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Acids and BasesChapter 12

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Learning Objectives...

Define acids in terms of the ions they produce in aqueous solution and their effects on indicators.

Describe the characteristic properties of acids as in reactions with metals, bases and carbonates.

State some uses of acids. Define alkalis in terms of the ions they produce in

aqueous solution and their effects on indicators. Describe the characteristic properties of bases as in

reactions with acids, metal ions and ammonium compounds.

Describe the pH scale as a measure of relative acidity and alkalinity.

Construct ionic equations.

Acids...

eth

an

oic

acid

in

vin

eg

ar

formic acid in bee sting

hydrochloric acid in gastric

juice

tartaric acid in grapes

citric acid in lime

tannic acid in

teala

cti

c

acid

in

yog

hu

rt

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Laboratory Acids

3 common laboratory acids: hydrochloric acid (HCl) sulphuric acid (H2SO4)

nitric acid (HNO3)

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Physical Properties of Acids

Acids have a sour taste. Found in many sweets and fruits.

Acids are hazardous. Dilute acids are irritants – can cause skin to

redden and blister. Acids change the colour of indicators:

Turns blue litmus red

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Chemical Properties of Acids

Acids react with metals Hydrogen gas is produced

Test: use a burning splint. Observation: Fire extinguishes with

a ‘pop’ sound

hydrochloric acid + magnesium magnesium chloride + hydrogen

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Chemical Properties of Acids

Acids react with carbonates and hydrogen carbonate Carbon dioxide gas is produced

Test: Bubble gas through limewater Observation: A white precipitate is formed.

hydrochloric acid + calcium carbonate calcium chloride + carbon dioxide + water

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Chemical Properties of Acids

Acids react with metal oxides and hydroxide Most metal oxides and hydroxides react slowly

with warm, dilute acids Forms salt and water.

sulphuric acid + copper oxide copper sulphate + water

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Uses of Acids

Most important industrial acid: sulphuric acid

Used mainly to manufacture agricultural fertilisers Fertiliser: ammonium sulphate Formed by reacting sulphuric

acid with ammonia

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Uses of Acids Sulphuric acid is also used to manufacture of

detergents, paints, dyes, artificial fibres and plastics.

Dilute sulphuric acid is used in vehicle batteries. Hydrochloric acid and sulphuric acid are used to

remove rust, which consists of iron (III) oxide.

A car battery

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Bases

Bases are the oxides or hydroxides of metals. E.g.: copper (II) oxide, sulphur dioxide, sodium

hydroxide Some bases are insoluble.

E.g.: copper (II) oxide cannot dissolve in water Some bases are soluble. They are known as

alkalis E.g.: sodium hydroxide

ALL ALKALIS ARE BASES, BUT NOT ALL BASES ARE ALKALIS

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Laboratory Alkalis

3 common laboratory alkalis: Sodium hydroxide, NaOH Calcium hydroxide, Ca(OH)2

Aqueous ammonia, NH3 (or NH4OH)

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Physical Properties of Alkalis

Alkalis feel slippery. Alkalis are hazardous:

Concentrated sodium hydroxide and potassium hydroxide are corrosive. They are also known as caustic alkalis.

Dilute alkalis are irritants. Alkalis change the colour of indicators:

Turns red litmus blue

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Chemical Properties of Alkalis

Alkalis react with acids Also known as the neutralization reaction It is the reaction between an acid and a base to

form salt and water only.

acid + base (or alkali) salt + waternitric acid + sodium hydroxide sodium nitrate +

water

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Chemical Properties of Alkalis

Alkalis react with ammonium compounds Heating alkali with a solid ammonium

compound will produce ammonia gas. Test: Use damp red litmus paper Observation: Litmus paper turns blue

ammonium chloride + sodium hydroxide sodium chloride + ammonia + water

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Applications of Neutralization

Controlling pH of the soil Plants need soil of a suitable pH to grow well. If the soil is too acidic, farmers add bases

e.g. calcium oxide (quicklime) or calcium hydroxide (slaked lime)

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Applications of Neutralization

Controlling pH of the soil

If soil is too alkaline, farmers add compost Consists of rotting

plant material As it decomposes, the

plants give off carbon dioxide

Dissolves in soil water to form carbonic acid

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Applications of Neutralization

Treatment of indigestion Gastric juices contain hydrochloric acid for

digestion. Overeating can cause the stomach to produce

too much acid, causing indigestion. To relieve the pain, the acid is neutralized with

a medicine containing mild alkali.

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Applications of Neutralization

Treatment of indigestion An example of mild alkali is magnesium hydroxide.

magnesium hydroxide + hydrochloric acid

Magnesium hydroxide is an example of an antacid. Antacids can also contain aluminium hydroxide,

magnesium carbonate and calcium carbonate.

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Applications of Neutralization

Toothpaste Bacteria in our mouths can feed on food

particles, thus releasing acid. Acid corrodes our teeth, causing tooth decay. Toothpastes are alkaline, commonly containing

magnesium hydroxide.

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Uses of Alkalis

Apart from neutralization, alkalis can also be used to dissolve grease

Soaps and detergents contain mild (weak) alkali. Floor and oven cleaners contain sodium

hydroxide, a strong alkali Ammonia is used in glass cleaners.

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Acidity and Alkalinity - pH

pH is the measure of theacidity or alkalinity of a solution

pHsmall caps big caps

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Acidity and Alkalinity - pH

The pH scale ranges from 1 to 14. pH 7 neutral

E.g. pure water pH < 7 acidic pH > 7 alkaline

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pH of Common Substances

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Measuring pH

Using Universal Indicator Consists of a mixture of indicators. Can come in solution form or pH

paper. Disadvantage: gives approximate

pH values

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Measuring pH

Using Universal Indicator When the paper is dipped into solution, it gives

different colours at different pH values. The pH is found by comparing the colour obtained with a colour chart.

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Measuring pH

Using a pH meter Advantage: gives accurate pH values How to use:

Dip a probe into the solution The meter shows the pH Can be a scale or a digital display

Some are small and portable

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pH Sensor and Computer

A pH sensor can be connected to a computer.

The pH reading will be displayed on the computer screen.

Dataloggers are portable computers with attached pH sensors to measure pH values outside the laboratory.

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Other Indicators

Some indicators do not give pH values. They can only show if the substance is acid or alkaline.

There are 3 common indicators:

blueredlitmus

yellowredmethyl orange

pinkcolourlessphenolphthalein

colour in alkalis

colour in acids

indicator

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Using Litmus Paper

Question!A solution is tested with a piece of red litmus paper and the paper stays red. Meiling says the solution is acidic. Liyana says that to be sure, the solution should also be tested with blue litmus paper. Who is correct?

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Importance of pH

pH and the body Substances in the body have different pH

values. In the digestive system, the following are

needed for good digestion: Acidic conditions in the stomach (pH ~ 1.5) Alkaline conditions in the small intestine (pH ~

8.4) Blood going to the heart and lungs contains

carbon dioxide, which makes the blood slightly acidic.

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Importance of pH

pH and food preservation Many fresh foods spoil because of bacteria present

in the food. Bacteria do not grow well in solutions of low pH. Acids are thus used to preserve foods.

Ethanoic acid (vinegar) used to preserve vegetables Benzoic acid is used in fruit juices, oyster sauce and

jams. Citric acid is used as preservative and flavouring.

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Importance of pH

pH in the garden Most plants grow best when soil is about pH 5.6. Some plants grow well in more extreme pH:

Orchids prefer acidic soil (pH 4 - 5) Common vegetables e.g. beans and peas prefer

neutral soil (pH 6 – 8) Water lilies prefer alkaline soil (pH 8 - 9)

Pink hydrangea grown in alkaline soil

Blue hydrangea grown in acidic soil

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Importance of pH

pH and hair Normal hair is weakly acidic (~ pH 5) Alkaline solutions make hair curly.

When perming hair, alkaline solutions are used. Alkaline solutions can damage hair by causing it

to become weaker and easier to break.

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Importance of pH

pH and hair To clean hair, shampoo is

used. It is alkaline to dissolve grease present in hair.

Shampoos must always be rinsed after used.

Hair conditioners contain weak acids to restore the pH of hair to its normal value.