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Chapter 18: Problem 30

Why is rainwater naturally acidic, even in the absence of polluting gases such as \(\mathrm{SO}_{2}\) ?

Short Answer

Expert verified
Rainwater is naturally acidic, even in the absence of polluting gases like \(\mathrm{SO}_{2}\), due to the reaction between carbon dioxide (CO\(_2\)) and water vapor in the atmosphere. This reaction forms carbonic acid (H\(_2\)CO\(_3\)), a weak acid that partially dissociates into protons (H\(_+\) ions) and bicarbonate ions (HCO\(_3^-\)) in water. The presence of free H\(_+\) ions in rainwater gives it a pH less than 7, making it slightly acidic. The pH of unpolluted rainwater typically hovers around 5.6 due to the presence of carbonic acid.

Step by step solution

01

Understand the components of the atmosphere

The Earth's atmosphere is composed of various gases, one of which is carbon dioxide (CO\(_2\)). Carbon dioxide, although present in relatively low concentrations, is an essential part of our atmosphere, taking part in various atmospheric reactions and the carbon cycle.
02

Reaction between carbon dioxide and water

Even without the presence of acid-producing pollutants like sulfur dioxide, rainwater can still become acidic naturally. This is due to the reaction between carbon dioxide and water vapor in the atmosphere. When carbon dioxide dissolves in water, it forms carbonic acid (H\(_2\)CO\(_3\)), according to the following reaction: \(CO_2 (g) + H_2O (l) \leftrightarrows H_2CO_3 (aq)\) This carbonic acid formation contributes to the slight acidity of rainwater.
03

Dissociation of carbonic acid

Carbonic acid is a weak acid, meaning it does not completely dissociate in water. However, even partial dissociation can lead to an overall acidic environment. Carbonic acid dissociates in water into protons (H\(_+\) ions) and bicarbonate ions (HCO\(_3^-\)) as follows: \(H_2CO_3 (aq) \leftrightarrows H^+ (aq) + HCO_3^- (aq)\) The presence of free H\(_+\) ions in rainwater causes it to have a pH less than 7, making it acidic. This acidity is mild and not harmful under normal circumstances.
04

The pH of natural rainwater

The pH of unpolluted rainwater typically hovers around 5.6 due to the presence of carbonic acid. This is slightly acidic, but not harmful to most ecosystems. However, it is important to note that the addition of pollutants like sulfur dioxide and nitrogen oxides can significantly decrease the pH of rainwater, leading to the formation of acid rain, which can be harmful to both humans and the environment. To summarize, rainwater is naturally acidic even in the absence of polluting gases like \(\mathrm{SO}_{2}\) because of the reaction between carbon dioxide and water in the atmosphere, forming carbonic acid. This weak acid dissociates in water, generating H\(_+\) ions and making the rainwater slightly acidic.

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Most popular questions from this chapter

An important reaction in the formation of photochemical smog is the photodissociation of \(\mathrm{NO}_{2}\) : $$ \mathrm{NO}_{2}+h \nu \longrightarrow \mathrm{NO}(g)+\mathrm{O}(g) $$ The maximum wavelength of light that can cause this reaction is $420 \mathrm{nm} .$ (a) In what part of the electromagnetic spectrum is light with this wavelength found? (b) What is the maximum strength of a bond, in \(\mathrm{kJ} / \mathrm{mol}\), that can be broken by absorption of a photon of 420 -nm light? \(?\) (c) Write out the photodissociation reaction showing Lewis-dot structures.

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