energy

• Atom


  The smallest particle of an element
• Molecule


  Two or more atoms joined together
• Molecules


  • H₂O
  • CO₂
  • Ne
• Atoms


  • H
  • O
  • N
  • Cl
  • C
  • He
• Diamond


  • Tetrahedral structure
  • Each atom forms bonds with 4 nearest neighbours
  • Bond angles 109.47°
  • No spare bonds
• Exothermic reaction


  Reaction that releases energy
• Endothermic reaction


  Reaction that absorbs energy
• Conservation of mass: matter cannot be created or destroyed, total mass of reactants = total mass of products
• Catalyst


  Substance that speeds up a reaction without being consumed
• Catalysts


  • Specific to reactions
  • Biological catalysts are called enzymes
• Catalyst action


  1. Lowers activation energy

  2. Allows reactions to happen quicker at lower temperature
• Activation energy


  Energy needed to start a reaction
• Catalysts have economic advantages (cheaper reactions) and environmental advantages (less fossil fuels burned)
• what is the difference between atom and molecule

  An atom is the smallest unit of matter that still has all of the properties of its element, whereas a molecule is a structure that contains multiple atoms bonded together
• Thermal Decomposition


  1. Thermal heat

  2. Decompose break down
• Thermal decomposition


  A single reactant breaks down into simpler substances after being heated
• Gas tests

  • Hydrogen - squeaky pop
  • Oxygen - lit splint relights
  • Carbon dioxide - cloudy limewater
• Thermal decomposition reactions
• Carbon dioxide
• Chlorine bleaches litmus paper white
• Water - bleaches cobalt paper (blue-white) or condenses in ice
• Bond energy

  The amount of energy needed to break one mole of a particular bond
• Calculating bond energy


  1. Add together the bond energies for all the bonds in the reactants - this is the 'energy in'

  2. Add together the bond energies for all the bonds in the products - this is the 'energy out'

  3. Calculate the energy change = energy in - energy out
• Exothermic reaction


  • Hydrogen and chlorine react to form hydrogen chloride gas
• Exothermic reaction


  • Energy change is negative, due to the fact that the energy released by the bonds formed is greater than the energy absorbed by the bonds broken
  • Energy is released to the surroundings
• Endothermic reaction


  • Hydrogen reacts with iodine to form hydrogen iodide
• Calculating bond energy for an endothermic reaction

  1. Energy in = 436 + 151 = 587 kJ/mol

  2. Energy out = 2 x (bond energy of H-I) = 2(H-I)

  3. Energy change = in - out = 587 - 2(H-I) = -3 kJ/mol

  4. Rearrange to: 2(H-I) = 587 + 3 = 590 kJ/mol

  5. Therefore: (H-I) = 590 ÷ 2 = 295 kJ/mol