Acids, Bases and Salts

Acids

An acid is                                                                                                                                                                                                                                                                 

                                                                                                                                                                                                                                                                                               

                                                                                                                                                                                                                                                                                               

                                                                                                                                                                                                                                                                                               

                                                                                                                                                                                                                                                                                               

When we study acids, we are actually looking at the chemistry of the aqueous hydrogen ion, H+(aq).

 

Examples of acids

We come across many acids in everyday life. A few are described below:

      Mineral acids:

hydrochloric acid      HCl(aq)                 (strong)  present in bile (stomach)

sulphuric acid                                                            (                    )                                                                        

                                        HNO3(aq)                             (                    )                                                                        

phosphoric acid                                                        (                    )                                                                        

 

      Organic acids:

citric acid                     C6H8O7(aq)    (weak)                                   present in fruit (e.g. lemon juice)

ascorbic  acid                                                              (                    )                                                                        

lactic acid                                                                    (                    )                                                                        

ethanoic acid                                                              (                    )                                                                        

methanoic acid                                                          (                    )                                                                        

 

     Mineral acids generally react faster than organic acids. Mineral acids are examples of                     acids. Organic acids are examples of                                  acids. There exceptions in each case.



Properties of acids, indicators and the pH scale

The word acid means ‘sour’ and all acids possess this property. They are also:

                             soluble in water

                             corrosive (they react with metals).

Alkalis are very different from acids:

                             they will remove the sharp taste from an acid

                             they have a soapy feel.

It would be too dangerous to taste a liquid to find out if it was acidic. Chemists use substances called indicators which change colour when they are added to acids or alkalis. Many indicators are dyes which have been extracted from natural sources. For example, litmus is a purple dye which has been extracted from lichens.

 

Indicator

Colour in acid

Colour in alkali

phenolphthalein

colourless

pink

methyl orange

pink

yellow

red litmus

red

blue

blue litmus

red

blue

methyl red

red

yellow

Indicators show if a substance is either acidic or alkaline but to obtain an idea of just how acidic or alkaline a substance is, chemists use another indicator known as universal indicator (UI). Universal indicator is a mixture of many other indicators. The colour shown by this indicator can be matched against a pH scale:

 

 

1

2

3

4

5

6

7

8

9

10

11

12

13

14

The pH scale was developed by a Scandinavian chemist called Sörenson who was employed by the Carlsberg brewery. The pH scale runs from below 0 to 14. A substance with a pH of less than 7 is acidic. One with a pH of greater than 7 is alkaline. One with a pH of 7 is said to be neither acidic or alkaline. It is neutral.

Another way in which the pH of a substance can be measured is by using a pH meter. The pH electrode is placed into the solution whose pH you want to measure and the pH is given on the digital display.



Reactions of acids

Acids react with metals that are higher than hydrogen in the reactivity series, metal carbonates, metal hydrogencarbonates, alkalis and bases.

 

      With reactive metals

Acids react with metals to form                                                                                                                                                                                                     

Example chemical equation:

                                                                                                                                                                                                                                                                                               

 

      With metal carbonates

Acids react with metal carbonates to form                                                                                                               

Example word equation:

                                                                                                                                                                                                                                                                                               

Example chemical equation:

                                                                                                                                                                                                                                                                                               

 

      With metal hydrogencarbonates

Acids react with metal hydrogencarbonates to form                                                                                                                                           

Example word equation:

                                                                                                                                                                                                                                                                                               

Example chemical equation:

                                                                                                                                                                                                                                                                                               

 

      With alkalis

Acids                                                    alkalis to form                                                                                                                                                       

Example word equation:

                                                                                                                                                                                                                                                                                               

Example chemical equation:

                                                                                                                                                                                                                                                                                               



      With bases

Acids react with bases to form                                                                                                                                     

Example word equation:

                                                                                                                                                                                                                                                                                               

Example chemical equation:

                                                                                                                                                                                                                                                                                               

More about acids

     An acid is a substance that releases hydrogen ions when it is dissolved in water. For example:

H2SO4(l)           2H+(aq) +             SO42-(aq)

sulphuric acid                                            hydrogen ions                    sulphate ions

 

These aqueous hydrogen ions are responsible for all the reactions that are typical of acids. For example, neutralising an alkali:

                                NaOH(aq)  +  HCl(aq)                                                       +                                                              

Ions present:                                                                                                                                                              

Ignoring spectator ions, which do not play any part in the reaction, we can sum up the chemistry in an ionic equation:

Ionic equation:                                                                                                                                                                                                                                      

     A solution of a strong acid forms a much higher concentration of hydrogen ions than a solution of a weak acid. Mineral acids are generally strong acids: they react faster than weak organic acids.

     Some covalently-bonded gases, such as hydrogen chloride (HCl) and non-metal oxides (e.g. SO3, P4O10, NO2)  dissolve to give acidic solutions.

For example, with hydrogen chloride:

Word equation:         

                                                                                                                                                                                       

Chemical equation:

                                                                                                                                                                                       

For example, with sulphur trioxide:

Word equation:

                                                                                                                                                                                       

Chemical equation:

                                                                                                                                                                                       



Bases

A base is                                                                                                                                                                                                                                                                    

                                                                                                                                                                                                                                                                                               

                                                                                                                                                                                                                                                                                               

                                                                                                                                                                                                                                                                                               

                                                                                                                                                                                                                                                                                               

Examples of bases

There are many  bases that we come across in everyday life.

calcium hydroxide                            Ca(OH)2                (strong) treatment for soil that is too

(slaked lime, limewater)                                                                  acidic

 

calcium oxide                                                                                                                                                                    

(                               )

magnesium hydroxide                                                                                                                                                   

(                               )

sodium hydroxide                                                                                                                                                           

(                               )

ammonia                                                                                                                                                                             

Definition of an alkali:

An alkali is a                                                                                                                                                                                                                                                        

                                                                                                                                                                                                                                                                                               

There are relatively few alkalis. The ones you are likely to encounter are:

                NaOH(aq)                            sodium hydroxide solution

                LiOH(aq)                                                                                             

                KOH(aq)                                                                                              

                NH3(aq)                                                                                               

                Ca(OH)2(aq)                                                                                       



Properties and reactions of bases

     Alkalis (soluble bases) change the colour of indicators

        e.g.                                                                                                                                                                                 

     Bases have a pH of                                                                                                                                                                                                                            

     A solution of an alkali in water contains                                                                                                                                                              

     Bases                                                                                              acids

Example word equation:

                                                                                                                                                                                                                                                                                   

Example chemical equation:

                                                                                                                                                                                                                                                                                   

 

When an alkali neutralises an acid, the same chemistry happens regardless of the identity of the acid and base. For example, the reaction between sodium hydroxide and hydrochloric acid can be described by the equation:

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

However, if we write the equation in terms of the ions present in solutions of reactants and products, we see a different picture:

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

Water is a covalently bonded molecule and does not normally form ions. Eliminating ions that occur on both sides of the equation, spectator ions, we see what really happens during a neutralisation reaction:

OH-(aq)  +  H+(aq)      H2O(l)

Thus, an aqueous hydroxide ion and an aqueous hydrogen ion react together to form water.

 



Salts

A salt is                                                                                                                                                                                                                                                                        

                                                                                                                                                                                                                                                                                               

                                                                                                                                                                                                                                                                                               

 

Examples of salts

NaCl

 

 

 

CuSO4

 

 

 

Mg(NO3)2

 

 

Common uses of salts

sodium chloride                                NaCl                                      table salt   (fish and chips!!)

magnesium sulphate                                                                                                                                                      

calcium sulphate                                                                                                                                                              

sodium nitrate                                                                                                                                                                   

silver bromide                                                                                                                                                                   

ammonium nitrate                                                                                                                                                                         

 

Definition of an acid salt:

A acid salt is                                                                                                                                                                      

                                                                                                                                                                                               

Example of an acid salt

NaHSO4

If an acid salt is added to water it will yield a hydrogen ion (as well as sodium and sulphate ions). It is therefore acidic but it is also a salt: it is an acid salt.



Rules for the solubility of salts

Many salts can be prepared in the laboratory using very simple methods. However, it is first necessary to work out the solubility of the desired salt as this governs the method used to prepare it.

The following salts are soluble in cold water:

     all nitrates: e.g. potassium nitrate, KNO3; copper nitrate, Cu(NO3)2; iron(III) nitrate, Fe(NO3)3; etc.

     all common sodium, potassium and ammonium salts: e.g. sodium carbonate, Na2CO3; potassium bromide, KBr; ammonium iodide, NH4I; etc.

     all chlorides except lead chloride, PbCl2; and silver chloride, AgCl.

     all sulphates except lead sulphate, PbSO4; barium sulphate, BaSO4; and calcium sulphate, CaSO4.

Salts containing other ions tend to be insoluble, although there are plenty of exceptions. It should be realised that even ‘insoluble’ salts are partially soluble, although the amount of a solid insoluble salt that actually dissolves in water may be extremely small.

 

Preparation of soluble salts

If the desired salt is soluble in cold water, four methods are available:

     acid + metal: this method can only be used with the less reactive metals. It would be very dangerous to use a metal such as sodium in this type of reaction. The metals used in this method of salt preparation are the MAZIT metals, that is, magnesium, aluminium, zinc, iron and tin.

     acid + carbonate: this method can be used with any metal carbonate and any acid, providing the salt produced is soluble. However, titration is generally a better method for preparing sodium, potassium and ammonium salts.

     acid + insoluble base: this method an be used to prepare a salt of an unreactive metal, such as lead or copper. It is not possible to use a direct reaction of the metal with an acid so the acid is neutralised using the particular metal oxide.

     acid + alkali (titration): this method is generally used for preparing the salts of very reactive metals such as potassium and sodium. It would certainly be too dangerous to add the metal directly to the acid. In this case, we solve the problem indirectly and use an alkali which contains the particular reactive metal whose salt you wish to prepare. It is also used for preparing ammonium salts.



Use of the ‘acid + metal’ method for the preparation of zinc sulphate

A typical experimental method is given below. Zinc sulphate can be prepared by adding excess granules of zinc to sulphuric acid:

Word equation:             

                                                                                                                                                                                               

Chemical equation:

                                                                                                                                                                                               

Ionic equation:

                                                                                                                                                                                               

Method:

1.     Half-fill an evaporating dish with dilute sulphuric acid and warm the acid. Why is the acid warmed?

2.     Add zinc slowly to the warm acid, whilst stirring, until no more reacts. How can you tell the reaction is finished?

3.     Filter off the excess zinc and keep the filtrate (containing zinc sulphate). Why is zinc used in excess, rather than the acid?

4.     Evaporate the salt solution (i.e. filtrate) to half its original volume by heating the solution.

5.     Allow the saturated solution to cool and form crystals of zinc sulphate. Why is the solution allowed to cool slowly, rather than boiled to dryness?

6.     Collect the zinc sulphate crystals by filtration and dry them between two filter papers. How do you collect the crystals?

Diagram of the apparatus required for the ‘acid + metal’ procedure:



Use of the ‘acid + carbonate’ method for the preparation of lead(II) nitrate

A typical experimental method is given below. Lead(II) nitrate can be prepared by adding an excess of lead(II) carbonate to nitric acid:

Word equation:

                                                                                                                                                                                               

Chemical equation:

                                                                                                                                                                                               

Method:

1.     Half-fill an evaporating dish with dilute nitric acid and warm the acid. Why is the acid warmed?

2.     Add lead(II) carbonate slowly to the warm acid, whilst stirring, until no more reacts. How can you tell the reaction has finished?

3.     Filter off the excess lead(II) carbonate and keep the filtrate (containing lead(II) nitrate). Why was the lead(II) carbonate added in excess and not the acid?

4.     Evaporate the salt solution (i.e. filtrate) to half its original volume by heating the solution.

5.     Allow the saturated solution to cool and form crystals of lead(II) nitrate. Why isn’t the solution boiled to dryness?

6.     Collect the lead(II) nitrate crystals by filtration and dry them between two filter papers.

 

Use of the ‘acid + insoluble base’ method for the preparation of copper(II) sulphate

The experimental procedure here is very similar to the ‘acid + carbonate’ method described above. For example, copper(II) sulphate can be prepared by adding copper(II) oxide to sulphuric acid.

Word equation:

                                                                                                                                                                                               

Chemical equation:

                                                                                                                                                                                               

Method:

1.     Half-fill an evaporating dish with dilute sulphuric acid and warm the acid.

2.     Add copper(II) oxide slowly until no more reacts.

3.     Filter off the excess copper(II) oxide and keep the filtrate (containing copper(II) sulphate).

4.     Evaporate the salt solution to half its original volume by heating the solvent.

5.     Allow the saturated solution to cool and form crystals of copper(II) sulphate.

6.     Collect the copper(II) sulphate crystals by filtration and dry them between two filter papers.



Use of the ‘acid + alkali’ (titration) method for the preparation of potassium chloride

In this neutralisation reaction both reactants are in solution, so a special technique is required called titration. For example, potassium chloride can be prepared from potassium hydroxide solution and hydrochloric acid:

Word equation:

                                                                                                                                                                                               

Chemical equation:

                                                                                                                                                                                               

Ionic equation:

                                                                                                                                                                                               

1.     Add 25cm3 of potassium hydroxide (the alkali) to a conical flask using a pipette and pipette filler. Why is a known amount of alkali used?

2.     Add one or two drops of indicator e.g. phenolphthalein (which is pink in alkali). Why is an indicator necessary?

3.     Titrate with hydrochloric acid (the acid!) using a burette until the indicator changes colour (i.e until phenolphthalein turns colourless). The solution is then neutralised.

4.     Note the volume of acid that was required to neutralise the alkali. Why is it important to know how much acid was added?

5.     Repeat the titration without indicator, adding exactly the same volumes of alkali and acid as noted above.

6.     Evaporate the resulting solution to give crystals of sodium chloride. How are the best crystals obtained in this step?

 

Diagram of the apparatus required for a titration

 



Space for Additional Notes on Acid, Bases and Salts

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A Review of the Acids, Bases and Salts Topic

Each topic in the IlIrd Form chemistry course is divided into a series of 'learning targets'. These are listed below for work on Acids, Bases and Salts. For each 'learning target' you should make an assessment of how you are progressing, in terms of increasing your knowledge and developing a clear understanding of the principles. You should assess your progress on a 1 - 3 scale as follows:

                1=           I feel confident about this aspect of the work and I am encountering few                       problems.

                2=           I am making reasonable progress, but I have encountered a few difficulties and                         feel that I need to go over these particular areas again.

                3=           I am finding this aspect of the topic difficult.

Remember, be fair to yourself - be honest!!



Learning Targets

a)      Know the names of some common acids and the formulae of nitric, sulphuric and

        hydrochloric acids                                                                                                                                                   

b)    Know the general reactions of acids with metal oxides, hydroxides, carbonates,

        hydrogencarbonates and metals                                                                                                                         

c)     Write equations for the general reactions of acids with metals, metal oxides etc                                 

d)      Know the effect of acids on indicators and what this tells you about the pH of acids                     

e)    Understand that water is vital for substances to be able to behave as acids                                         

f)     Understand that ALL acids produce H+(aq) in water and this is the chemical

        definition of an acid                                                                                                                                                

g)    Understand what is meant by the word 'neutralisation'                                                                             

h)    Know that bases are substances that neutralise acids to give salt and water                                       

i)     Know that most metal oxides and hydroxides are bases                                                                             

j)      Know that alkalis are soluble bases eg. sodium hydroxide                                                                        

k)    Understand that ALL alkalis produce the OH-(aq) ions in water                                                             

l)     Understand that the pH scale is a measure of the concentration of H+(aq) and

        OH-(aq) ions in aqueous solution                                                                                                                       

m)   Understand that salts are derived from acids and, in terms of formulae, the 'H’ part

        of the acid is replaced by a metal. Hence sulphates are made from sulphuric acid etc                      

n)    Know the names of some common everyday salts and their uses                                                            

o)    Understand that the method chosen to make a salt in the lab depends on its solubility

        and hence know the solubilities of some common salts eg. all nitrates are soluble                            

p)    Understand that even ‘insoluble’ salts are partially soluble, although the amount

        that dissolves in water may be very small                                                                                                        

q)    Understand how to make a soluble salt from an insoluble base/ carbonate/ metal                          

r)     Understand why the method in q) would not work for a soluble base (alkali)                                     

s)     Know how to prepare a crystalline sample of a soluble salt by titration                                                

Ideally, all of your responses will be ‘1’. However, this is rarely the case first time through! If you have written a ‘2’ anywhere, you may wish to read through your notes again or look at the relevant page in your text book. If you have written ‘3’ as a response to any of the questions, see me for further help. Alternatively, you may wish to attend the Chemistry Surgery which takes place in the Chemistry Department during Pursuits Periods (Mondays and Fridays, 2.00- 2.45pm). A member of the department will be on hand to sort out your difficulties.