CHEMISTRY P1

• Exothermic reactions

  Heat energy released, energy released to surroundings
• Endothermic reactions


  Take in heat energy from surroundings
• Activation energy


  Minimum energy required for particles to collide
• Bond energy


  Energy needed to break one mole of a particular covalent bond
• Breaking bonds is endothermic, bond formation is exothermic
• Calculating energy change

  Energy required to break bonds - energy required to form bonds
• If the result is negative, the reaction is exothermic
• Electrochemical cells

  Use chemical reactions to produce electricity
• Factors affecting voltage

  Temperature, type and concentration of electrolyte, metal used for electrodes
• Rechargeable cells


  Chemical reactions are reversed when external current is supplied
• Non-rechargeable cells

  Voltage produced until reactants used up, chemical reactions are irreversible
• Simple cell


  Two different metals in contact with an electrolyte
• Batteries

  Two or more cells connected in series to provide greater voltage
• Fuel cells

  Supplied by external fuel (e.g. hydrogen) and oxygen, fuel is oxidised electrochemically to produce potential difference
• Fuel cells release energy as electrical energy rather than thermal, at lower temperature
• Hydrogen fuel cell reaction

  1. At anode: H2 ➔ 2H+ + 2e- (oxidation)

  2. At cathode: O2 + 4H+ + 4e- ➔ 2H2O (reduction)

  3. Overall: 2H2 + O2 ➔ 2H2O
• Solid


  Fixed shape, vibrate in fixed position, cannot be compressed
• Liquid


  Flow, take shape of container, cannot be compressed
• Gas


  Flow, completely fill container, move quickly, highest energy
• Simple particle model doesn't present forces, particles presented as solid spheres with mostly empty space
• Energy needed to change state

  Stronger the forces of attraction, more energy is needed
• Predicting physical state

  Temp < melting point = SOLID

  Temp between mp and bp = LIQUID

  Temp > boiling point = GAS
• Phase changes

  • Solid ➔ Liquid (MELTING)

  Liquid ➔ Solid (FREEZING)

  Liquid ➔ Gas (EVAPORATING/BOILING)

  Solid ➔ Gas (SUBLIMATION)

  Gas ➔ Liquid (CONDENSATION)

  Gas ➔ Solid (DEPOSITION)
• Ion

  Atom with positive or negative charge
• Forming positive ions

  Metals lose electrons from outer shells, have more protons than electrons
• Forming negative ions


  Non-metals gain electrons, have more electrons than protons
• Ionic compound


  Giant structure of ions held by strong electrostatic forces
• Ionic compounds

  • High melting and boiling points (solid at room temp)

  Conduct electricity when liquid, molten, or dissolved in water

  Soluble in water
• Covalent bond


  Formed when 2 atoms share electrons (non-metals)
• Covalent compounds

  • Consist of small molecules

  Intermolecular forces are weak

  Low melting and boiling points

  Cannot conduct electricity
• Giant covalent structure


  Many atoms joined by covalent bonds
• Giant covalent structures

  • High melting and boiling points

  Cannot conduct electricity (except graphite)
• Silicon dioxide (SiO2)


  Contains silicon and oxygen atoms in a regular arrangement, strong bonds
• Silicon dioxide

  • High melting and boiling point

  Cannot conduct electricity

  Giant covalent structure
• Diamond


  Each carbon atom joined to 4 other carbon atoms in a regular tetrahedral network
• Diamond


  • Hard, rigid, cannot conduct electricity
• Graphite

  Each carbon atom forms 3 covalent bonds, forming layers of hexagonal rings
• Graphite

  • Has one delocalised electron allowing it to conduct electricity

  Weak forces between layers allow them to slide over each other, useful as a lubricant
• Allotrope


  Different structural forms of the same element in the same physical state
• Graphene

  Single layer of graphite, giant structure with delocalised electrons