National Chemistry

 

 8 & 9 Acids and Reactions of acids

 

8.1  The pH scale runs continuously from 1 (strong acid) through 7 (neutral) to 14 (strong alkali).

     - if a substance dissolves in water (pH = 7) then it may change the pH of the water making an acid or an alkali.

 

8.2  substance dissolving                 solution               pH            indicator                       ions

          non-metal oxide                       acid                 below 7           red            LOTS -H+(aq) in excess

          water                                    neutral                   7                 green         FEW- H+ (aq) = OH-(aq)

          metal oxide                           alkali                  above 7          purple    LOTS - OH- (aq) in excess

 

     (remember group I = alkali metals)

     neutral solutions may have lots of ions but H+(aq) = OH-(aq)

 

8.3  Sulphur dioxide and oxides of nitrogen (pollution caused by burning fossil fuels) react with water in the atmosphere to produce acid rain.

Acid rain can damage buildings, stones, metals (speeds up corrosion/rusting) by chemical reactions - and can damage plant and animal life by lowering pH - (make it more acidic).

 

8.4  Acids are usually called 'ACIDS'  sulphuric acid H2SO4; hydrochloric acid HCl ; nitric acid HNO3  vinegar

         Alkalis are usually called HYDROXIDES =   sodium hydroxide NaOH;  potassium hydroxide KOH

              calcium hydroxide Ca(OH)2- lime water, soaps, toothpaste, cleaners

 

8.5  Diluting an acid or alkali with water reduces the acidity or the alkalinity (makes less acid or alkaline) and shifts the pH closer to 7 - decreases concentration of H+(aq)    or   OH-(aq).

 

8.6  All acids have excess H+(aq) and when an electric current is passed through an solution the H+(aq) are attracted to the -ve electrode where they gain electrons  (reduction) to produce hydrogen gas.

     Hydrogen gas burns with a pop (explodes) when a lighted splint is added to it.

 

8.7 Calculations involving

 

                                  mass in question  =       mole  

                                                                  mass of 1 mole

 

concentration is written as "mol/litre" which is a division

 

                               concentration =     mole

                                                             litre

 

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9.1  When acids react the pH will increase, moving towards pH = 7 which is neutral

                   = NEUTRALISATION.

      When alkalis react the pH will decrease, moving towards pH = 7  which is neutral              

                    =NEUTRALISATION.

 

      ACID + ALKALI   is a NEUTRALISATION reaction

      ACID + METAL OXlDE  is a NEUTRALISATION reaction

      ACID + CARBONATE  is a NEUTRALISATION reaction

     H+(aq) forming water  is a NEUTRALISATION reaction

 

            a base neutralises an acid -  a base that dissolve is called an alkali

 

      - other examples

            Reducing acidity in soil or lakes by adding lime;

            treatment of acid indigestion.

 

9.2  Acids react to form salts. hydrochloric acid (HCl) produces chlorides (Cl-)

                                               sulphuric acid (H2SO4 produces sulphates (SO4)2-

                                               nitric acid (HNO3) produces nitrates (NO3-)

                - the positive metal ion comes from the substance which neutralises the acid  replacing

the hydrogen ion in the acid

 

9.3  Acid reacts with

                (i) ALKALI (metal hydroxide) ----->   SALT + WATER

               (ii) METAL OXIDE-                ------>  SALT + WATER

               (iii) CARBONATE                  -------> SALT + WATER + CARBON DIOXIDE

 

              (iv) METALS (not copper, silver, gold)------->SALT + HYDROGEN

 

     Acid rain attacks buildings and rocks which are carbonate containing stone and structures made of iron.

 

examples  

(i)    ACID   +  ALKALI (metal hydroxide) ----->   SALT + WATER

hydrochloric acid   +   sodium hydroxide  ---->  sodium chloride   +  water

              HCl          +         NaOH              ---->       NaCl               +     H2O

remove spectator ions

               H+           +             OH-            ---->                         H2O

 

(ii)              ACID   +         METAL OXIDE  ----->   SALT + WATER

           nitric acid   +            lithium oxide   ---->  lithium nitrate    +  water

              HCl         +              Li2O             ---->       LiNO3             +     H2O

 

(iii)           ACID     +        CARBONATE    ----->   SALT       + WATER   +  CARBON DIOXIDE

    sulphuric acid   +  calcium carbonate    ---->  calcium sulphate   +  water + carbon dioxide

              H2SO4          +         CaCO3       ---->              CaSO4      +   H2O  +      CO2

remove spectator ions

            An acid reacts with a carbonate by (hydrogen ion plus carbonate ion)

 

                         2H+(aq)    +    CO32-(aq) ----->          CO2(g)    +    H2O(l)

 

 

 

      When an acid reacts with a metal      2H+(aq)+   2e------->     H2(g)     (reduction)

and the metal is  oxididsed

 

 

 

 

9.4  Two soluble solutions can react to produce a solid precipitate (insoluble salt) -called                              

                       PRECIPITATION.   example 2

 

          silver nitrate               +             sodium chloride    ----->   silver chloride     +       sodium nitrate

            soluble                                         soluble                           insoluble                      soluble

       Ag+(aq) +  NO3-(aq)     +               Na+(aq) + Cl-(aq)----->   Ag Cl(s) +      Na+(aq)   +   NO3-(aq)

 

spectator ions  NO3-(aq) and Na+(aq) do not take part in the reaction so are called spectator ions    

 

9.5  It is easier to prepare a salt by neutralising an acid with an insoluble base

- once   the acid is neutralised (no reaaction occurring  -no gas produced or no more solid  reacting because no more H+ left)  excess solid can be filtered away. Water then evaporated.

 

With two solutions reacting it is difficult to find the neutralisation point and  therefore easy to add too much.

National 4/5

Acids and Bases

 

These notes are for the Scottish National Chemistry course taught in fourth year in most Scottish schools.

 

The notes available here are concise notes. They are NOT to be considered as material to learn from - they are for revision.

 

 The notes are arranged under the following topics

 

National 3          Unit 1

                          Unit 2

                          Unit 3

 

National 4 and 5  Unit 1            Rates of Reaction

                                                  Atomic Structure

                                                  Bonding and Properties

                                                  Acids and bases

 

National 4 and 5  Unit 2       Fuels and Homologous Series

                                                Consumer Products

                                                Carbohydrates

 

National 4 and 5  Unit 3         Metals

                                                   Plastics

                                                   Fertilisers

                                                   Nuclear Chemistry

                                                  Chemical analysis

 

 

 

 

 

 

 

 

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