module 3

• standard enthalpy of formation is the enthalpy change when 1 mole of compound is formed from its elements under standard condition
• bond enthalpy is the energy required to break 1 mole of bonds
• a weak acid is incompletely ionised in water, so it has a low pH
• a strong acid is completely ionised in H+ ions
• conditions used in the haber process

  1. pressure - 80-1000 atm
  2. temperature - 400-550 C
  3. catalyst - iron
• le Chatalier's principle is a change in conditions that will cause a shift in the position of equillibrium
• features of a dynamic equilibrium:

  1. takes place in a closed system
  2. forward rate = reversed rate
  3. reversible reaction
• haber process converts nitrogen (N2) from the air into ammonia (NH3)
• exothermic = a reaction that gives out heat/energy to the surroundings or in which the reactants react with a decrease in internal enthalpy/energy
• enthalpy profile diagram for exothermic reactions

  
• activation energy = minimum energy that molecules/particles need to have to react and so the reaction can occur
• endothermic = a reaction that absorbs heat/energy from its surroundings or in which the products are formed with an increase in internal enthalpy/energy
• enthalpy profile diagram for endothermic reactions

  
• heterogeneous catalysts are catalysts in a different phase/state to the reactants
• homogenous catalysts are catalysts in the same phase/state as the reactants
• the function of a catalytic converter is to convert toxic gases into less toxic ones
• enthalpy change of combustion = enthalpy change for the complete combustion of 1 mol of a substance
• standard conditions

  1. 100kPa / 1 atm
  2. 25C / 298K
• chlorine radical equations:

  Cl + O3 -> ClO + O2

  ClO + O -> Cl + O2

  overall:

  O3 + O -> 2O2
• enthalpy profile diagram of normal reaction and effect of a catalyst

  
• bond enthalpy values are positive when bond breaking is endothermic
• bond enthalpy values are negative when bond making is exothermic
• hydrogen is important in manufacturing of:

  1. Haber process
  2. production of margarine
  3. production of ammonia
• margarine is made by reacting vegetable oils with hydrogen under high pressure at around 200°C using nickel as a catalyst.
• the Haber process uses nitrogen from the air, hydrogen from natural gas or oil, and steam to produce ammonia.
• ammonia can be used as fertiliser (NH4+) or as feedstock for other chemicals such as plastics and explosives.
• nitric acid is produced through oxidation of ammonia using oxygen gas, which requires an iron catalyst.
• ammonia synthesis (Haber process): N2(g) + 3H2(g) → 2NH3(g)
• ammonia can be used as fertiliser (for crops) and also as fuel (in cars)
• catalytic converters use platinum group metals such as palladium, rhodium, and platinum to convert toxic gases into less damaging ones during car exhaust emissions.
• platinum group metals have very low melting points so they must be supported on an alumina substrate which has a higher melting point than the metal itself.
• carbon dioxide is formed during combustion reactions involving carbon-containing compounds.
• carbon monoxide is toxic because it binds strongly to haemoglobin in red blood cells, preventing them from carrying oxygen throughout the body.
• oxidation of ammonia to form nitric acid: 4NH3(aq) + 5O2(g) → 4NO(g) + 6H2O(l)
• nitrate ion (NO3-) is formed when NO reacts with water.
• platinum group metals are expensive but they have high melting points and good electrical conductivity
• iron is cheap and abundant, making it useful for construction purposes
• the three main reactions that occur inside a catalytic converter are reduction, oxidation, and combustion
• law of conservation of energy states that energy cannot be created or destroyed
• catalytic converters are heterogenous catalysts