06 acids bases salts

8/6/2019 06 Acids Bases Salts

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Acids, Bases and Salts 1

Acids, Bases & Salts

Acids, bases and salts are three categories of important chemicals.

The word acidmeans sour. In fact all acids are sour. Acids are produced whenacidic oxides, e.g. CO2, SO2 are reacted with water. All acids are soluble in water.

Acidic solutions are neutralised by bases1. These are another group of chemicals.Bases which are soluble in water are called alkalis.

Acids and alkalis react with another type of chemicals called indicators. A colourchange is obtained when alkalis and acids react with indicators.

Colour change given by acids and alkalis with indicators

Indicator Acid Neutral Alkaline

Methyl Orange pink orange yellow

Litmus red purple blue

Phenolphtalein colourless colourless pink

Indicators are usually obtained from lichens and other plant material.

Universal indicator does not give one colour change but a series of colour

changes. This shows that not all acids and bases are of the same strength.

The strength of acids and alkalis is measured using a special scale - the pH scale.

The pH scale

The term pH is derived from the German word which means power ofhydrogen.

The pH scale varies from 0-14. The pH value is directly proportional to the

concentration of H+

ions (for acids) or OH-

ions (for alkalis). This will bediscussed later.

All acids have a pH less than 7. A pH value of 0 indicates a very strong acid,whilst a pH of 6 would indicate a weak acid.
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All alkalis have a pH greater than 7. A pH value of 8 indicates a very weak alkaliwhilst a pH value of 14 indicates a very strong alkali.

A solution that has a pH value of 7 is neutral, i.e. neither acidic nor alkaline.

Indicators such as methyl orange and phenolphthalein show only one colour with

acids and one with alkalis. Hence they cannot indicate how acidic and howalkaline a solution is. Universal indicator though, can do this.

As can be seen from the previous diagram, rough pH values can be obtained bythe use of Universal indicator. Since the latter changes colour at different pHvalues, a colour change can indicate the pH of a solution. More accurate pHreadings are given with a pH meter.

Acids

Weak and strong acids

Definitions of acids have varied over the years as scientists have discoveredmore about them. The most common definitions used today are:

a.An acid is a substance that reacts with a base to form a salt and water only.

b. An acid is a substance which releases hydrogen ions (H+) when dissolved inwater.

It is the latter property that gives acids their acidity and hence their properties.

When acids are in solution oxonium ions, i.e. H3O+ are produced. These oxonium

ions are usually shown as H+(aq) to simplify matters.

These oxonium ions are formed when the acid is ionised2in water. For example,

HCl(g) + H2O(l) --> H+(aq) (or H3O

+(aq)) + Cl-(aq)
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It is the oxonium ion that is the cause for acidic properties.

The basicity of acids is an indication of how many H+ ions are formed from onemolecule of acid.

HCl(g) --> H+

(aq) + Cl-

(aq) Monobasic

HNO3(g) --> H+(aq) + NO3-(aq) Monobasic

H2SO4(g) --> 2H+(aq) + SO4

2-(aq) Dibasic

Some acidic properties

Acids:

1.

are sour2. are soluble in water3. are often corrosive4. change the colour of indicators5. are neutralised by bases6. react with some metals to release hydrogen7. react with carbonates to release carbon dioxide

Not all acids have the same strength. Note that strength does not have anythingto do with concentration. Concentrated and dilute refer only to the proportionof water and acid in the solution. The strength of an acid depends on the

concentration of H+ ions formed in solution. Strong acids produce a highconcentration of H+ ions whereas weak acids produce a low H+ concentration insolution.

Strong acids ionise completely when in solution, e.g. sulphuric acid.

H2SO4(l) + water --> 2 H+(aq) + SO4

2-(aq)

Weak acids do not ionise completely when in solution, e.g. ethanoic acid

CH3COOH(l) + water CH3COO-

(aq) + H+

(aq)

Two other weak acids include carbonic acid and sulphurous acid.

H2O(l) + CO2(g) H2CO3(aq)

H2O(l) + SO2(g) H2SO3(aq)


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The next table lists some common weak and strong acids.

Weak and strong acids

Acid Formula Strength Occurrence

Nitric acid HNO3Verystrong

Found in the lab

Sulphuric acid H2SO4 Strong Found in the lab

Hydrochloricacid

HCl Strong Produced by the stomach

Ethanoic acid CH3COOH Weak Found in vinegar

Citric acid C6H8O7 WeakFound in the juice of citrus fruits (e.g.

lemons and oranges)

Tartaric acid C3H6O6 Weak Found in grape juice

Carbonic acid H2CO3 Weak Found in lemonade

Some reactions of acids

Some reactions are common to all acids. These include:

a. Dilute acids react with reactive metals (i.e. metals which are more

electropositive than hydrogen - see Topic 10) to release hydrogen gas and forma salt.

Mg(s) + 2HCl(aq) --> MgCl2(aq) + H2(g)

b. Dilute acids react with metal carbonates and hydrogencarbonates to form asalt, carbon dioxide and water.

PbCO3(s) + 2HNO3(aq) --> Pb(NO3)2(aq) + CO2(g) + H2O(l)

NaHCO3(s) + HCl(aq) --> NaCl(aq) + CO2(g) + H2O(l)

c. Dilute acids react with metal oxides to form a salt and water. The oxidesusually have to be warmed.

CuO(s) + H2SO4(aq) --> CuSO4(aq) + H2O(l)

d. Dilute acids react with alkalis to form a salt and water. This is called aneutralisation reaction, because the acid and alkali neutralise each other out.


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NaOH(aq) + HCl(aq) --> NaCl(aq) + H2O(l)

Alkalis and bases

One should not confuse the terms alkali and base. A base is a substance thatreacts with an acid to produce a salt and water only. For example most metaloxides and hydroxides are bases. Many of these bases, unlike acids, are insolublein water.

Some insoluble and soluble bases

Insoluble bases Soluble bases

Copper (II) oxide (CuO) Sodium hydroxide (NaOH)

Iron (III) oxide (Fe2O3) Potassium hydroxide (KOH)

Copper (II) hydroxide (Cu(OH)2) Calcium hydroxide (Ca(OH)2)

Lead (II) oxide (PbO) Ammonia solution (NH3(aq))

An alkali is a solution of a base in water. Therefore all soluble bases form alkalinesolutions. Alkalis contain hydroxide ions OH-(aq) in solution and will neutraliseacids. Alkalis have a soapy feel. This is because they react with natural oils onthe skin to form a soap. It is this soap that gives them their slippery feel.

Just as there are weak and strong acids, there are weak and strong alkalis. The

strength of an alkali depends on the amount of OH- ions in solution. The more analkali ionises the stronger it is.

KOH(s) + water --> K+(aq) + OH-(aq)

If an alkali does not ionise completely, a weak alkali is given,

Mg(OH)2(s) + water Mg2+(aq) + 2OH-(aq)

On the pH scale, strong alkalis have a pH value of over 11 whilst weak alkalis

have a pH value between 7 and 11.

NOTE: There are some substances that are neither acidic nor alkaline. They areneutral and have a pH of 7. These do not have any effect on indicators. E.g.common salt, sodium nitrate and cane sugar give neutral solutions.


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Salts

Normal salts and acid salts

A salt is a substance formed when either all or part of the ionizable hydrogen ofan acid is replaced by a metallic or ammonium ion.

If all the ionizable hydrogen is removed then a normal salt is formed, e.g.Na2SO4 from H2SO4, and NaCl from HCl.

If only some of the ionizable hydrogen is removed, then an acid salt is formed,e.g. NaHSO4 from H2SO4, and NaHCO3 from H2CO3.

Normal and acid salts

Acid salts Normal salts

Potassium hydrogensulphate KHSO4 Potassium sulphate K2SO4

Sodium hydrogencarbonate NaHCO3 Sodium carbonate Na2CO3

Calcium hydrogencarbonate Ca(HCO3)2 Calcium carbonate CaCO3

Sodium hydrogensulphite NaHSO3 Sodium sulphite Na2SO3

Note:

monobasic acids cannot form acid salts since they have only one ionizablehydrogen;

CH3COONa, sodium ethanoate, is a normal salt because the remaining hydrogensin the CH3 group are not ionizable.

Preparation of salts

Soluble salts are prepared in solution. The solution is then evaporated and thesalt crystals form. Insoluble salts on the otherhand are prepared by precipitation.

Some soluble and insoluble salts

Soluble Insoluble


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All sodium, potassiumand ammoniumsalts

All nitrates

All chlorides exceptLead (II) and silver chlorides (lead (II)

chloride is soluble in hot water)

All sulphates exceptLead (II) and barium sulphates (calcium

sulphate is only slightly soluble)

Carbonates of lithium, sodium,potassium and ammonium

All other carbonates

Oxides and hydroxides of lithium,sodium, potassium, calcium and

ammonium

All other oxides and hydroxides (calciumand magnesium hydroxides are only

slightly soluble)

Sulphides of sodium, potassium andammonium

All other sulphides

Salts can be prepared by:

1. Synthesis (or direct combination of elements) e.g.

Zn(s) + S(s) --> ZnS(s)

2 Fe(s) + 3Cl2(g) --> 2FeCl3(s)

2. The action of an acid on (i) a metal, (ii) an insoluble metal oxide, hydroxide,or carbonate, (iii) an alkali or soluble carbonate.

(i) Mg(s) + H2SO4(aq) --> MgSO4(aq) + H2(g)

(ii) CuO(s) + H2SO4(aq) --> CuSO4(aq) + H2O(g)

Mg(OH)2(s) + H2SO4(aq) --> MgSO4(aq) + 2 H2O(l)

PbCO3(s) + 2 HNO3(aq) --> Pb(NO3)2(aq) + CO2(g)

(iii) 2 NaOH(aq) + H2SO4(aq) Na2SO4(aq) + 2 H2O(l)

Na2CO3(aq) + H2SO4(aq) Na2SO4(aq) + H2O(l) + CO2(g)


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3. Precipitation

PbNO3(aq) + H2SO4(aq) --> PbSO4(s) + HNO3(aq)

Lets take a closer look at two of these methods - neutralisation and

precipitation.

Neutralisation

Acids can be neutralised by any of the following methods to form salts:

a. Reaction of a dilute acid with an alkali

b. Reaction of a dilute acid with a metal

c. Reaction of a dilute acid with a base

d. Reaction of a dilute acid with a carbonate

a. Reaction of a dilute acid with an alkali

This method is ideal for the preparation of the normal salt of reactive metals (itwould be too dangerous to add the metal directly to the acid).

Since both the reactants are in solution, a special technique is required. This iscalled a titration.


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1. An indicator is added to a conical flask containing a known volume of thealkali. When the indicator changes colour, neutralisation would have occurredand the experiment is stopped.

2. Acid is dropped from a burette onto the conical flask until the indicator

changes colour. The volume of acid that neutralises the known amount of alkaliis noted.

3. The same volume of acid and alkali are the mixed without the indicator, andthen evaporated to obtain the salt by crystallisation.

HCl(aq) + NaOH(aq) --> NaCl(aq) + H2O(l)

b. Reaction of a dilute acid with a metal - e.g. dilute sulphuric acid andmagnesium

This method is suitable for less reactive metals (commonly used for magnesium,zinc and iron).

1. Zinc is added to dilute sulphuric acid until an excess of zinc remains.

2. Excess zinc is removed by filtration and the salt, in this case zinc sulphate, isevaporated slowly.

3. In this way a hot concentrated solution is produced. This is tested by dippinga cold glass rod in it. If crystals form on its end, then the solution is ready tocrystallise and it is left to cool.

4. The crystals produced are filtered and dried in air.

H2SO4(aq) + Mg(s) --> MgSO4(aq) + H2(g)


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c. Reaction of a dilute acid with a base - e.g. dilute sulphuric acid andcopper (II) oxide

This is usually suitable to produce normal salts of unreactive metals such ascopper and lead.

1. The procedure used is the same as that in (b) although the reactants mayneed to be warmed.

CuO(s) + H2SO4(aq) --> CuSO4(aq) + H2O(l)

d. Reaction of a dilute acid with a carbonate - e.g. dilute nitric acid and

lead (II) carbonate

This can be used to make the normal salts of many metals.

1. The procedure used is the same as that in (b).

Precipitation

This method is suitable for producing insoluble salts. Solutions of two solublesalts are mixed and an insoluble salt is formed.

For example to prepare lead (II) sulphate, lead (II) nitrate and sodium sulphatemay be used.

Pb(NO3)2(aq) + Na2SO4(aq) --> PbSO4(s) + 2 NaNO3(aq)

The precipitate of PbSO4 is filtered off, washed with distilled water and dried.

This method is also called double decomposition. It can be summarised asfollows:

soluble salt (XA) + soluble salt (YB) insoluble salt (XB) + soluble salt (YA)

Other examples,

Pb(NO3)2(aq) + 2 HCl(aq) --> PbCl2(s) + 2 HNO3(aq)

AgNO3(aq) + HCl(aq) --> AgCl(s) + HNO3(aq)


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BaCl2(aq) + H2SO4(aq) --> BaSO4(s) + 2 HCl(aq)

Preparation of an acid salt

When preparing an acid salt, twice the volume of acid required to produce thenormal salt is used. For example,

NaOH(aq) + H2SO4(aq) --> NaHSO4(aq) + H2O(l)

Water of crystallisation and associated terminology

Most salts produce crystals that have water incorporated in them. This water iscalled water of crystallisation. Salts that possess this water of crystallisation are

called hydrates or as said to be hydrated. Salts which lose this water ofcrystallisation are called anhydrous.

Some hydrates

Name Formula

Sodium carbonate crystals NaCO3.10H2O

Sodium sulphate crystals Na2SO4.10H2O

Copper (II) sulphate crystals CuSO4.5H2OIron (II) sulphate crystals FeSO4.7H2O

Water of crystallisation can be lost by heating the hydrated salt. For examplewhen blue crystals of copper (II) sulphate pentahydrate (CuSO4.5H2O) areheated, they leave a white powder of anhydrous copper (II) sulphate.

CuSO4.5H2O(s) CuSO4(s) + 5H2O(l)

If water is added to the anhydrous copper (II) sulphate, the blue hydrated

compound forms again. A lot of heat energy is given out in the process (i.e. thereaction is very exothermic3). This reaction is used to test for the presence ofwater.

Some hydrated salts lose some or all of their water of crystallisationspontaneously when left in air, e.g. sodium carbonate decahydrate
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(Na2CO3.10H2O) or washing soda. Such salts are said to effloresce (The processis called efflorescence).

Other salts absorb water vapour from the air to form solutions. This process iscalled deliquescence and the substances which behave so are said to

deliquesce, e.g. sodium hydroxide, anhydrous calcium chloride.

Other substances if left in the air, absorb water vapour but do not form solutionsor change their state. For example, concentrated sulphuric acid absorbs moisturefrom the air and becomes diluted whilst still remaining a liquid. These substancesare called hygroscopic, e.g. calcium oxide, concentrated sulphuric acid.

When are acids not acids?

A solution of hydrogen chloride (HCl) in water is acidic. But besides water,

hydrogen chloride is also soluble in other solvents such as methylbenzene(C6H5.CH3) (also known as toluene).

Hydrogen chloride does not behave the same in both solvents. Whereas inaqueous solution hydrogen chloride exhibits acidic properties, in methylbenzeneit does not (or very little). Why is this so ?

The answer to this behaviour of hydrogen chloride is obtained when one looks atthe solvent.

Water is a proton acceptor such that in water, hydrogen chloride is ionised, i.e. it

forms ions. On the otherhand, methylbenzene is NOT a proton acceptor and in ithydrogen chloride is not ionised and remains mostly in the molecular state. Inthis state it does not exhibit any acidic properties since acidity depends on H+ions.

For the same reasons hydrogen chloride in water is a conductor of electricitywhereas in methylbenzene it is not.

Also, if ammonia gas is passed in a solution of hydrogen chloride inmethylbenzene, a white precipitate of ammonium chloride is given.

Everyday acids and alkalis

Vinegar

Vinegar is also known as acetic acid or more scientifically ethanoic acid. It isformed by the oxidation of ethanol, which is an alcohol. This also occurs as anatural process by bacterial action when they oxidise alcohol to form vinegar.


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This is why some wines often have a sour taste. Ethanoic acid is a typical organicacid. The characteristic group or functional group of organic acids is the COOHgroup. In fact the formula of ethanoic acid is CH3COOH. Vinegar is a weak acidbecause it does not ionise fully.

CH3COOH(l) + H2O(l) CH3COO-(aq) + H3O+(aq)

Bleach solution

Bleaching powder is manufactured by passing chlorine over moist calciumhydroxide. It has a complex structure which can be simplified to CaOCl2.H2O.Bleach solution is alkaline. It is also an oxidising agent and adds oxygen to

whiten objects.

Quicklime and slaked lime

Quicklime or calcium oxide is a white solid. It does not melt easily, even at veryhigh temperatures. It merely incandesces and gives out a powerful light. Once itwas used for this purpose (lime-light). It is a base and in water it forms analkaline solution called slaked lime, or calcium hydroxide.

Slaked lime has several uses such as:

to recover ammonia from ammonium chloride in the Solvay process;to treat acid soils;to make mortar which is used in building;

to soften temporary hard water;to make bleaching powder;to make milk of lime which is used as whitewash.

06 acids bases salts

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