CH2 Acids and Salts

7/30/2019 CH2 Acids and Salts

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Acids

What makes a substance an acid ?

Look at the formulae of the following strong acids:

H2SO4 sulphuric acid HCl hydrochloric acidHNO3 nitric acid H3PO4 phosphoric acid

What they all have in common is hydrogen in their formula and moreimportantly in solutions these substances split up into ions.

Providing hydrogen ions, H+(aq), is what makes all these acids.

Substances such as ammonia (NH3) and methane (CH4) also have hydrogen in

their formula, but they dont produce hydrogen ions in solution so they arent acids.

The pH of a solution is used to indicate how

acidic it is. A pH of less than 7 is acidic and

a solution of more than 7 is alkaline. ExactlypH7 is called neutral.

Indicators such as Universal Indicator

turn different colours to show the pH of

the solution they are added to.


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Bases

Bases are substances which can neutralise acids reacting with them to form water.

Bases may be insoluble in water (e.g. metal oxides such as calcium oxide) or

soluble in water (e.g. some metal hydroxides sodium hydroxide) in which case wecall them alkalis.

An alkali is a soluble base, which makes a solution alkaline by providing hydroxide

ions (OH-) in the solution.

What happens during a neutralisation: When an acidic solution is neutralised byan alkaline solution, it is the hydrogen ions from the acid which are reacting with the

hydroxide ions from the alkali. We can write an ionic equation to show how these

ions react:

H+(aq)

+ OH-(aq)

H2O

(l)

hydrogen ion + hydroxide ion water


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Reactions of acids

Acids react with some metals, metal oxides and metal hydroxides. These

reactions are used to make soluble salts. The salt solutions can then be

crystallised to produce solid salt.

The particular salt produced in any reaction between an acid and a metal, base or

alkali depends on:

- the acid used

hydrochloric acid produces chlorides nitric acid produces nitrates sulphuric acid produces sulphates

the metal in the base or alkali

e.g. copper oxide will react with sulphuric acid to make copper sulphatemagnesium will react with hydrochloric acid to make magnesium chloridesodium hydroxide will react with nitric acid to make sodium nitrate

Ammonia, NH3, dissolves in water to produce an alkaline solution of ammoniumhydroxide, containing ammonium ions, NH

4

+

(aq)

. These react with acids to

produce ammonium salts.

copper sulphate,

made in thereaction of

copper oxide

with sulphuric

acid


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Acids + Metals

Acids react with metals according to the general equation:

ACID + METAL SALT + HYDROGEN

The metal is added to the acid until no more fizzing takes

place (i.e. all the acid has reacted). The excess metal is then

filtered off, and the solid salt can be obtained by crystallising

the salt solution.

Not all metals are suitable, some are too reactive (e.g. the

metals in Group 1 Sodium, Lithium, Potassium etc), and

others are not reactive enough (e.g. copper, silver, gold)

The hydrogen which is produced in these reactions is seen as

bubbles being produced. A lit splint will ignite the hydrogenwith a squeaky pop, showing the gas to be hydrogen.

examples:

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

Zn(s) + H2SO4(aq) ZnSO4(aq) + H2(g)


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Acids + Metal Oxides

Acids react with insoluble bases such as metal oxides according to the general

equation:

ACID + METAL OXIDE SALT + WATER

The insoluble metal oxide is added until no more

reacts (because all the acid has been used up).

As it reacts with the acid it dissolves, forming a

solution of the salt.

When no more metal oxide dissolves, the remaining

unreacted metal oxide is filtered off. Gentle heat is

often needed to encourage the reaction to go at a

reasonable rate.

examples:

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

CaO(s) + 2 HCl(aq) CaCl2(aq) + H2O(l)

MgO(s) + 2 HNO3(aq) Mg(NO3)2(aq) + H2O(l)


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Acids + Alkalis (metal hydroxides):

Acids react with solutions of metal hydroxides according to the general equation:

ACID + METAL HYDROXIDE SALT + WATER

Both acid and metal hydroxide

are likely to be colourlesssolutions, and the salt will bea colourless solution too. This

makes it difficult to see that areaction is taking place, or

when the reaction is finished

because the acid has been

completely neutralised by the

added alkali.

examples:

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

HNO3(aq) + KOH(aq) KNO3(aq) + H2O(l)

2HCl(aq) + Ca(OH)2(aq) CaCl2(aq) + 2H2O(l)

An indicator can be added to the solution, so that its colour change can be used totell when the acid has been completely neutralised. Once the exact amount of acid

needed to neutralise the alkali has been measured, the experiment can be repeated

without the indicator, to make a pure solution of the salt, which can then be

crystallised.


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Ammonium salts:

When ammonia dissolves in water it produces ammonium ions and hydroxideions. This makes an ammonia solution alkaline.

NH3(g) + H2O(l) NH4+

(aq) + OH-(aq) - also written as NH4OH(aq)

The hydroxide ions in ammonia solution can be neutralised by the hydrogen ions in

acids, forming water and an ammonium salt. The solid salt can be obtained from the

solution by crystallisation.

These salts are very important as fertilisers.e.g. the fertiliser Nitram is ammonium nitrate.

examples:

NH4OH(aq) + HNO3(aq) NH4NO3(aq) + H2O(l)

2 NH4OH(aq) + H2SO4(aq) (NH4)2SO4(aq) + 2H2O(l)


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Making insoluble salts:Many salts are not soluble in water. A different method, precipitation, is used toprepare them.

The name of the insoluble salt tells you which metal ion and which other ion will beneeded to form the salt. For example to make insoluble lead iodide, we need lead

ions and iodide ions.

Pb2+(aq) + 2I-(aq) PbI2(s)

When a solution containing lead ions is mixed with a solution containing iodide ions,the two solutions react and a precipitate of the insoluble salt (lead iodide) settlesout. A precipitate is a solid, so it can be filtered off, washed then dried.

The two solutions we mix together need to be of soluble salts.

For metals: the nitrates are a good choice because all nitrates are soluble. In this

case wed choose lead nitrate.

For the other ions: sodium or potassium salts are a good choice because they aresoluble. In this case we could choose potassium iodide.

Pb(NO3)2(aq) + 2 KI(aq) PbI2(s) + 2 KNO3(aq)

lead potassium lead potassiumnitrate iodide iodide nitrate


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Examples:

Making silver bromide, AgBr Ag+(aq) + Br-(aq) AgBr(s)

Wed choose to react silver nitrate a soluble salt containing silver ions, withsodium bromide a soluble salt containing bromide ions. Sodium nitrate remainsin solution after the reaction, and the insoluble silver bromide can be filtered off.

AgNO3(aq) + NaBr(aq) AgBr(s) + NaNO3(aq)

Making barium sulphate, BaSO4 Ba2+

(aq) + SO42-

(aq) BaSO4(s)

Wed choose to react barium nitrate a soluble salt containing barium ions, with

sodium sulphate a soluble salt containing sulphate ions. Sodium nitrate remainsin solution after the reaction, and the insoluble barium sulphate can be filtered off.

Ba(NO3)2(aq) + Na2SO4(aq) BaSO4(s) + 2NaNO3(aq)


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Removing harmful ions from water:

Precipitation reactions can be very useful, since they can turn an unwanted metal

ion which is dissolved into a solid precipitate which can be filtered off.

Heavy metal ions such as barium or lead are harmful in drinking water, and cause

problems for aquatic life if released into rivers in the effluent water from industrial

processes.

A precipitation reaction can be used to remove them.

The hydroxides of heavy metals are insoluble, so by raisingthe pH of the water (adding hydroxide ions) the metal ions

will react with the hydroxide ions forming a precipitate of the

metal hydroxide. Sodium hydroxide solution is commonly

used to raise the pH of water.

e.g. lead ions + hydroxide ions lead hydroxide

Pb2+(aq) + 2OH-(aq) Pb(OH)2(s)

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CH2 Acids and Salts