Gas Chemistry

• Gases in the atmosphere

  Atmosphere describes the collection of gases that surround earth.

  • The ’air’ atmosphere near the surface
  • The composition of air has remained reasonably constant for about 200 million years
  • 78% nitrogen
  • 21% oxygen
  • Noble gases - Argon 1%
  • water vapour and CO2 - 0.03 - 0.04 %
• Collection of gases 


  Gas collection depends on solubility of gas in water and if soluble, depends on density compared with air
• Collecting insoluble gases 


  Gases that are insoluble in water or have low solubility can be collected by collection of water:

  
• Collecting soluble gases 


  Gases that are soluble in water are collected by displacement of air and so the density of gas compared with air must be considered

  • soluble gases that are denser than air are collected by downward delivery
  • soluble gases that are less dense than air are collected by upward delivery
• Soluble gas collection 


  Upward and Downward Delivery

  
• Physical Properties of Nitrogen

  • Colourless, odourless gas
  • insoluble in water
  • diatomic gas, N2
  • Unreactive gas - because of triple covalent bond between nitrogen atoms in N2 molecules. Strong triple covalent bonds requires substantial energy to break before nitrogen atoms can react
• Uses of nitrogen

  • liquid nitrogen is used as a coolant
  • food packaging as nitrogen creates and inert atmosphere to keep food fresh
• Physical properties of ammonia

  • NH3 is a colurless, odourless gas that is soluble in water
  • less dense than air
• Test for ammonia

  • Method > dip glass rod in concentrated hydrocholric acid and put this in a sample of the gas
  • Result > If present, white smoke of ammonium chloride is observed
  • NH3 gas reacts with hydrogen chloride gas, forming ammonium chloride
  • NH3 + HCl -> NH4Cl ( white smoke )
• Uses of ammonia

  • used to produce fertilisers by reacting with acids such as hydrochloric and nitric:
  • NH3 + HCl -> NH4Cl
  • NH3 + HNO3 -> NH4NO3
  • The ammonium salts are very soluble in water so they can dissolve in soil water and be absorbed by the plants
• Physical properties of hydrogen

  • Colourless, odourless Gas
  • insoluble in water
  • less dense than air
  • diatomic, H2
• Preparation of hydrogen 


  Prepared using zinc or magnesium and dilute hydrochloric acid

  • Zn + 2HCl -> ZnCl2 + H2
  • Mg + 2HCl -> MgCl2 + H2
  • Hydrogen is collected over water as it is insoluble
  • Reaction of dilute HCl with Mg is more vigorous
• preparation of hydrogen 


  Diagram

  
• Test for hydrogen

  • method: apply a lit splint
  • test result: burns with squeaky pop
  • 2H2 + O2 -> 2H2O
• Uses of hydrogen

  • meteorological (weather) balloons
  • hardening oils to form solid fats in margarine and spreads
  • as a clean fuel
• Hydrogen as a clean fuel

  Described as a clean fuel because the only product of combustion (water) is non-polluting

  • Can be produced from electrolysis of water. This requires electricity. This can be generated from combustion of fossil fuels or renewable resources of energy which increases the potential of hydrogen as a clean fuel
  • used to power veichles and would be supplied in liquid form which requires energy and specialised storage to protect against risk of explosion
  • tech is advancing to improve supply, storage and use of hydrogen
• Physical properties of oxygen

  • Colourless, odourless gas
  • slightly soluble
  • slightly denser than air
  • diatomic, O2
• Lab prep for oxygen 


  Made using hydrogen peroxide and manganese (IV) oxide using same apparatus as hydrogen.

  Manganese (IV) oxide is a catalyst used to speed up decomposition of hydrogen peroxide

  • 2H2O2 -> 2H2O + O2
• Test for oxygen 


  Method: apply a glowing splint

  Result: glowing splint relights
• Use of oxygen

  • Medicine
  • welding
• Reaction with carbon 


  Observations: black carbon burns with an orange, sooty flame forming colourless gas - CO2 :

  • C + O2 -> 2CO
  • If limited supply of oxygen, combustion produces CO which is toxic
  • 2C + O2 -> 2CO
• Reaction with sulfur 


  Observation : yellow, solid sulfur melts to a red liquid and burns with a blue flame, giving colourless, pungent gas - sulfur dioxide

  • S + O2 -> SO2
• Reaction with magnesium 


  Observation: grey solid magnesium burns with a bright, white light, releasing heat and producing white solid - magnesium oxide

  • 2Mg + O2 -> 2MgO
• Reaction with iron 


  Observation: Grey, solid iron filings burn with orange sparks, producing a black solid - Fe3O4

  • 3Fe + 2O2 -> Fe3O4
• Reaction with copper 


  Observation: red brown solid glows red, (may be blue-green flame) and forms a black solid - copper (II) oxide

  2Cu + O2 -> 2CuO
• Oxides


  Formed are either basic or acidic
• Basic oxides


  • Oxides of metals that react with acid to produce a salt and water
  • Examples: MgO, Fe3O4, CuO
• MgO and CuO


  Basic oxides, insoluble in water so they don't produce an alkaline solution when added to water
• Acidic oxides


  • Oxides of non-metals that will react with alkalis to form a salt and water
  • Examples: CO2, SO2
• K2SO3


  Potassium sulfite, a sulfite ion is SO3 2-
• Acidic oxides


  React to form an acidic solution
• Carbonic and sulfuric acid


  Weak acids, pH 3-6
• Universal indicator


  Changes from orange to yellow
• Preparation of Carbon Dioxide

  CO2 is prepared from calcium carbonate and hydrochloric acid using the same apparatus as hydrogen

  Zinc is replaced by calcium carbonate:

  • CaCO3 + 2HCl -> CaCl2 + CO2 -> H2O
  • collected over water due to low solubility
• Physical properties of carbon dioxide

  • Colourless, odourless gas
  • low suitability
  • denser than air
• Test for carbon dioxide

  Method: bubble the gas through limewater

  Test result: colourless solution becomes milky

  • CO2 + Ca(OH)2 -> CaCO3 + H2O
  • CaCO3 + CO2 + H2O -> Ca(HCO3)2
• Reaction with water 


  Carbon dioxide reacts with water to form weak, carbonic acid :

  • CO2 + H2O -> H2CO3
  • Acid can’t be isolated from the solution and as often written as CO2 (aq). Causes acidity in fizzy drinks
• Reaction with limewater 


  CO2 + Ca(OH)2 -> CaCO3 + H2O
• Uses of carbon dioxide

  • Used in fire extinguishers - doesn’t support combustion - denser than air so covers burning fuel
  • Used in making carbonated drinks because it has low solubility in water. When bottle is opened, gas is released with fizz. Gives the drink an acidic taste due to carbonic acid present