Acidic Oxides9.3.2 The Acidic Environment

In this keynote, we will be covering the following:9.3.2 While we usually think of the air around as neutral, the atmosphere contains acidic oxides of carbon, nitrogen and sulfur. The concentrations of these acidic oxides have been increasing since the industrial revolution.

Recall these common laboratory acids (you should remember these):

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The most familiar laboratory bases are soluble hydroxide compounds which are sometimes called alkalis:

A quick review...

Notice all common lab acids are solution containing H+ ions, and laboratory bases are solutions containing OH-.

You should also be familiar with neutralisation reactions:

For example, the reaction between sulfuric acid and potassium hydroxide

Ionic Equation

These are all spectator ions, so we remove them from the equation to get the net ionic equation...

We saw earlier all common lab acids are solution containing H+ ions, and laboratory bases are solutions containing OH-. These ions always react to form water, so the acid and base neutralise each other.

The common laboratory acids and alkali-bases we have used so far are not the only acids and bases. Most of the oxide compounds of the elements show some acid-base behaviour too, and the periodic table reveals another pattern.

- Non metal oxides, including semi metals, are usually acidic.

- Oxides of metals are basic.

- Some oxides are amphoteric (they can acts as either acids or bases) but we will look at this concept in greater detail later

- Neutral oxides are neither acidic or basic

Acid-base properties of Oxides

- Non metal oxides, including semi metals, are usually acidic.

- Oxides of metals are basic.

- Some oxides are amphoteric (they can acts as either acids or bases) but we will look at this concept in greater detail later

- Neutral oxides are neither acidic or basic

Task: Complete the following questions in your exercise book.

Most of the oxides of the metallic elements are considered basic because they can neutralise acids, forming water and a “salt”.

Basic Oxides of the Metals

For example...

Many of the oxide compounds of the non-metal elements are acidic because they will:

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Acidic Oxides of the Metals

- react with water to form an acid

AND/OR

- react with a base by neutralising it to form water and a ‘salt’

There are 3 main acidic oxides that are of concern in the environment:

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Acidic Oxides and the environment

All three are gases produced by both natural processes and by human activities. All 3 react with water in the environment to form acids.

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Sources of Acidic Oxides and the environment

Each of these major acidic oxides can cause different major environmental problems:

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Environmental Impacts

Carbon dioxide is the weakest acid of these 3 oxides, and its acidity is not the problem. It’s biggest impact is on contributing to the greenhouse effect.

The acidity of nitrous oxides is of minimal concern, however, they do contribute to photochemical smog and ozone pollution. However, nitrogen dioxide (NO2)contributes to acid rain.

The acidity of SO2 is a problem - it reacts with the environment to produce acid rain. It produces sulfurous acid - a strong acid that contributes to the creation of acid rain

Lakes and wetlands can become acidic enough to kill plants and animals and disrupt the food- chains and the normal ecological balance.

Forests can be killed by acidity of the rain, and the leaching of the soils by acids.

Environmental Impacts

In the 1960’s (when Acid Rain became known) many forests were seriously damaged, including the famous “Black Forest” of Germany. In Canada, 15,000 lakes were known to be “dead” in an ecological sense.

Since then, emissions of SO2 have been limited and damage reduced, but the threat of Acid Rain is still a serious one, especially in rapidly industrialising countries such as China.

!The environmental damage around Queenstown was caused by the release of SO2 from the Mt.Lyell copper mine and smelter. Although pollution ceased over 20 years ago, the environment has still not recovered.

Evidence for Acidic oxide pollutionWe know that CO2 levels in the atmosphere have increased because measurements have been collected over many years. The measurements of SO2 and NOx gases have not been collected for as long, and these gases are rapidly “washed” out of the air by rain, so the evidence for their presence is not so certain.

What we can be certain about are the localised effects of SO2 pollution in places where it is, or was, prevalent. Some examples from the 1960’s were mentioned before. Closer to home and to the present, is the evidence of devastation around Queenstown, Tasmania.

Task: answer the following questions.

Task: answer the following questions.