molecules. An exception is noble gases (helium, neon, argon, etc.), which exist as individual atoms.
Gases have an unlimited ability to mix with each other. At the same time, some of them (chlorine, ammonia) are toxic, which requires instrumental or organoleptic control of air composition in industrial settings to avoid inhalation poisoning.
Basic laws and calculations
The behavior of real gases within certain limits is described by the ideal gas model. Key for calculations is Avogadro's law (1811), from which it follows that the ratio of volume to the amount of substance is a constant value for all gases.
To calculate the amount of substance ($n$), two main formulas are used:
- Under normal conditions (STP): the molar volume of a gas ($22.4$ L/mol) is used.
- At any values of temperature and pressure: the Mendeleev-Clapeyron equation ($pV = nRT$) is applied.
The universal gas constant ($R$) in SI units is $8.31$ J/(mol·K). Calculations also actively use the concept of relative density of a gas, which is equal to the ratio of the molar masses of the test and reference gases (e.g., relative to hydrogen).
Composition of gas mixtures and partial pressure
Gas mixtures are described via mass ($\omega$) and volume ($\varphi$) fractions of components. An important property of gases is that the volume fraction numerically coincides with the mole fraction.
Each gas in a mixture exerts a pressure independent of the presence of other components. This is the partial pressure. According to Dalton's law, the total pressure of a mixture is equal to the sum of the partial pressures of all gases. The partial pressure of the required component can be calculated by multiplying the total pressure by its volume fraction.
For mixtures, the average molar mass ($\bar{M}$) is also calculated. For example, for air this calculated value is $28.8$ g/mol, but taking into account impurities (carbon dioxide, argon), the real value is taken as $\approx 29$ g/mol.
Volume changes during reactions
At constant temperature and pressure, the change in volume of a reacting system depends solely on the change in the number of moles of gaseous reactants. The volumes of solid and liquid substances in such problems are neglected.
- If the number of moles of gaseous products is greater than that of reactants (e.g., during the decomposition of solids with gas evolution), the total volume increases.
- Wenn Gase in feste Verbindungen übergehen (wie bei der Reaktion von Ammoniak und Chlorwasserstoff unter Bildung von festem Ammoniumchlorid), verringert sich das Systemvolumen um ein Vielfaches. Am Ende des Prozesses wird das wahre Volumen nur durch den Überschuss eines der Ausgangsgase bestimmt.
Briefly: answers to topical questions
Which toxic gases require mandatory control of air composition in industrial settings?
Special instrumental control of air purity in factories and laboratories is used to detect hazardous impurities.
Hazardous gases indicated in the sources include:
- Chlorine (Cl2) — a toxic gas; some chlorine impurities can be detected by smell.
- Ammonia (NH3) — a toxic gas; some ammonia impurities can be detected by smell.
- Methane (CH4) — listed as a toxic gas; flammable methane also forms explosive mixtures with air.
Volatile liquids are also classified as hazardous substances: hydrogen cyanide (HCN) and carbon disulfide (CS2). The route of entry of such substances into the body is inhalation, through involuntary breathing of the substance mixed with air.
Which gases are classified as permanent gases?
Permanent gases are substances that do not turn into a liquid solely upon increasing pressure.
This group includes:
- Oxygen (O2) — a permanent gas.
- Nitrogen (N2) — a permanent gas.
- Hydrogen (H2) — a permanent gas.
- Methane (CH4) — a permanent gas.
- Helium (He) — a noble permanent gas.
To liquefy these gases, mandatory preliminary lowering of the temperature below the critical mark is required, and only after that will increasing the pressure lead to their transition into the liquid state.
Which gas mixtures are used in medical practice for inhalation anesthesia?
In medical practice, mixtures of vapor-forming anesthetics with medical gases are used for inhalation anesthesia.
The following combinations are most commonly used:
- Halothane — used in combination with nitrous oxide and oxygen.
- Enflurane — used in combination with nitrous oxide and oxygen.
- Sevoflurane — administered in combination with nitrous oxide and oxygen.
A mixture of nitrous oxide and oxygen is also used for ataralgesia against the background of spontaneous breathing and is combined with intravenous anesthetics (sodium hydroxybutyrate, etomidate, propofol with ketamine) for combined general anesthesia.
What is the mechanism of the toxic effect of carbon monoxide on the human body?
The mechanism of the toxic effect of carbon monoxide is based on the disruption of oxygen transport and cellular respiration.
The toxic effect is realized through the following processes:
- Formation of carboxyhemoglobin (HbCO) — carbon monoxide binds to hemoglobin, blocking oxygen transport to tissues, which causes oxygen starvation (anoxemia).
- Blockade of cytochrome c oxidase — inhibition of complex IV of the electron transport chain occurs, leading to the development of tissue hypoxia.
An increase in carboxyhemoglobin levels to 70–75% causes severe hypoxia and death.
To avoid confusing volume fraction with mass fraction in gas mixtures, remember the rule: "Gas volume is a mole." The volume fraction is numerically equal to the mole fraction, since according to Avogadro's law, volume depends only on the number of particles, not on their mass.
Class and exam questions
Why is humid air lighter than dry air at constant pressure?
Equal volumes contain an equal number of particles. Water molecules have a mass of 18 g/mol, and when they replace heavier nitrogen and oxygen molecules (average mass 29 g/mol), the overall air density decreases.
What is the difference between a gas and a volatile liquid?
Gases transition to a liquid state at room temperature only at pressures above 1 atmosphere. Volatile liquids (e.g., medical ether) exist in liquid form under normal conditions, but have a high saturated vapor pressure (from 0.1 atm).
How does vapor differ from a permanent gas in the context of liquefaction?
Vapors (e.g., ammonia or carbon dioxide) can be converted into a liquid simply by increasing the pressure. Permanent gases (hydrogen, oxygen) must first be cooled below their critical temperature, otherwise compression will not lead to liquefaction.
What else to study on this topic
- Calculation of the mass of a solid reactant from the volume of the evolved gas
- Identification of an unknown gas by relative density
- Algorithms for solving excess and limiting reactant problems in mixtures
- Calculation of the average molar mass of the initial mixture from combustion products
- Gas toxicology and air purity monitoring methods
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