National Chemistry

The chemical reactions described below are:

Combustion    addition      cracking        addition polymerisation

redox      neutralistion        precipitation hydrolysis       condensation      

+

 

Specific reactions

Fermentation,  photosyntheisis, respiration, Haber, Oswald

REDOX

 Oxidation is loss of electrons;  Reduction is gain of electrons  =      OIL RIG

 

 

When non- metals react they gain electrons which makes them negatively charged - this is reduction

                Cl2     +       2e  ----->    2Cl-

 

These half-equations are found on page 7 of the data book - the top of the page has written "Reduction reaction" - therefore they can be copied out

 

 

When metals react they lose electrons which makes them positively charged    - this is oxidation

               Cu(s)------ >    Cu2+ (aq) + 2e  

 

These half-equations are also found on page 7 of the data book - since the top of the page says "reduction reactions"  to get the oxidation reaction you need to write it the other way round.

 

Reduction is written the Right way; Oxidation is written the Opposite way

 

 

 

 

 

 

 

 

 

 

 

 

 

Oxidation and reduction must occur together - something loses electrons so something gains electrons - this is called REDOX

 

 

 

 

 

 

 

 

 

 

 

Oxidation and reduction occur when

(a) ionic solutions are broken doen by electrolysis

  positive ion goes to negative electrode where it gains electrons (look for ut in data book)  AND

   negative ion goes to positive electrode where it loses electrons (look for ut in data book)  

 

(b)  displacement as another substance is pushed out

     eg. Ca(s)      +      Zn2+(aq)  ----->       Zn(s)      +      Ca2+(aq)

                       ZINC IS DISPLACED BY CALCIUM

 

             Ca(s)------ >    Ca2+ (aq) + 2e       

                                             (equation in opposite direction from page 7 of data book)

 

        and  another substance is REDUCED    (gains electrons)

 

            Zn2+(aq)    + 2e------ >   Zn(s)          (equation in right direction as in data book)

 

The metal higher up diplaces the metal lower down the activity series on page 7 - this is because the more reactive metal reacts faster.

 

(c) Corrosion

  Where a metal loses electrons to attack by the environment - ie oxidation

 

            Sn      ----->  Sn2+     +    2e

 

Usually investigated as rusting of iron (or steel)

 

             Fe         ----->  Fe2+     +    2e          followed by

 

              Fe2+    ----->  Fe3+     +    e             (both oxidation)

 

The reduction part is

 

               2H2O      +   O2    +    4e    ----->    4OH-

 

 

 

Watch out for

(i) electrons in the equations

(ii) being asked to write one -just copy it from data book (opposite way for oxidation)

(iii) ferroxyl turning blue to show Fe2+ ions formed

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                   

       

 

 

 

 1. Combustion

Also known as burning

Come across as fuels burning in oxygen (actually burning in air but the nitrogen does not react)

Eg   carbon burns in oxygen to produce carbon dioxide  

      C   +   O2   -->  CO2

Hydrogen burns in oxygen to give water

     2H2    +    O2    ---->    2H2O

Hydrocarbons, such as  propane burn to produce carbon dioxide and water

C3H8   +  5 O2    --->    3CO2    +   4H2O

 

If a substance burns to produce CO2 and H2O then the substance must have contained C and H – the O coming from the air.

 

Watch out for:

(i) Incomplete combustion producing poisonous carbon monoxide

Commonly this occurs in a car engine where there is a shortage of oxygen.

(ii). When burned in an enclosed space only 20% of the air is used up ie the oxygen is used up leaving 80% behind. This will be all nitrogen.

Specific reactions

PHOTOSYNTHESIS

Occurs in green (with chlorophyll) plants in the light.

 

                   carbon dioxide  +  water  -----light----&------>  glucose        +     oxygen

                        6CO2          +  6H2O  ---chlorophyl------>   C6H12O6   +     6O2

 

RESPIRATION

Occurs in animals when glucose releases with oxygen to release energy energy

Respiration also occurs in plants at night (ie dark)

 

                      glucose        +     oxygen---------->        carbon dioxide  +  water

                     C6H12O6      +     6O2     ---------->             6CO2          +    6H2O

 

Respiration and photosynthesis are opposites

 

Watch out for

(i) Reaction which uses up CO2

(ii) reaction which produces O2

 

FERMENTATION

The fermentation of glucose - using the biological catalyst (enzyme) yeast produces the  alcohol (ethanol) and carbon dioxide.

 

        glucose    ---enzyme in yeast-------->   ethanol       +   carbon dioxide

 

        C6H12O6                                              2C2H5OH  +    2CO2

 

watch out for

(i) CO2 being formed (gas making lime water milky)

 

NITROGEN CHEMISTRY

                                       400 atmospheres

            N2       +    3H2      -------------------- >      NH3          Haber produced ammonia

                                               400oC

 

            NH3  +     O2                  ------ >         NO2          Oswald produces nitrogen dioxide

 

Watch out for

(i) test for ammonia - turns pH paper blue (alkali)

(ii) uses N2 or H2

(iii) uses O2

 

CONDENSATION

Two momosaccharides reract together to make one disaccharide and water - a CONDENSATION reaction

 

                 C6H12O6    +   C6H12O6      ----->    C12H22O11

 

About a hundred of glucose molecules react together to form starch and water. The formation of water by an organic reaction is termed CONDENSATION. Because starch is a polymer this is called CONDENSATION POLYMERISATION

 

                 n x glucose            --------------->    starch                    + n x water

                n x  C6H12O6        ---------------->   (C6H10O5)n          +   n H2O

 

(n=big number; approximately a 100 for starch)

 

watch out for

(i) mention of starch

 

 

HYDROLYSIS

During digestion starch molecules are broken down in the body by enzymes - biological catalysts - into small glucose molecules which can pass through the gut wall.

This is called hydrolysis.

 

                        starch           +  water            ----enzyme----->    glucose

               (C6H10O5)n          +    H2O            ----enzyme----->  n x  C6H12O6  

 

Hydrolysis by enzymes is at body temperature (37oC)

 

Disaccharides can also be broken down by hydrolysis

 

                   disaccharide                    monosaccharide        monosaccharide

                       maltose                           glucose                        fructose

 

                   C12H22O11        ----->    C6H12O6                 +   C6H12O6  

 

Hydrolysis can also be catalysed by acid at higher temperature (usually surrounded by boiling water (100oC)

 

Watch out for

(i) mention of digestion

(ii) water as a reactant

(iii) mention of starch

 

Beware

Hydrolysis and Condensation seem to be opposites of one another.

HYDROlysis nedds water (hydrogen oxide)

Condensation makes water - think steam in your kitchen

PRECIPITATION

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

 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)

 

Barium sulphate is a favourite insoluble compound in questions

 

remember to use page 5 of data book

NEUTRALISATION

ACID +

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

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

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

 

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

               2H+           +             CO3 2-      ---->                                      H2O  + CO2

 

watch out for

(i) all neutralisation reactions MUST have an acid (or H+(aq)) before the arrow as a reactant

(ii) if question asks about making acid rain (or H+(aq)) then acid must come after the arrow

 

In neutralisation pH increases - moves to 7.

CRACKING

Catalytic cracking is used to change long chain hydrocarbons into short chain hydrocarbons which are more useful.

Cracking must produce unsaturated hydrocarbons as there are not sufficient  hydrogens to saturate all of the molecules

                                hexane        ---->       butane                               +     ethene

                                 C6H14                       C4H10                                     C2H4

                           saturated                        saturated                                 unsaturated...

ADDITION

Alkenes (double 'e'; double bond, double the number of hydrogens) undergo addition reactions. these reactions occur at the C=C carbon to carbon double bond.

The reacting diatomic molecule simply adds onto the double bond

 

eg          C2H4        +   H2   ----------->   C2H6

 

               C2H4        +   Br2   ----------->   C2H4Br2

                                orange                     colourless

 

 

When orange bromine reacts it becomes colourless. This is used as a test for a C=C.

 

Every C=C needs two bromine atoms.

(i) C=C is good clue

(ii) adding  chemicals other than bromine (eg  HCl,  H2O)

(iii) ene

(iv) molecules with two C=C double bonds - need 2Br per double bond

 

ADDITION POLYMERISATION

 

                 monomer with C=C     ---------->  polymer

 

Watch out for

(i) poly

Page 7 shows that   metal ions can gain electrons   -  reduction

 

                   Zn2+(aq)    + 2e------ >   Zn(s)

 

and

 

        non-metal ions can lose electrons - oxidation

 

                             2Cl-      --------->      Cl2    +  2e

Redox as one substance is OXIDISED (loses electrons)

 

         Ca(s)------ >    Ca2+ (aq) + 2e       

                                             (equation in opposite direction from page 7 of data book)

 

        and  another substance is REDUCED    (gains electrons)

 

            Zn2+(aq)    + 2e------ >   Zn(s)          (equation in right direction as in data book)

Reactions to produce gases

1.   HYDROGEN

       reactive metal plus water

       mid reactive metal plus acid

 

2. CARBON MONOXIDE

     heating a carbonate

     carbonate reactibg with acid

 

3. AMMONIA

    ammonium compound with soda lime or sodium hydroxide

    nitrogen plus hydrogen in presence of iron catalyst at 400 atmospeheres pressure and 400oC - Haber process