The Acidic EnvironmentYEAR 12 CHEMISTRY

ACIDS AND BASES

Acids are compounds that:

have a low pH (below 7) taste sour turn blue litmus paper redreact with bases to neutralize them

and produce saltsrelease H2 gas in reactions with

active metalsaqueous solutions conduct electricity furnish H+

burn the skin if strongdissolve carbonates

These are some common acids. What others can you think of? Make a list in your notes.

ACIDS AND BASESBases are compounds that:

have a high pH (above 7) taste bitter turn red litmus paper bluereact with acids to neutralize them

and produce saltsare slippery feelingaqueous solutions conduct electricityburn skin if strongreact with fats to form soap furnish OH-

What other bases/alkalis do you know?

ACIDS AND BASESSome common laboratory acidsHCl hydrochloric acid HClO4

perchloric acidHBr hydrobromic acid HNO3

nitric acidHI hydroiodic acid H2SO4

sulfuric acid

Five of these acids are classified as monoprotic acids. They only have one hydrogen which they are able to donate. Sulfuric acid is classified as a diprotic acid because it has two acidic hydrogens that it can donate. Similarly, an acid which has three donatable hydrogens would be classified as triprotic.

ACIDS AND BASESSome common laboratory basesNaOH sodium hydroxide Ca(OH)2 calcium hydroxideKOH potassium hydroxide Mg(OH)2 magnesium

hydroxideNH3 ammonia Na2CO3 sodium carbonate

Bases are often found in everyday products such as many cleaning products (sodium hydroxide), antacid products (magnesium hydroxide )and fertilisers (ammonia). It is a common misconception that bases are not as dangerous as acids. In fact, many bases can be as much or more corrosive than many acids.

pH Indicators A simple explanation of pH is that it

is a measure of acidity/basicity (more in-depth explanation to follow)

Many substances change colour as they are exposed to different pH levels. These can be used to “indicate” the pH of substances when the colour ranges are known

Some natural products such as litmus, cabbage, grapes and tea are natural indicators while others such as phenolphthalein and methyl orange are synthetic

pH indicators are themselves acids or bases as they donate or accept protons (more on this later)

Notice the variety of ranges where different indicators change colours. Some have more than one change.

pH of some common substances

pH IndicatorsSome specific examples:1. Litmus red5 8blue2. Phenolphthalein colourless8.3 10red3. Methyl orange red3.1

4.4yellow4. Bromothymol blue yellow6 7.6blue

Note: colour changes within these ranges are gradual

A pH problem

Indicator ColourLitmus BluePhenolphthalein ColourlessMethyl orange YellowBromothymol blue Blue

An unknown solution produces the colours above. What is the pH range of this solution?

8-8.3

Uses of IndicatorsSoil testing Most plants cannot survive outside pH

5.5-7.5 Different plants require different pH

level

2 methods for testing soil pH Mix small sample with universal

indicator and sprinkle BaSO4 powder on top – read colour

Mix soil with water in a test tube and add indicator – read colour

Changing soil pH Too acidic – add NH3, CaO (lime),

CaCO3 Too basic – add manure, pine bark, peat

Uses of Indicators

Pool acidity

Pool should be pH of 7.4 Tested with a meter or an indicator such

as phenol red

Changing the pH Too acidic – add CaOCl2 (pool chlorine),

Na2CO3 Too basic – add HCl soln, NaHSO4

Non-metal OxidesSources1. Atmospheric O2 is very reactive and reacts with many

substances to form oxides2. Natural formation

1. CO2 – from respiration (“burning” sugars for energy)2. NO2 – from lightning strikes (N2 + 2O2 in the air 2NO2)3. SO2 – released from volcanoes or H2S + O2 SO2 + H2O (H2S

produced by bacterial decomposition of organic matter)3. Human causes (bushfires and burning fossil fuels)

1. CO2 – fossil fuel combustion product2. NO – high temperature combustion product3. NO2 – NO is easily oxidised in the air (NO + O2 NO2)4. SO2 – burning coal that contains S as an impurity5. SO3 – SO2 is easily oxidised in the air

Acidic Non-metal OxidesMany non-metal oxides react with water in the

atmosphere to produce acids;

CO2 + H2O H2CO3 (carbonic acid)SO2 + H2O H2SO3 (sulfurous acid)SO3 + H2O H2SO4 (sulfuric acid)

2NO2 + H2O HNO3 + HNO2 (nitric and nitrous acid)

These non-metal oxides are all gasesTheir acidic products all contribute to the acidity of

rain

Oxide Trends in the Periodic TableOxides tend to increase in acidity from left to rightIn general:Metal oxides are basic (left side)Non-metal oxides are acidic (right side)

Exceptions:Amphoteric oxides (i.e. Al, Be, Ga, Sn, Pb)

Why this trend?This is due to electronegativity increasing from left to

right (see following slides for more details)

Oxides on the left side of the PTElectrons are transferred to the O2-

This is due to the ionic nature of these bonds because of a large difference in electronegativities, therefore ions are formed in solutionFor example: element electronegativity

Na 0.93O 3.44

This means: Na2O(aq) Na+ + O2- and O2- + H+ OH- (readily)

This overall consumption of H ions leads to an increase of pH (i.e. Basic)

Oxides on the right side of the PTElectrons are shared with the O2-

This is due to the covalent nature of these bonds because of a small difference in electronegativities, thus no ions are formedFor example: element electronegativity

S 2.58O 3.44

This means: Due to a partially positive S central atom, SO3(aq) + H2O H+ + HSO4-

This overall production of H ions leads to an decrease of pH (i.e. acidic). (See following slide for details of this reaction)

Sulfur trioxide forms an acid

O

SOO H+

OH--

H+

water Sulfuric acid solution

Sulfur trioxide

O

SOHO

O

Industrial Pollution ResearchThe Industrial

Revolution brought about many changes in the 19th Century. One of these changes involves an increase in the release of sulfur dioxide SO2 and oxides of nitrogen such as NO2.

1. Summarise the industrial origins of SO2 and NO2 and evaluate reasons for concern about their release into the environment

2. Consider health issues, visibility issues, effects on the environment (rivers, soil, plants), and effects on buildings