acids and bases

ACIDS, BASES & SALTS

8.1 The characteristic properties of acids and basesCore• Describe the characteristic properties of acids as reactions with metals, bases, carbonates and effect on litmus and methyl orange• Describe the characteristic properties of bases as reactions with acids and with ammonium salts and effect on litmus and methyl orange• Describe neutrality and relative acidity and alkalinity in terms of pH measured using Universal Indicator paper (whole numbers only)• Describe and explain the importance of controlling acidity in soilSupplement• Define acids and bases in terms of proton transfer, limited to aqueous solutions• Describe the meaning of weak and strong acids and bases8.2 Types of oxidesCore• Classify oxides as either acidic or basic, related to metallic and non-metallic characterSupplement• Further classify other oxides as neutral or amphoteric8.3 Preparation of saltsCore• Demonstrate knowledge and understanding of preparation, separation and purification of salts as examples of some of the techniques specified in section 2.2.2 and the reactions specified in section 8.1Supplement• Demonstrating knowledge and understanding of the preparation of insoluble salts by precipitation• Suggest a method of making a given salt from a suitable starting material, given appropriate information

CONTENTS

• Acidity and alkalinity

• Indicators

• pH

• Acids

• General methods for making salts

• Making salts from metal oxides

• Making salts from metal carbonates

• Making salts from metals

• Making salts from alkalis – by titration

• Making insoluble salts – by precipitation

• Questions

ACIDS, BASES & SALTS

ACIDITY AND ALKALINITY

• water is a neutral substance

• when substances dissolve in water, the solution may become… ACIDIC or ALKALINE or stay NEUTRAL

ACIDITY AND ALKALINITY

• water is a neutral substance

• when substances dissolve in water, the solution may become… ACIDIC or ALKALINE or stay NEUTRAL

• hydrogen ions H+(aq) make solutions acidic

• hydroxide ions OH¯(aq) make solutions alkaline

• solutions with equal numbers of H+(aq) and OH¯(aq) are neutral

INDICATORS

• show by a colour change if a solution is acidic, alkaline or neutral

• some can even show how acidic or how alkaline a solution is

INDICATORS



• well known indicators include…

INDICATORS




LITMUS

acidic RED alkaline BLUE neutral PURPLE

INDICATORS




LITMUS

acidic RED alkaline BLUE neutral PURPLE

UNIVERSAL

acidic RED alkaline VIOLET neutral GREEN

pH SCALE

pH SCALE

Used to compare the relative acidity (and alkalinity) of solutions.The value is related to the concentration of H+ions, in solution.

pH SCALE


“The greater the concentration of hydrogen ionsin solution, the lower the pH”

pH SCALE



<— 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 —> strongly weakly neutral weakly strongly acidic acidic alkaline alkaline

pH SCALE




HCl CH3COOH H2O NH3 NaOH HYDROCHLORIC ETHANOIC AMMONIA SODIUM

ACID ACID HYDROXIDE

pH SCALE




HCl CH3COOH H2O NH3 NaOH HYDROCHLORIC ETHANOIC AMMONIA SODIUM

ACID ACID HYDROXIDE

Acidic solutions pH < 7Alkaline solutions pH > 7Neutral solutions pH = 7

pH SCALE

Despite what some people think, the pH scale is not restricted tobetween 0 and 14. It extends beyond those values, but is not asrelevant; e.g. the pH of the dilute HCl and H2SO4 found on the laboratory shelves is -0.3 and -0.6 respectively.

pH SCALE

Despite what some people think, the pH scale is not restricted tobetween 0 and 14. It extends beyond those values, but is not asrelevant; e.g. the pH of the dilute HCl and H2SO4 found on the laboratory shelves is -0.3 and -0.6 respectively.

Weak acids have higher pH values than stronger ones of the sameconcentration.

Ethanoic acid CH3COOH has a larger pH thanhydrochloric acid HCl of the same concentration.

MEASURING pH

Indicator Add a few drops of universal (full-range) indicator tothe solution and compare the colour against a

chart.

The colour you “see” depends not only on how many

drops of indicator you add but also on your perception

of colour.

MEASURING pH

Indicator Add a few drops of universal (full-range) indicator tothe solution and compare the colour against a

chart.

The colour you “see” depends not only on how many

drops of indicator you add but also on your perception

of colour.

pH meters Electrical/electronic devices which measure theconductivity of the solution.

Having calibrated the apparatus against a solution of

known pH, you place the probe in any solution and read

off the pH value on a scale or digital read-out.

ACIDS

Definition ACIDS ARE PROTON (H+ ion) DONORS

Categories Acids can be arranged into groups according tohow many H+ ions they produce from each molecule

ACIDS



ACID SALTS ION EXAMPLE

hydrochloric HCl chloride Cl¯ NaCl

nitric HNO3 nitrate NO3¯ NaNO3

ethanoic CH3COOH ethanoate CH3COO¯ CH3COONa

THE ABOVE ARE MONOPROTIC (MONOBASIC) ACIDS

ACIDS



ACID SALTS ION EXAMPLE

hydrochloric HCl chloride Cl¯ NaCl

nitric HNO3 nitrate NO3¯ NaNO3

ethanoic CH3COOH ethanoate CH3COO¯ CH3COONa

THE ABOVE ARE MONOPROTIC (MONOBASIC)ACIDS

sulphuric H2SO4 hydrogensulphate HSO4¯ NaHSO4

sulphate SO42-

Na2SO4

“carbonic” H2CO3 hydrogencarbonate HCO3¯ NaHCO3

carbonate CO32-

Na2CO3

THE ABOVE ARE DIPROTIC (DIBASIC)ACIDS

ACIDS


Categories Acids can also arranged into groups according tothe extent they split into ions when put in water.

ACIDS



STRONG ACIDS COMPLETELY dissociate (split up) into ionse.g. sulphuric, hydrochloric, nitric

ACIDS




WEAK ACIDS PARTIALLY dissociate (split up) into ionse.g. ethanoic, citric, carbonic

ACIDS




WEAK ACIDS PARTIALLY dissociate (split up) into ionse.g. ethanoic, citric, carbonic

This means that if you take similar concentrations of HCl andCH3COOH, the concentration of H+ ions in the solution of CH3COOHwill be less as only a few of its molecules will have split up into ions.Its pH will consequently be higher.

ACIDS

Reactions The hydrogen ions produced by acids can be replacedby other positive ions (metal ions or ammonium

ions)to produce salts. This occurs when dilute acids

reactwith metals, oxides of metals, hydroxides of metals,carbonates, hydrogencarbonates and ammonia.

ACIDS



reactwith metals, oxides of metals, hydroxides of metals,carbonates, hydrogen carbonates and ammonia.

HYDROCHLORIC ACID

ZINCCHLORIDE

SODIUMCHLORIDE

COPPERCHLORIDE

AMMONIUMCHLORIDEZINC

SODIUM HYDROXIDE

COPPER OXIDE

AMMONIA

MAGNESIUMCHLORIDE

MAGNESIUMCARBONATE

ACIDS



reactwith metals, oxides of metals, hydroxides of metals,carbonates, hydrogen carbonates and ammonia.

SULPHURIC ACID

ZINCSULPHATE

SODIUMSULPHATE

COPPERSULPHATE

AMMONIUMSULPHATEZINC

SODIUM HYDROXIDE

COPPER OXIDE

AMMONIA

MAGNESIUMSULPHATE

MAGNESIUMCARBONATE

ALKALIS

ALKALIS

Definition ALKALIS ARE SOLUBLE BASES Their solutions contain hydroxide ions, OH¯(aq)

ALKALIS


STRONG ALKALISsodium hydroxide (NaOH)potassium hydroxide (KOH)

WEAK ALKALIS ammonia solution (ammonium hydroxide) (NH4OH)

ALKALIS




Making salts using alkalis cannot be done by the methods used forinsoluble bases because you cannot tell when all the acid has beenneutralised – the excess alkali is soluble.

ALKALIS





The salts are made by titration using an indicator

ACID + ALKALINE HYDROXIDE ——> NEUTRAL SALT + WATER SOLUTION SOLUTION

ALKALIS





The salts are made by titration using an indicator

ACID + ALKALINE HYDROXIDE ——> NEUTRAL SALT + WATER SOLUTION SOLUTION

The indicator tells you when the acid has been neutralised

What is an oxide?

An oxide is a compound of oxygen and another element.

Most oxides can be grouped into four types:

acidic oxides basic oxides amphoteric oxides neutral oxides

Acidic oxides

Oxides of non-metal

Acidic oxides are often gases at room temperature.

Acidic oxides

React with water to produce acids.

Example: sulphur trioxide + water

sulphuric acid

SO3 + H2O H2SO4

Acidic oxides

React with bases and alkalis to produce salts

Example: carbon dioxide + sodium hydroxide sodium

carbonate + water

CO2 + 2NaOH Na2CO3 + H2O

Examples of acidic oxides

Acidic Oxide FormulaAcid Produced with

Water

sulphur trioxide

SO3 sulphuric acid, H2SO4

sulphur dioxide SO2sulphurous acid,

H2SO3

carbon dioxide CO2 carbonic acid, H2CO3

phosphorous(V) oxide

P4O10phosphoric acid,

H3PO4

Basic oxides

Oxides of metalBasic oxides are often solids at room temperature.

Most basic oxides are

insoluble in water.

Calcium oxide (quicklime)

Basic oxides

React with acids to produce salts and water only.

Example: Magnesium oxide + hydrochloric acid

magnesium chloride + water

MgO + 2HCl MgCl2 + H2O

This is a neutralisation reaction

Examples of basic oxides

Basic Oxide Formula

magnesium oxide MgO

sodium oxide Na2O

calcium oxide CaO

copper(II) oxide CuO

Amphoteric oxides

Oxides of metalCan behave as acidic oxides

or as basic oxides

Zinc oxide

Amphoteric oxidesReact with acids and alkalis to produce

salts Example: aluminium oxide (Al2O3)

aluminium oxide as a base

aluminium oxide as an acid

hydrochloric acid

aluminium

chloride

wateraluminium oxide

+ +

aluminium oxide

sodium hydroxide

sodium aluminate

water+ +

Neutral oxides

Non-metals that form oxides that show neither basic nor acidic properties.

Insoluble in water.

Examples of neutral oxides

Neutral Oxide Formula

water H2O

carbon monoxide CO

nitric oxide NO

MAKING SALTS - POSSIBILITIES

IS THE SALT SOLUBLE OR INSOLUBLE?

DOES THE METAL REACT WITH DILUTE

ACIDS?

IS IT SAFE?

IS THE METAL OXIDE OR CARBONATE

SOLUBLE IN WATER?

MIX SOLUTIONS OF TWO SUITABLE SALTS TO PRECIPITATE THE

INSOLUBLE SALT

ADD EXCESS METAL TO THE ACID THEN

FILTER OFF THE EXCESS METAL

TITRATE THE METAL HYDROXIDE WITH

ACID

FILTER OFF THE SALT

ADD EXCESS METAL OXIDE OR METAL

CARBONATE TO ACID THEN FILTER OFF THE

EXCESS SOLID

CRYSTALLISE THE SALT FROM THE FILTRATE

NONO

SOLUBLE

NO

YES

YESYES

INSOLUBLE

PREPARATION OF SOLUBLE SALTS

Salts can be made by NEUTRALISATION of acids;

• acidic solutions contain aqueous hydrogen ions H+(aq)• sulphuric acid produces a sulphate• hydrochloric acid produces a chloride• nitric acid produces a nitrate

Salts can be made by NEUTRALISATION of acids;

• acidic solutions contain aqueous hydrogen ions H+(aq)• sulphuric acid produces a sulphate• hydrochloric acid produces a chloride• nitric acid produces a nitrate

Other chemicals are formed, it depends on what is neutralising the acid.

OXIDES SALT + WATER

HYDROXIDES SALT + WATER

CARBONATES SALT + CARBON DIOXIDE + WATER

METALS SALT + HYDROGEN(not all metals are suitable; some are too reactive; others are not reactive enough)

PREPARATION OF SOLUBLE SALTS

PREPARATION OF SALTS BY NEUTRALISATION

MAKING SALTS – FROM INSOLUBLE BASES



ACIDS?

IS IT SAFE?


SOLUBLE IN WATER?


INSOLUBLE SALT




ACID

FILTER OFF THE SALT



EXCESS SOLID


NONO

SOLUBLE

NO

YES

YESYES

INSOLUBLE

Acid with excess insoluble oxide


1 Place dilute acid in a beaker and warm it gently without letting it boil (reactions are faster at higher temperatures)




2. Remove the heat




2. Remove the heat

3. Carefully and slowly add small amounts of the solid while stirring (most reactions of this type are exothermic i.e. they give off energy - the energy released would make the solution boil over)




2. Remove the heat

3. Carefully and slowly add small amounts of the solid while stirring (most reactions of this type are exothermic i.e. they give off energy - the energy released would make the solution boil over)

4. Continue adding the solid until no more dissolves (this ensures all the acid has been used - it is easier to filter off excess solid than extract excess acid).



5. Filter the solution into an evaporating dishto remove excess solid




6. Transfer the dish to a tripod and carefully warm the mixture to remove some of the water. Too much heat will drive off any water of crystallisation.





7. When some of the water has been evaporated, let the solution to cool; (allows crystallisation takes place)







8. When sufficient crystals have appeared, filter them off, wash with a little cold water and transfer them to a filter paper. Place another filter paper over the top and press gently to help remove some of the water.







9. Allow the crystals to dry naturally in air. (heat can cause the crystals to decompose and lose their water of crystallisation)



Acid with excess insoluble carbonate



The method used is similar to that used with acids and metal oxides.The main differences is that it is not necessary to use heat and thatcarbon dioxide gas is given off so you have to make sure the solutiondoesn’t effervesce (fizz) too much.




1 Place dilute acid in a beaker





2. Carefully and slowly add small amounts of the solid carbonate while stirring – make sure there is not too much effervescence.





2. Carefully and slowly add small amounts of the solid carbonate while stirring – make sure there is not too much effervescence.

3. Continue adding the solid carbonate until no more dissolves (this ensures all the acid has been used - it is easier to filter off excess solid than extract excess acid).

MAKING SALTS – FROM METALS



ACIDS?

IS IT SAFE?


SOLUBLE IN WATER?


INSOLUBLE SALT




ACID

FILTER OFF THE SALT



EXCESS SOLID


NONO

SOLUBLE

NO

YES

YESYES

INSOLUBLE


Acid with excess metal



The method used is similar to that used with acids and metal oxidesor metal carbonates. However, because hydrogen is produced duringthe experiment, Bunsen burners must not be used while the chemicalreaction is taking place.

Not all metals are suitable: some are too reactive K, Na, Casome don’t react Cu






2. Carefully and slowly add small amounts of the metal while stirring – make sure there is not too much effervescence.






2. Carefully and slowly add small amounts of the metal while stirring – make sure there is not too much effervescence.

3. Continue adding the metal until no more dissolves (this ensures all the acid has been used - it is easier to filter off excess metal than extract excess acid).

4. Filter the solution into an evaporating dishto remove excess metal



MAKING SALTS – FROM SOLUBLE BASES (ALKALIS)



ACIDS?

IS IT SAFE?


SOLUBLE IN WATER?


INSOLUBLE SALT




ACID

FILTER OFF THE SALT



EXCESS SOLID


NONO

SOLUBLE

NO

YES

YESYES

INSOLUBLE

AMMONIUM SALTS

Ammonia is very soluble in water giving an alkaline solution whichcan be neutralised by acids to produce AMMONIUM SALTS

e.g. AMMONIA + HYDROCHLORIC —> AMMONIUMSOLUTION ACID CHLORIDE

AMMONIUM SALTS

Ammonia is very soluble in water giving an alkaline solution whichcan be neutralised by acids to produce AMMONIUM SALTS

e.g. AMMONIA + HYDROCHLORIC —> AMMONIUMSOLUTION ACID CHLORIDE

If ammonium hydroxide is specified you can write the equation

AMMONIUM + HYDROCHLORIC —> AMMONIUM + WATERHYDROXIDE ACID CHLORIDE

Ammonium salts make very important fertilisers

MAKING SALTS BY TITRATION

This method is limited to the soluble salts of ammonia or alkali metals e.g. ammonium chloride NH4Cl

sodium chloride NaClpotassium sulphate K2SO4

sodium nitrate NaNO3





Method React an acid with an alkali (hydroxides of Group Imetals and ammonia). The method involves

TITRATION.





Method React an acid with an alkali (hydroxides of Group Imetals and ammonia). The method involves

TITRATION.

1 Place a measured quantity of acid in a conical flask.2 Add three or four drops of a suitable indicator.3 Place the alkali in a burette. Note the level of the liquid.4 Add the alkali to the acid until the indicator just changes colour Note the new level of liquid.5 Repeat experiment with exactly the same volumes but no indicator.6 Evaporate a small amount of the water in an evaporating basin and let the solution cool to allow crystals to form.


Example word equations:-

sodium hydroxide + hydrochloric acid —> sodium chloride + water

sodium hydroxide + sulphuric acid —> sodium sulphate + water

potassium hydroxide + nitric acid —> potassium nitrate + water

ammonium hydroxide + nitric acid —> ammonium nitrate + water







Now complete these:-








potassium hydroxide + hydrochloric acid —> potassium chloride + water








potassium hydroxide + hydrochloric acid —> potassium chloride + water sodium hydroxide + nitric acid —> sodium nitrate + water








potassium hydroxide + hydrochloric acid —> potassium chloride + water sodium hydroxide + nitric acid —> sodium nitrate + water

ammonium hydroxide + hydrochloric acid —> ammonium chloride + water

NEUTRALISATION - IONIC EQUATION

In all alkali – acid reactions, the same ionic reaction takes place…



sodium hydroxide + dil. nitric acid —> sodium nitrate + water NaOH(aq) + HNO3 (aq) —> NaNO3 (aq) + H2O (l)




Na+(aq) + OH¯(aq) + H+(aq) + NO3¯(aq) —> Na+(aq) + NO3¯(aq) + H2O(l)




Na+(aq) + OH¯(aq) + H+(aq) + NO3¯(aq) —> Na+(aq) + NO3¯(aq) + H2O(l)

cancel ions H+(aq) + OH¯(aq) —> H2O(l)

from from acid alkali

MAKING SALTS - POSSIBILITIES



ACIDS?

IS IT SAFE?


SOLUBLE IN WATER?


INSOLUBLE SALT




ACID

FILTER OFF THE SALT



EXCESS SOLID


NONO

SOLUBLE

NO

YES

YESYES

INSOLUBLE

PREPARATION OF INSOLUBLE SALTS


METHOD Insoluble salts can be made by mixing appropriate solutions of ions so that a precipitate is formed.



EXAMPLES silver chloride silver nitrate and sodium chloridebarium sulphate barium chloride and sodium

sulphatelead iodide lead nitrate and potassium iodide



PREPARATION OF SILVER CHLORIDE

To make silver chloride you need… (i) a soluble silver salt silver nitrate plus (ii) a soluble chloride sodium chloride.





Prepare a solution of sodium chloride.1





The sodium chloride lattice breaks up and dissolves in the water.2





Prepare a solution of silver nitrate.3





The silver nitrate lattice breaks up and dissolves in the water.4





Mix the two solutions.5





All four ions are now mixed together.6





The silver ions and chloride ions come together and theinsoluble silver chloride forms a white precipitate.

7





Because sodium nitrate is soluble in water their ions remain insolution. The precipitate is then filtered, washed and dried.

7

SALTS - QUESTIONS

SALTS - QUESTIONS

1. Work out what you get if the following chemicals are mixed.

a) zinc and sulphuric acidb) copper carbonate and nitric acidc) sodium hydroxide and hydrochloric acidd) lead oxide and nitric acide) magnesium and hydrochloric acid f) ammonium hydroxide and sulphuric acidg) magnesium carbonate and sulphuric acid

SALTS - QUESTIONS


a) zinc and sulphuric acid zinc sulphate + hydrogenb) copper carbonate and nitric acid copper nitrate + carbon dioxide +

waterc) sodium hydroxide and hydrochloric acid sodium chloride + waterd) lead oxide and nitric acid lead nitrate + watere) magnesium and hydrochloric acid magnesium chloride + hydrogen f) ammonium hydroxide and sulphuric acid ammonium sulphate + waterg) magnesium carbonate and sulphuric acid magnesium + carbon + water

sulphate dioxide

SALTS - QUESTIONS




sulphate dioxide

2. Answer the following...Which ion is found in (i) acidic solutions; H+(aq) (ii) alkaline solutions?

What is formed when you mix these two ions together?

What name do we give to this type of reaction?

When making salts from metals, oxides and carbonates, how do you knowwhen all the acid is used up?

When making salts from soluble Group I hydroxides, what doyou use to check when all the acid is used up?

SALTS - QUESTIONS




sulphate dioxide

2. Answer the following...Which ion is found in (i) acidic solutions; H+(aq) (ii) alkaline solutions? OH¯(aq)

What is formed when you mix these two ions together? WATER

What name do we give to this type of reaction?NEUTRALISATION

When making salts from metals, oxides and carbonates, how do you knowwhen all the acid is used up? WHEN THE SOLID NO LONGER DISSOLVES

When making salts from soluble Group I hydroxides, what doyou use to check when all the acid is used up? AN INDICATOR

acids and bases

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