2.6Chemical Bonding, Application Of Chemical Reactions and Organic Chemistry
Reversible Reactions, Industrial Processes and Important Chemicals — WJEC GCSE Chemistry Revision Notes & Topical Questions
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Complete PDF with figures and original page layoutPage 1 WJEC GCSE Chemistry Your notes 2.6 Reversible Reactions, Industrial Processes & Important Chemicals Contents Reversible Reactions The Haber Process Choosing the Haber Process Conditions Tes…
Section 1 of 15Reversible reactions
- Reaction going to completion
- the reactants are used up to form products and the reaction stops
- Reversible reaction
- products can react or decompose to re-form the reactants, so the reaction occurs in both the forward and reverse directions
- Notation
- the symbol ⇌ is used, with half arrowheads
- Example
- the Haber process, N2 + 3H2 ⇌ 2NH3
- Energy
- if the forward reaction is exothermic, the reverse is endothermic, with the same amount of heat in each direction
Section 2 of 15Hydrated copper(II) sulfate as a reversible reaction
- Equation
- hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water
- Forward (hydration)
- adding water to white anhydrous crystals gives blue crystals, and it is exothermic
- Reverse
- heating the blue crystals gives a white powder, and it is endothermic
Section 3 of 15The Haber process: stages
- Product
- ammonia, in five stages
- Hydrogen from methane (natural gas) and nitrogen from the air are mixed and pumped into the compressor.
- The gases are compressed to about 150–200 atmospheres.
- They pass into a tank with layers of iron catalyst at 350–450 °C, where some react: N2(g) + 3H2(g) ⇌ 2NH3(g).
- Unreacted gases and ammonia pass into a cooling tank, where the ammonia liquefies and is removed to pressurised storage.
- Unreacted hydrogen and nitrogen are recycled to the start.
- Exam knowledge
- the sources of hydrogen and nitrogen, the iron catalyst and the conditions
- (not vanadium oxide or enzymes)
- the sources of hydrogen and nitrogen, the iron catalyst and the conditions
Section 4 of 15The Haber process: choosing conditions (Higher tier)
- Compromise
- conditions balance yield, rate and cost, since the best yield and fastest rate conflict
- Temperature: 450 °C
- The forward reaction is exothermic, so a lower temperature would increase yield.
- At low temperature the rate is too slow, so 450 °C gives a lower yield but faster production.
- Pressure: 200 atm
- High pressure favours the forward reaction because there are 2 molecules of gaseous product against 4 of reactants.
- Very high pressure is dangerous and needs expensive equipment, so 200 atm is a compromise.
- Catalyst
- iron speeds up the reaction
Section 5 of 15Testing for ammonia
- Test
- ammonia is a gas with a pungent smell that turns damp red litmus paper blue
- Method
- hold the paper near the mouth of the tube, not touching the sides,
- so traces of sodium hydroxide do not turn it blue when testing ammonia from ammonium ions
- Difference
- ammonium is the aqueous cation NH4+, and ammonia is the gas NH3
- Smell
- the pungent smell is not a chemical test, but the litmus paper change is
Section 6 of 15The Contact process
- Product
- sulfuric acid
- Sulfur is oxidised to sulfur dioxideS + O2 → SO2.
- Sulfur dioxide is oxidised to sulfur trioxide with a vanadium(V) oxide, V2O5, catalyst: 2SO2 + O2 ⇌ 2SO3, at 450 °C and about atmospheric pressure, with oxygen from air.
- Sulfur trioxide is absorbed in 98% sulfuric acid to make oleum (disulfuric acid): SO3 + H2SO4 → H2S2O7.
- Oleum is added to waterH2O + H2S2O7 → 2H2SO4.
- Why not absorb in water
- a fine mist of sulfuric acid forms, which is hard to condense and dangerous
Section 7 of 15Contact process conditions (Higher tier)
- Temperature450 °C
- The forward reaction is exothermic, so a higher temperature shifts the equilibrium left and lowers the yield.
- A lower temperature would raise the yield but slows the reaction, so 450 °C is a compromise.
- Pressurejust above atmospheric
- Higher pressure would shift right because there are fewer product molecules.
- The equilibrium mixture already contains about 96% sulfur trioxide, so extra energy and cost are not worthwhile.
- Higher pressure also liquefies sulfur dioxide.
- Exam skill
- word and balanced equations for each stage and the conditions must be recalled
Section 8 of 15Uses of sulfuric acid
- Products
- fertilisers, paints, dyes, detergents, fibres and plastics
Section 9 of 15Concentrated sulfuric acid as a dehydrating agent
- Dehydration
- removing the elements of water, such as H and O from sugar, or removing water of crystallisation
- With sucrose (C12H22O11)
- the H : O ratio is 2 : 1, as in water
- The sugar caramelises and turns brown, then quickly black as carbon forms.
- The reaction is very exothermic, so water evaporates and some vapour traps in the carbon, making it grow out of the container as a column.
- With hydrated copper(II) sulfate
- Concentrated acid is added dropwise because of its strength.
- Blue hydrated crystals become white anhydrous copper(II) sulfate.
- Syllabus note
- the observations are needed, but not the equations
Section 10 of 15Fertilisers and nutrients
- Purpose
- fertilisers supply substances plants need to grow healthily
- Nitrogenous fertilisers
- they supply nitrogen as well as minerals such as potassium and phosphorus
- Nitrogen
- it makes chlorophyll and protein and promotes healthy leaves
- Potassium
- it promotes growth and healthy fruit and flowers
- Phosphorus
- it promotes healthy roots
Section 11 of 15Soluble ions in fertilisers
- Nitrogen sources
- ammonium ions (NH4+) and nitrate ions (NO3-)
- Phosphorus source
- phosphate ions (PO43-)
- Potassium source
- most potassium compounds dissolve to give K+ ions
| Fertiliser compound | Formula |
|---|---|
| Ammonium nitrate | NH4NO3 |
| Ammonium phosphate | (NH4)3PO4 |
| Potassium sulfate | K2SO4 |
| Ammonium sulfate | (NH4)2SO4 |
- Composition
- different fertilisers contain different proportions of nitrogen, potassium and phosphorus
Section 12 of 15Producing fertilisers
- Method
- neutralising an acid with ammonia, a gaseous alkali, forms an ammonium salt, which is the fertiliser
- Ammonium nitrate
- NH3(g) + HNO3(aq) → NH4NO3(s)
- Ammonium sulfate
- 2NH3(g) + H2SO4(aq) → (NH4)2SO4(s)
Section 13 of 15Test for ammonium ions
- Warm the solution gently with sodium hydroxide solution, which supplies OH- ions.
- Ammonia gas is released.
- Test the gas with damp red litmus paper, which turns blue.
Section 14 of 15Benefits of fertilisers
- Replenishing nitrogen
- fertilisers restore nitrogen in fields for optimum growth, because plants use it for proteins and chlorophyll
- Other nutrients
- most contain phosphorus and potassium as well
- Practical
- they are easy and quick to apply
Section 15 of 15Eutrophication
- Cause
- fertiliser washes into rivers, lakes and streams, and rain leaches soluble minerals from soil into waterways
- The soluble minerals stimulate rapid growth of single-celled, microscopic plants.
- This forms a continuous surface layer called an algal bloom.
- Sunlight cannot reach the plants below, so they cannot photosynthesise and oxygen levels fall.
- The short-lived algae die.
- Aerobic bacteria break down the dead organisms and use up even more oxygen.
- Fish and other animals die in large numbers because they need high oxygen levels.
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