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

 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.

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

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)

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.

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

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

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

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