Recap of Lecture 5

• Learned about the box model concept and steady-state

• Applied the box model to estimating concentrations of indoor pollutants

• Give an overview of the acid rain phenomenon

• Review acidity and the concept of pH

• Practice calculations involving the dissolution of acids in water

Overview of Acid Rain Phenomenon
 
 

• Most common term for acidification of the environment, which can occur from a wide variety of processes, including precipitation, fog, dry deposition, aerosol sedimentation

• Acids in the environment originate largely from pollutants such as nitrogen and sulfur oxides, which, in turn are generally by-products of combustion activities

• Research into environmental acidity started in the late 1800’s in Britain and Scandinavia; widespread effects seen as early as the 1950’s

• The first large-scale environmental issue, requiring cooperation among states and countries (e.g., US and Canada)

• Effects include "death" of many mountain lakes and severe damage to forests

• Good case study in regulation and international cooperation

The Basics: Acidity and pH
 

Need a refresher on acid/base chemistry? Try this tutorial!
 
 

What is an acid?

A substance that, when dissolved in water, increases the concentration of hydrogen ion (H⁺). Similarly a base is a substance that, when dissolved in water, increases the concentration of the hydroxide ion (OH^-).
 
 

In an acidic solution, [H⁺] > 1.00 x 10^-7 M (M º moles L^-1)

In a neutral solution, [H⁺] = 1.00 x 10^-7 M

In a basic solution, [H⁺] < 1.00 x 10^-7 M
 
 

The pH scale

For convenience, the pH (potential hydrogen) scale was developed. This relates the [H⁺] to an easier number to manipulate, via:

So, if the [H⁺] of a solution is 3 x 10^-4 M, the pH would be

Self-ionization of water

To some small degree, water will break down into ions:

H₂O « H⁺ + OH^-                     K_w = 1 x 10^-14

Where K_w = [H⁺][OH^-], the ion product or equilibrium constant for this reaction.

One can show from this relationship that

Problem: The pH of blood is 7.40. What is the hydrogen ion concentration? What is the hydroxide ion concentration?

 pH = -log[H⁺], so [H⁺] = 10^-pH

[H⁺] = 10^-7.4 = 3.98 x 10^-8 M

To determine the [OH^-], one can take either of two paths:

(a.) pH + pOH = 14.0, so pOH = 14.0 - pH = 6.6
     Then [OH^-] = 10^-pOH = 10^-6.6 = 2.51 x 10^-7 M

(b.) [H⁺][OH^-] = 1 x 10^-14, so [OH^-] = 1 x 10^-14/[H⁺]
     Then [OH^-] = 1 x 10^-14/3.98 x 10^-8 = 2.51 x 10^-7 M
 
 

Acids (and Bases) in Water
 
 

• Called aqueous solutions; ions given subscript (aq)

Since the water concentration does not change, it can be removed from the equation:

The chemical equation is typically shortened to read:

Problem: A 0.01 M solution of hydrofluoric acid (HF) is used in an experiment. If K_a = 6.8 x 10^-4, what is the pH of this solution?
 
 

Then,

If x is small compared to 0.01, we can disregard it in the denominator.

Atmospheric Aqueous Chemistry

• Atmospherically relevant aqueous chemistry usually involves interactions between gases and water, in addition to the liquid-phase reactions.
• When a gas is involved, we use its partial pressure rather than concentration:

Problem: Nitrogen oxides can interact with water to produce nitrous acid:

Suppose there were a raindrop in contact with these gases; what would its pH be? [NO = 1 x 10^-9 atm]
 

First, set up equilibrium expressions for the two processes:

Both expressions contain [HNO₂], so we can solve one and substitute into the other:

[HNO₂] = K₁ P_NO ; then K₂  = [H+][NO₂-]/K₁ P_NO

However, this still leaves us with an equation with two unknowns. From the second chemical reaction, we can see that for every H+, we must have an NO₂-. In other words, [H+] = [NO₂-]. This allows us to simplify the equation for K₂ to:

Solving for [H+]:

Problem: Vitamin C (ascorbic acid) is a diprotic acid, H₂C₆H₆O₆. What is the pH of a 0.10 M solution? How much ascorbate (C₆H₆O₆^2-) is in solution? K₁ = 7.9 x 10^-5 and K₂ = 1.6 x 10^-12
 

Try this one at home... first part of the problem is similar to the HF problem above. The second part is more challenging. Answers given in next lecture.
 
 
 
 
 

What are the acids in the atmosphere?

Inorganic acids

• Sulfuric acid (H₂SO₄): Source – sulfur-containing gases such as SO₂, H₂S, and (CH₃)₂S (dimethyl sulfide)

• Nitric acid (HNO₃): Source – nitrogen oxide emissions from combustion and soils

• Hydrochloric acid (HCl): Source – Volcanoes, industry

• Carbonic acid (H₂CO₃): Source – carbon dioxide, mostly natural, but has increased due to fossil fuel burning

• Sulfurous acid (H₂SO₃): Source – sulfur dioxide (as above)

Organic acids - typically biological in origin

• Formic acid (HCOOH): Source – by-product of oxidation of complex hydrocarbons; also made by ants

• Acetic acid (CH₃COOH): Also a by-product of oxidation

Short answer – it depends on the location!!

North America:

Similar to the US

China: