POTENTIAL PAPER 3

• Enthalpy change (ΔH)


  The amount of heat energy taken in or given out during a chemical reaction
• Enthalpy change units


  Joules (J) or kilojoules (kJ) per mole (mol)
• Positive enthalpy change


  Reaction is endothermic (absorbs heat from surrounding)
• Negative enthalpy change

  Reaction is exothermic (releases heat to surrounding)
• Types of Enthalpy Change

  • Enthalpy change of reaction (ΔH°r)
  • Enthalpy change of formation (ΔH°f)
  • Enthalpy change of combustion (ΔH°c)
  • Enthalpy change of neutralisation (ΔH°n)
• Enthalpy change of reaction (ΔH°r)

  The enthalpy change that accompanies a reaction in the molar quantities shown in the chemical equation
• Enthalpy change of formation (ΔH°f)


  The enthalpy change when one mole of a compound is formed from its constituent elements
• Enthalpy change of combustion (ΔH°c)

  The enthalpy change when one mole of a substance undergoes complete combustion with oxygen
• Enthalpy change of neutralisation (ΔH°n)

  The enthalpy change when one mole of water is formed from the reaction of an acid and an alkali
• Electrolysis


  A process that uses an electric current to drive a non-spontaneous chemical reaction
• Anode


  The positive electrode where oxidation occurs
• Cathode


  The negative electrode where reduction occurs
• Electrolyte


  The substance that is broken down, often an ionic compound dissolved in a suitable solvent
• Determining melting point


  1. Place a small amount of product in a capillary tube and melt in your chosen apparatus, measure the temperature using a thermometer

  2. An impure product will melt over a range of temperatures (usually lower than the pure product) and the pure product will melt at a either a single temperature or over a much smaller temperature range
• Example of a Green Synthesis: Aspirin


  • The synthesis of salicylic acid from willow bark is an example of a green chemistry process
  • It is a highly atom economical process with little to no waste
  • It takes place at room temperature and pressure, demonstrating energy efficiency
  • Reducing agents used, like sodium formate, are less hazardous, aligning with the principle of less hazardous chemistry
• Greener Alternatives


  • Use of Renewable Feedstocks
  • Green Catalysis
  • Green Solvents
• Monitoring Green Processes


  • Real-Time Analysis for Pollution Prevention
  • Green Analytical Techniques
• Amine hydrolysis


  1. Amides can be hydrolysed to form carboxylic acids and amines or ammonia

  2. Under strong acidic conditions, amides can undergo acid-catalysed hydrolysis, producing a carboxylic acid and an ammonium ion

  3. Under basic conditions, they undergo base-catalysed hydrolysis to produce a carboxylate ion and an amine
• Hydrolysis of Esters


  1. Esters can be hydrolysed; this means they can be split apart using water

  2. The reaction is faster in acidic or alkaline conditions

  3. In acidic conditions, a carboxylic acid and alcohol are formed

  4. In alkaline conditions, an alcohol and carboxylate salt are formed
• Standard hydrogen electrode (SHE)

  The universal reference for all half-cell potentials, set at 0 volts
• Positive electrode potential


  Indicates a strong tendency to be reduced (gain electrons)
• Negative electrode potential


  Means the species has a tendency to lose electrons and get oxidised
• Anode


  The electrode where oxidation occurs in a voltaic cell
• Cathode


  The electrode where reduction occurs in a voltaic cell
• Electrolytic Cells


  Use electrical energy to drive a non-spontaneous redox reaction
• Electrode potentials are affected by factors such as temperature, pressure, and concentration of the ions in solution
• The Nernst equation can be used to calculate electrode potentials when these conditions deviate from their standard values
• Rusting


  A corrosion process primarily affecting iron and its alloys, leading to the formation of an orange-brown flaky coating that is commonly known as rust
• Rust


  A mixture of various hydrated iron(III) oxides and hydroxides such as Fe2O3.xH2O and Fe(OH)3
• The process of rusting accelerates in the presence of electrical conductors like saltwater, acid or base, and slows in a dry and oxygen-free environment
• Rusting Mechanism


  1. Involves an anode reaction that oxidizes iron to iron(II) ions, and a cathode reaction that reduces oxygen in the presence of water

  2. The iron(II) ions are further oxidized to iron(III) ions which react with oxygen and water to produce rust
• Prevention and Protection Against Rusting

  • Galvanising
  • Cathodic protection
  • Physical and chemical barriers such as paint, oil or oxidation-resistant alloys
• Transition metals


  Elements that have partially filled d orbitals
• Important transition metals


  • Copper
  • Iron
  • Nickel
• Transition metals


  • Have typical metallic properties such as good conductivity of heat and electricity, and malleability
  • Don't react vigorously with water or oxygen
  • Often exhibit multiple oxidation states due to the presence of electrons in the d orbital
• Colour of transition metal complexes

  Due to d-d electronic transitions that absorb certain wavelengths of light
• Transition metals and their compounds often act as good catalysts due to their ability to adopt multiple oxidation states and to form complexes