Question

Nitric acid in acid rain forms when gaseous nitrogen dioxide pollutant reacts with gaseous oxygen and liquid water to form aqueous nitric acid. Write a balanced chemical equation for this reaction.

Short answer

2NO2(g) + O2(g) + 2H2O(l) -> 2HNO3(aq)

Step-by-step solution

Write unbalanced chemical equation

First, write the unbalanced chemical equation with the known reactants and products. Reactants are nitrogen dioxide (NO2 gas) and oxygen (O2 gas), while the product is nitric acid (HNO3 aqueous). The unbalanced equation is: NO2(g) + O2(g) + H2O(l) -> HNO3(aq)

Balance oxygen atoms

To balance the oxygen atoms, observe that there are two oxygen atoms in NO2, two in O2, and one in H2O on the reactant side, and three in HNO3 on the product side. We can start by writing the oxygen from O2 as half a molecule, which gives: NO2(g) + 1/2O2(g) + H2O(l) -> HNO3(aq)

Balance hydrogen atoms

To balance the hydrogen atoms, note that there are two hydrogens in H2O and one in HNO3. Since we have one HNO3 molecule on the product side, we only need one H2O molecule to provide the sufficient number of hydrogen atoms. Thus, the hydrogen atoms are already balanced.

Balance nitrogen atoms

To balance the nitrogen atoms, observe there is one nitrogen atom in NO2 and one in HNO3. They are already balanced, with one nitrogen atom on each side.

Verify the balanced equation

Verify that all atoms are balanced on both sides of the equation. The balanced chemical equation is NO2(g) + 1/2O2(g) + H2O(l) -> HNO3(aq), with each element having the same number of atoms on both sides of the equation.

Finalize the balanced equation

It's unusual to have a fraction in a balanced chemical equation, so we multiply the entire equation by 2 to remove the fraction: 2NO2(g) + O2(g) + 2H2O(l) -> 2HNO3(aq)

Key concepts

Understanding Chemical Reactions

A chemical reaction is a process where substances, known as reactants, transform into new substances called products. This transformation involves breaking and forming of chemical bonds, leading to a change in the properties of the substances involved.

When writing a chemical equation to represent a reaction, it's crucial to remember that mass cannot be created or destroyed (a principle known as the Law of Conservation of Mass). This law dictates that the number of atoms of each element must be the same on both sides of the equation, which is where the skill of balancing chemical equations comes into play.

As exemplified in the exercise with nitrogen dioxide reacting with oxygen and water to produce nitric acid, balancing the oxygen, hydrogen, and nitrogen atoms is essential to ensure the equation accurately represents the reaction. While balancing, remember it's more conventional to avoid fractions in the final equation by multiplying through by whole numbers.

The Role of Stoichiometry in Chemical Equations

The term stoichiometry takes from the Greek words 'stoicheion' (element) and 'metron' (measure), essentially meaning the measurement of elements. In chemistry, stoichiometry refers to the quantitative relationship between reactants and products in a chemical reaction.

Stoichiometry is vital because it tells us how much of each reactant is needed to create a certain amount of product. This allows scientists and engineers to predict the outcomes of reactions, design experiments, and even scale up processes for industrial applications. The balanced chemical equation provides the molar ratio of reactants and products, essential for stoichiometric calculations.

For instance, in the balanced equation from the exercise, the stoichiometry shows that two molecules of nitrogen dioxide and one molecule of oxygen react with two molecules of water to produce two molecules of nitric acid, maintaining the principle that matter is neither created nor destroyed.

Environmental Implications of Acid Rain

When discussing acid rain, it's important to recognize that it's a result of human activities leading to the formation of acidic compounds, which fall to the ground as rain, fog, snow, or dust. Acid rain is primarily caused by emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) into the atmosphere, where they react with water, oxygen, and other chemicals to form sulfuric and nitric acid.

Effects of Acid Rain

Acid rain can have detrimental effects on aquatic ecosystems, soil chemistry, plant life, and infrastructure. It can lead to the acidification of lakes and streams, harming aquatic life; it can leach important nutrients from soils; and it can corrode buildings and cultural monuments.

Prevention and Control

Efforts to prevent and control acid rain involve reducing SO2 and NOx emissions through clean air legislation, using cleaner sources of energy, and implementing technologies like flue-gas desulfurization in power plants.

Understanding the chemical equations that represent the formation of acidic compounds, similar to the exercise's nitric acid formation, can help in assessing environmental impact and mitigating the effects of acid rain.