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Topic 5 – National Chemistry Summary Notes

Acids and Alkalis

Experiment – Collect some samples of rain water

The pH Scale

The pH scale is a continuous range of numbers from below 0 to above 14.

Acids have a pH number of less than 7

Neutral substances have a pH number equal to 7 – pure water is a

good example

Alkalis have a pH number of more than 7

Note a substance becomes more acidic or alkaline the further it is from

a pH number of 7

Testing the pH of Solutions

Experiment – pH of Common Solutions

Universal indicator, pH paper or a pH meter can be used to find the pH of

solutions.

When universal indicator is used the following colours are observed:

Acidic solutions (pH below 7) – red, orange, yellow

Neutral solutions (pH 7) – green

Alkaline solutions (pH above 7) – blue, purple

Notes:

to test the pH of a solid substance it must be soluble in water

the pH of water can only be affected by the addition of soluble

substances

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Common Acids and Alkalis

Acids and alkalis are commonly used in the home and in the laboratory.

The following tables give some examples of common acids and alkalis.

Laboratory

Acids Alkalis

hydrochloric acid sodium hydroxide

sulphuric acid lime water

nitric acid ammonia solution

Home

Acids Alkalis

Vinegar bleach

lemon juice washing powder

Coca-Cola baking soda

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Food and Drink and Acids

Many of the foods we eat are naturally acidic, for example, oranges and

lemons. Many of the manufactured food and drink you like have acids

added to them. They are added for lots of reasons. The three main

reasons are:

1. To act as a preservative – to stop the food going bad quickly

2. To change the flavour

3. To make baked products rise in the oven

The most common cause of tooth decay is by acidic foods and drinks. In

general, foods and drinks with a pH below 5.0–5.7 have been known to

trigger tooth decay. Numerous clinical and laboratory reports link tooth

decay to excessive consumption of drinks. Soft fizzy drinks and wine have

been shown to cause tooth decay.

Experiment – Chicken Bones and Acid

Go home tonight and look at some food labels and read the ingredients

list to find out if any acids are present.

Ethanoic acid, citric acid and benzoic acid are acids you may have come

across when looking at food labels. Try to find out why these are added

to certain foods and complete the table below with your findings.

Food

Acid Present Use

ethanoic

citric

benzoic

Experiment – Lemon Juice & Apples

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Pupil Activity

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The Effect of Dilution on The pH of Acids and Alkalis

Experiment

1. Collect a bottle of acid or a bottle of alkali, a bottle of pure water,

a plastic dropper, universal indicator and a colour chart

2. Copy and complete the table below

Acid or Alkali

Test Tube

Instructions

pH Number

1 10ml of acid or alkali

2 1ml from test tube 1 + 9ml water

3 1ml from test tube 2 + 9ml water

4 1ml from test tube 3 + 9ml water

5 1ml from test tube 4 + 9ml water

Now add 5 drops of universal indicator to each of the test tubes

The pH of a substance is a measure of the hydrogen ion concentration.

LI 5 E

Acids contain a greater concentration of hydrogen ions than

hydroxide ions.

Alkalis contain a greater concentration of hydroxide ions than

hydrogen ions.

Neutral solutions contain an equal concentration of hydrogen and

hydroxide ions.

remember to wash out the dropper with water between each instruction

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Pure water is an example of a neutral solution. Pure water contains mainly

molecules with a very small but equal number of hydrogen (H+) and

hydroxide (OH-) ions, i.e. a very small proportion of water molecules

dissociate into an equal number of hydrogen and hydroxide ions.

H2O (l) H+(aq) + OH-

(aq)

The higher the concentration of hydrogen ions in an acid, the lower

the pH number

The higher the concentration of hydroxide ions in an alkali, the

higher the pH number

The reason this happens is that when we add water we are mainly adding

molecules, not ions, and are therefore decreasing the concentration of

the hydrogen or hydroxide ions in the acid or alkali respectively.

Experiment – Comparing Conductivities

The pH number of an acidic solution rises towards 7 as it is diluted

The pH number of an alkaline solution falls towards 7 as it is diluted

Note: Diluted just means water has been added

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Solubility of Oxides

Experiment

Name of Oxide

Solubility in Water Acid/Alkali/Neutral

Using pg 5 of the data booklet we can see that:

Group 1 metal oxides are soluble in water and produce alkaline

solutions

Some Group 2 metal oxides are soluble in water and also produce

alkaline solutions

Non-metal oxides that are soluble in water produce acidic

solutions

Note: Insoluble oxides will not affect the pH of water

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E When added to water, soluble metal oxides produce metal hydroxide

solutions, increasing the hydroxide ion concentration.

Soluble non-metal oxides increase the hydrogen ion concentration.

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Problems with Non-Metal Oxides

Carbon, Sulphur and Nitrogen Oxides

Three important non-metal oxides that are soluble in water are sulphur

dioxide, carbon dioxide and oxides of nitrogen. These gases are produced

naturally as outlined below:

carbon dioxide – when living things breathe out

sulphur dioxide – volcanic eruptions

oxides of nitrogen – lightning storms

However, the increased production of these oxides, for example when

fossil fuels are burned, causes environmental problems.

When produced these oxides react with water in the atmosphere to

produce acid rain.

Experiments

1. Using a digital pH meter, test the pH of the rain water you

collected at the start of this Topic.

2. Cress Seed Growth & Acid Rain.

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The following table shows some examples of the damaging effects of acid

rain.

Problem Damaging Effect

buildings erosion of buildings, e.g. nose

missing from a statue

steel structures corrodes steel bridges and car

bodies

animal life increasing acidity of loch water can

cause fish to die

soils and plant life

makes soil too acidic for plants to

grow

Carbon Dioxide

Carbon dioxide gas is the primary gas responsible for Global warming (the

rise in the average temperature of the Earth’s atmosphere and oceans)

and the resulting alarming changes in our climate.

In addition to burning of fossil fuels, another large contribution of

carbon dioxide in the atmosphere is made by cement manufacturing.

When cement is made the process starts with heating calcium carbonate.

This releases carbon dioxide into the atmosphere. In addition, the energy

required to produce cement can also release carbon dioxide into the

atmosphere.

Ocean acidification is the name given to the on-going increase in the

acidity of the Earth’s oceans caused by the uptake of carbon dioxide

from the atmosphere.

Note: The term ‘carbon footprint’ refers to the amount of carbon

dioxide we emit individually in any one year period.

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A neutralisation reaction is one in which an acid reacts with a base to

form water and a salt.

The course of a neutralisation reaction can be followed by using a pH

indicator such as universal indicator. However, if the base is insoluble

then an indicator is not required as the following will occur when the

reaction has finished:

some of the insoluble base will be left at the bottom of the beaker

no more fizzing will be seen

In a neutralisation reaction the pH of the substance moves towards 7, i.e.

a neutral pH.

Bases

Bases are substances which neutralise acids giving a salt and water.

Bases are:

Metal hydroxides

Metal oxides

Metal carbonates

Note: bases which dissolve in water form alkalis

Experiment

1. Collect a bottle of acid and a bottle of alkali, universal indicator, a

colour chart and two syringes, one labelled acid the other alkali

2. Measure 10ml of acid into the syringe labelled acid

3. Transfer this to a beaker

4. Add 5 drops of universal indicator and swirl

5. Slowly add alkali from the other syringe to the beaker, swirling

after each addition, stopping when the colour becomes green

add the alkali slowly

from the syringe

LI 7 Neutralisation Reactions

acid + a few drops of

universal indicator

acid + alkali + a few drops of

universal indicator

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Acid + Metal Hydroxide

acid + metal hydroxide salt + water

Acid + Metal Oxide

acid + metal oxide salt + water

Acid + Metal Carbonate

acid + metal carbonate salt + water + carbon dioxide

Naming Salts

The name of the salt formed in a reaction can be easily worked out as

shown in the following tables.

Acid

Salt

sulphuric acid metal sulphate

nitric acid metal nitrate

hydrochloric acid metal chloride

Acid Metal

Hydroxide/Oxide/Carbonate

Salt

sulphuric acid sodium hydroxide sodium sulphate

hydrochloric acid zinc oxide zinc chloride

nitric acid calcium carbonate calcium nitrate

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Full Examples

1. sulphuric + sodium sodium sulphate + water

acid hydroxide

2. hydrochloric + zinc zinc chloride + water

acid oxide

3. nitric + calcium calcium + water + carbon dioxide

acid carbonate nitrate

Formula of Acids

H2SO4 – sulphuric acid (hydrogen sulphate)

HNO3 – nitric acid (hydrogen nitrate)

HCl – hydrochloric acid (hydrogen chloride)

Everyday Examples of Neutralisation Reactions

Adding lime, an alkali, neutralises the excess acid when soil is too

acidic

Adding lime to lakes to reduce acidity

Indigestion is caused by too much acid in the stomach. Treat by

taking acid indigestion remedies, these are alkaline and therefore

neutralise the excess acid in the stomach

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To find out if the salt to be made is soluble or insoluble look at page 5 of

the data book.

Soluble Salts

The easiest way to make a soluble salt is to neutralise the relevant acid

with an excess of an insoluble metal oxide or carbonate. An indicator is

not required as the following will occur when the reaction has finished:

some of the insoluble base will be left at the bottom of the beaker

no more fizzing will be seen

The excess metal oxide or carbonate can then be filtered off and the

salt can then be obtained by evaporating the solution left. No accurate

masses or volumes are required since the excess metal oxide or carbonate

is easily removed by filtering.

Experiment – Preparation of A Salt

Insoluble Salts

The easiest way to make an insoluble salt is by a precipitation reaction.

Precipitation is the reaction in which two solutions react to form an

insoluble product. The insoluble product is called a precipitate.

Example

To make the insoluble salt, barium sulphate, just look at page 5 of the

data book and react together two soluble compounds, one containing

barium and the other containing sulphate. For example,

barium chloride + sodium sulphate barium sulphate + sodium chloride

(vs) (vs) (i) (vs)

Notice the metals just swap places.

The insoluble barium sulphate can be removed from the sodium chloride

by filtration and then dried.

Note the other product formed must be soluble.

LI 8 Making Salts

remember to read over

the notes you have made

on this experiment in your

jotter

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Spectator Ions

Spectator ions do not take part in the reaction.

Example 1

hydrochloric + lithium lithium + water (word eq)

acid hydroxide chloride

H+ + Cl- + Li+ + OH- Li+ + Cl- + H2O (ionic eq)

The lithium and chloride ions are exactly the same on both sides of the

equation and therefore have not taken part in the reaction, i.e. they are

spectator ions.

The reacting species are determined by the omission of the spectator

ions.

H+ + Cl- + Li+ + OH- Li+ + Cl- + H2O

In this reaction the hydrogen and hydroxide ions are the reacting species

and react together to form water.

i.e. H+ + OH- H2O

Example 2

hydrochloric + sodium sodium chloride + water + carbon dioxide

acid carbonate

2H+ + 2Cl- + 2Na+ + CO32- 2Na+ + 2Cl- + H2O + CO2

The chloride and sodium ions are the spectator ions leaving the hydrogen

ions and carbonate ions to form water and carbon dioxide.

2H+ + 2Cl- + 2Na+ + CO32- 2Na+ + 2Cl- + H2O + CO2

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(when state symbols are included in an ionic equation this is known as an ionic state equation)

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Titration Calculations

Titration is an analytical technique used to determine the accurate

volumes involved in chemical reactions such as neutralisation.

An indicator is used to show the end-point of the reaction, i.e. when the

reaction is finished.

When carrying out titration calculations use the following formula

Acid Alkali

V x C x H+ = V x C x OH-

V= volume of acid or alkali

C= concentration of acid or alkali

H+ = number of hydrogen ions in acid formula

OH- = number of hydroxide ions in alkali formula

Example

What volume of a sodium hydroxide solution, with a concentration of

4moll-1, is neutralised by 40cm3 of 1moll-1 sulphuric acid?

Acid Alkali

H2SO4 NaOH

VxCxH+ = VxCxOH-

40x1x2 = Vx4x1

80 = 4V

80/4 = V

20cm3 = V

The volume of the sodium hydroxide solution = 20cm3

In titration calculations

the volume does not need

to be converted to litres

as long as it is the same

unit for both the acid and

the alkali

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Rough Titres

Whenever doing titrations always ignore the rough titre to get the volume

of acid required and average the rest.

Titre

Volume of Acid

(ml)

Rough

25.8

1st

25.4

2nd

25.2

The volume of acid required in this titration is 25.3ml, i.e. the average of

the 1st and 2nd titres.

Experiment - Titration

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Energy Changes of Chemical Reactions

All chemical reactions involve energy changes. Sometimes these signs of a

chemical reaction are obvious like an explosion or flames, sometimes

they are more subtle like a small temperature change.

Experiments

1. Adding 50 ml of 1 mol/l hydrochloric acid to 50 ml of 1 mol/l sodium

hydroxide. Measuring temperature before and after.

2. Dissolving 5g of ammonium nitrate in 50 ml of water. Measuring

temperature before and after.

Results

Reaction Temperature

before (0C)

Temperature

after (0C)

Temperature

change (0C)

Neutralisation

Average of acid and

alkali

Dissolving

ammonium nitrate

Endothermic or Exothermic?

Some reactions, like the neutralisation reaction above, release heat to

their surroundings. These reactions are described as being exothermic.

Some reactions, like dissolving ammonium nitrate, take in heat from

their surroundings. These reactions are described as being endothermic.

You may not have been aware of any reactions that made their

surroundings colder but you have met reactions that produce heat. Can

you name any other exothermic reactions? Write them below:

_______________________________________________________

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Topic 5 Pupil Self Evaluation

Acids and Alkalis

If there is an E in any part of the notes or the success criteria is in italics, then this is extension level work

Number

Learning Intention

Success Criteria

1 I will find out about the pH scale I can state if a pH number represents:

An acid, alkali or neutral substance

A more or less acidic or alkaline substance

2 I will test some substances with

universal indicator to find out their pH

I can:

Test the pH of a substance

State the colours observed when an acid, alkali or neutral

substance is tested using universal indicator

3 I will find out the names of some

common laboratory and household acids

and alkalis

I can name some common laboratory and household acids and alkalis

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4 I will find out:

why acid is added to some food

and drink

one health problem caused by

this addition

I can state:

the three main reasons acids are added to some food and drink

one health problem caused by this addition

5E I will find out:

The most concentrated ion in

acids and alkalis

About the dissociation of pure

water

about dilution (adding water) of

acids and alkalis

I can:

state the ion there is a high concentration of in an acidic or

alkaline solution

State the concentration of the ions present in pure water and

other neutral solutions

State what happens to the pH number and acidity and alkalinity

of an acidic or alkaline solution when they are diluted

explain why this happens

6 I will find out about:

the solubility of metal and non-

metal oxides

the effects soluble non-metal

oxides have on the environment

the term ‘carbon foootprint’

I can:

state whether a metal oxide or non-metal oxide produces an

acidic or an alkaline solution if soluble in water

state what happens to the hydrogen and hydroxide ion concentrations when metal oxide or non-metal oxides are soluble in water

explain how non-metal oxides produce acid rain

explain the damaging effects of acid rain on buildings, steel

structures, animal life and soil and plant life

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state the gas responsible for ocean acidification and mainly

responsible for Global warming and state the various ways this

gas can be produced

give the definition of the term ‘carbon footprint’

7 I will find out about neutralisation

reactions

I can:

State the definition of a neutralisation reaction

State how the course of a neutralisation reaction can be

followed

Give examples of a base

State the products formed when an acid reacts with a base

Name the salt formed in a neutralisation reaction

8 I will find out the best way to make

soluble and insoluble salts

I can describe the best way to make a soluble or insoluble salt

9E I will find out about spectator ions

I can, given a reaction, state the reacting species

10E I will find out how to carry out:

the titration technique

titration calculations

I can carry out:

the titration technique

titration calculations

11 I will find out that reactions can take

in heat as well as release heat

I can:

state that a reaction which gives out heat is called an

exothermic reaction

state that a reaction that takes in heat is called an endothermic

reaction

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