The Ozone Story

• What is electronegativity?

  The ability of an atom to attract the bonding electrons in a covalent bond towards itself
• What does the electronegativity of an atom depend on?

  size and nuclear charge
• Why does electronegativity increase along a period?


  Atomic radius decreases and charge density increases
• Why does electronegativity decrease down a group?

  Shielding increases and atomic radius increases so charge density decreases
• What is the value called used to compare the electronegativity of atoms?

  Pauling Electronegativity Values
• What is the most electronegative element?

  Fluorine
• The Pauling values can be used to calculate electronegativity differences in a covalent bond.

  The greater the electronegativity difference, the greater the bond polarity
• What are the 3 types of intermolecular bonds?

  Instantaneous dipole -induced dipole

  Permanent dipole-permanent dipole

  Hydrogen bonding
• How do permanent dipoles form?

  If two atoms that are bonded have sufficiently different electronegativities, a polar bond forms. The more electronegative atom draws more of the - charge towards itself and away from the other atom producing a - and + region
• Why is hydrogen fluoride a polar molecule?

  Fluorine is a lot more electronegative than hydrogen which means the electrons are drawn towards the fluorine atom
• Polar molecules: arise when there’s an overall difference in polarity across the molecule, due to the arrangement of polar bonds and geometry of the molecule. Polar Molecules must have polar bonds however a molecule with polar bonds may not b a polar molecule
• Polar molecules with a permanent dipole can align to form a lattice of molecules similar to an ionic lattice
• When can induced dipoles form?

  Induced dipoles can form when the electron orbitals around a molecule are influenced by the distributions of electrons on another particle
• What are Induced dipole bonds also known as?

  Van der Waals forces
• Van Der Waals forces are the weakest type of intermolecular force. They act as an induced dipole between molecules
• The strength of van Der Waals forces varies depending on the Mr of the molecule and its shape. The larger the Mr the stronger the intermolecular forces
• Where do van der waals forces act?

  Between organic alkane chains and are affected by chain length and the presence of branching. Chain length increase = bigger Mr so stronger forces and higher boiling point
• What affects van der waals forces?

  Mr and branching. Branched chains are less able to pack tightly together therefore distance is increased and attractive forces are weakened. Therefore branched alkanes have lower boiling points than straight alkanes
• Group 7:

  Simple covalent molecules with weak van der waals forces. Strength of these forces increase as Mr of the molecule increases so it increases down the group (higher melting and boiling points) this is why fluorine is a gas at room temp and why iodine is a solid
• What is a permanent dipole?

  a type of intermolecular force that acts between molecules with a polar bond. + and - regions on adjacent molecules attract each other and hold the molecules together in a lattice like structure. These are stronger than induced dipole-dipole.
• Hydrogen Bonding:

  The strongest type of intermolecular force. Only act between hydrogen and the three most electronegative atoms (N, O, F).
• How does hydrogen bonds arise?

  The line pair of electrons on the 3 most electronegative atoms form a bond with the + hydrogen atom from another molecule. Often shown as a dotted line.
• Molecules held with hydrogen bonds have much higher melting and boiling points compared to similar sized molecules without. This shows how the type of intermolecular force heavily influences the physical properties of a substance
• Water is a simple molecular structure but has an unusually high melting and boiling point for its size due to the presence of hydrogen bonds
• Why does ice have a much lower density than liquid water?

  The hydrogen bonds hold the molecules in a rigid structure with a lot of air gaps
• Why do alcohols have a much higher melting and boiling points than alkanes with a similar Mr value?


  The lone electron pair on the oxygen atom is able to form hydrogen bonds with a hydrogen atom on the hydroxyl group of a neighbouring alcohol molecule.
• Why can hydrogen bonds make alcohols and water good solvents?Good solvents for compounds that are able to form hydrogen bonds in solution.
• Why can alcohols and water be bad solvents?

  bad for dissolving some polar molecules such as halogenoalkanes which cannot form hydrogen bonds
• What is activation energy?

  The minimum energy required for a reaction to take place between two colliding reactant particles
• Enthalpy changes:

  
• What must you have for successful collisions?

  Energy must be greater than or equal to activation energy and the reacting particles orientation must be correct
• You can make the likelihood of successful reactions higher by increasing concentration, pressure, temp or adding a catalyst
• What are the other benefits of using catalysts to increase the chance of successful collisions?


  They also increase the sustainability of a reaction by lowering temperatures and reducing energy demand from the combustion of fossil fuels. This means less CO2 emissions
• The reaction profile of a catalysed reaction

  the dip in the energy profile represents a he intermediate formed during the reaction. It’s less stable in energy than the reactants and products

  
• Whats a homogeneous catalyst?

  Catalysts that are in the same phase as the reactants and products
• What is the Maxwell-Boltzmann distributions?

  A plot of the number of molecules against kinetic energy
• The maxwell-boltzmann distribution:

  
• Changing the reaction conditions will alter the shape of the curve because the number of particles with energy equal to or greater than the activation energy changes. The total area under the curve represents the total number of molecules so it must remain consistent
• What’s the effect of temperature to the maxwell-boltzmann distribution?

  shifts to the right when temp increases because a greater proportion of molecules have greater kinetic energy

  
• Effect if conc, pressure and SA on maxwell-boltzmann distribution:

  Increasing these components of reactants means that successful collisions occur more frequently however energy of individual particles is unchanged therefore the shape doesn’t change