Acids bases and salts

Measuring pH

Ph is a measure of the hydrogen ion concentration of a solution.

The ph scale is a number scale which indicates whether a solution is acidic, neutral or alkaline.

o Acidic solutins have ph less than 7o Neutal solutions have a ph of 7o Alkaline(basic) solutions have a ph greater than 7

The lower the ph value, the stronger the acid is.

The greater the ph value, the stronger the alkali is.

The ph meter measures the ph of a solution precisely. Most general indicator papers or indicator solutions can only be used to determine whether a solution is acidic, alkaline or neutral.

Indicators are dyes which are one colour in an acidic solution and another colour in an alkaline solution.

Full range or universal indicators are mixtures of indicators. They are more useful than single indicators because they give approximate ph values of solutions being tested.

The colour changes of some common indicators

Indicator litmus Methyl orange Screened methyl orange

Phenolphthalein

Colour with acid Red Red Light red ColourlessColour with base blue yellow Green pink

IGNORE LEFTMOST COLUMN AND COLOURS

Acids

An acid is a solution that dissolves in aqueous solutions to produce free hydrogen ions:

HCl (aq) + water H+ (aq) + Cl aq) H2SO4 (aq) + water 2H+ (aq) + SO42- (aq)

All acids have hydrogen in their formulae. However, not all hydrogen compounds are acids.

Physical properties of acids

o They are corrosiveo Taste souro Turn blue litmus redo Soluble in watero Have ph values less than 7o Aqueous solutions of acids conduct electricity

Chemical properties of acid

o They react with reactive metals to produce a salt and hydrogen gaso The react with bases (metal oxides and hydroxides) to produce salt and water onlyo They react with carbonates and hydrogen carbonates to yield salt, carbon dioxide and water

For a substance to behave as an acid, water must be present. If no water is present the substance is described as being anhydrous and will not display characteristic behaviours of acids. That is, pure hydrogen chloride gas will have no effect on dry blue litmus paper. However, when the paper is damp, it changes colour from blue to red. Other examples of anhydrous acids are: sulphur dioxide, nitrogen dioxide and carbon dioxide. These compounds dissolve in water to form acidic solutions.

Classifying acids

Mineral acid Organic acidsThese were originally obtained from minerals. These were originally obtained from plant and

animal materials.Examples examplesHydrochloric acid HCl Tartaric acid C4H6O6 – in grapesSulphuric acid H2SO4 Ethanoic acid CH3COOH - in vinegarNitric acid HNO3 Lactic acid C3H6O3 - in milkPhosphoric acid H3PO4 Citric acid C6H8O6 – citrus fruits

Strong and weak acids

A strong acid is one that is completely ionized in aqueous solutions while a weak acid is only partially ionized in aqueous solutions. Most mineral acids are strong acids (table above) while some are weak such as; sulphurous acid, carbonic acid and nitrous acid. All organic acids are weak.

Basicity/proticity of acid

Basicity describes the number of acidic hydrogens that are present in an acid.

A monobasic / monoprotic acid yields one free H+ ion for each molecule of acid in aqueous solution. For example: HCl and HNO3 . All organic acids are monobasic. CH3COOH

A dibasic / diprotic acid yields two free H+ ions for each molecule of acid in aqueous solution. For example H2SO4

A tribasic/ triprotic acid yields three free H+ ions for each molecule of acid in aqueous solution. For example H3PO4

Concentration of acids

Concentration describes the solute content of a solution in a given volume of solution. An acid is described as ‘concentrated’ if it contains a very large amount of solute molecules dissolve in water. Conversely, an acid is described as being ‘dilute’ if contains a small amount of solute molecules dissolved in water.

A weak acid can be concentrated or dilute.

A strong acid may be concentrated of dilute.