Consider the reaction N2 + O2 = 2NO. What is the partial pressure of N2 under normal atmospheric pressure?
The correct answer and explanation is:
Under normal atmospheric conditions, the partial pressure of nitrogen gas (N₂) in air is approximately:
0.78 atm
Explanation (≈300 words)
Air is a mixture of gases, primarily nitrogen (N₂) and oxygen (O₂), with small amounts of other gases like argon and carbon dioxide. At normal atmospheric pressure (1 atmosphere or 1 atm), each gas in the mixture contributes to the total pressure in proportion to its mole fraction, a concept explained by Dalton’s Law of Partial Pressures.
Dalton’s Law states:
Pₜ = P₁ + P₂ + P₃ + … + Pₙ
Where:
- Pₜ is the total pressure,
- P₁, P₂, … are the partial pressures of the component gases.
In dry air at sea level:
- Nitrogen (N₂) makes up approximately 78% of the air by volume (or mole fraction),
- Oxygen (O₂) is about 21%,
- The remaining 1% includes argon, carbon dioxide, etc.
So, the partial pressure of nitrogen can be calculated by:
P(N₂) = Mole fraction of N₂ × Total pressure
P(N₂) = 0.78 × 1 atm = 0.78 atm
This is the partial pressure of nitrogen in the air before the reaction:
N₂ + O₂ ⇌ 2NO
This reaction is important in atmospheric chemistry and combustion. It is endothermic and generally occurs at high temperatures (e.g., inside engines or lightning strikes), not under ambient conditions. Under normal temperature and pressure (STP), the formation of nitric oxide (NO) from nitrogen and oxygen is extremely slow, so atmospheric concentrations of NO are typically low unless energy is added.
To summarize: Under standard atmospheric pressure, the partial pressure of nitrogen gas is 0.78 atm, reflecting its dominant presence in Earth’s atmosphere.