When nitrogen dioxide ($\text{NO}_2$) reacts with oxygen ($\text{O}_2$), nitric acid is formed, but this process typically requires the presence of water ($\text{H}_2\text{O}$) as well. This reaction is a key part of the formation of acid rain.
The Chemical Process Explained
Nitrogen dioxide is a common air pollutant, often resulting from the combustion of fossil fuels. While $\text{NO}_2$ can react with oxygen directly (e.g., dimerization), the reaction that leads to a stable acidic product like nitric acid involves water.
The overall reaction describing the formation of nitric acid from nitrogen dioxide, oxygen, and water is:
$\text{4NO}_2\text{(g)} + \text{O}_2\text{(g)} + \text{2H}_2\text{O(l)} \rightarrow \text{4HNO}_3\text{(aq)}$
In this reaction:
- Four molecules of nitrogen dioxide gas react with one molecule of oxygen gas.
- Two molecules of liquid water are involved.
- This produces four molecules of aqueous nitric acid.
This process often occurs in the atmosphere, where nitrogen dioxide from sources like vehicle exhaust or power plants mixes with oxygen and water vapor (or liquid water in clouds/rain).
Why This Reaction Is Significant
Formation of Acid Rain
One of the most significant outcomes of this reaction is the contribution to acid rain. Nitric acid formed in the atmosphere dissolves in water droplets, increasing their acidity. When this water falls as rain, snow, or fog, it is known as acid precipitation.
- Acid rain can harm ecosystems by acidifying lakes and streams, damaging forests, and eroding buildings and monuments.
- The nitrogen compounds in nitric acid can also contribute to nutrient pollution (eutrophication) in water bodies.
Atmospheric Chemistry
This reaction is part of the complex series of chemical transformations that occur in the atmosphere involving nitrogen oxides. These processes influence air quality and climate.
Key Reactants and Product
Let's look at the substances involved:
| Substance | Chemical Formula | Role in Reaction | Common Source |
|---|---|---|---|
| Nitrogen Dioxide | $\text{NO}_2$ | Reactant | Fossil fuel combustion |
| Oxygen | $\text{O}_2$ | Reactant | Earth's atmosphere |
| Water | $\text{H}_2\text{O}$ | Reactant | Atmosphere (vapor/liquid) |
| Nitric Acid | $\text{HNO}_3$ | Product | Atmospheric reaction |
Note: While $\text{NO}_2$ and $\text{O}_2$ react directly under certain conditions, the presence of water is crucial for the formation of stable nitric acid via this pathway.
Summary
In summary, when nitrogen dioxide reacts with oxygen, particularly in the presence of water, nitric acid is formed. This atmospheric reaction is a major contributor to acid rain and has significant environmental implications.
