Chemistry OZ

• Electronegativity

  The ability of an atom to attract the bonding electrons in a covalent bond towards itself
• Electronegativity


  • Increases along a period as atomic radius decreases and charge density increases
  • Decreases down a group as shielding increases and atomic radius increases so charge density decreases
• Pauling electronegativity values


  Can be used to compare the electronegativity of atoms, a higher value indicates greater electronegativity, Fluorine has the highest value of 4.0
• Electronegativity difference in a covalent bond

  The greater the difference, the greater the bond polarity and the greater the degree of ionic character
• Permanent dipole


  Formed when two atoms with sufficiently different electronegativities are bonded, the more electronegative atom draws more of the negative charge towards itself
• Polar molecule


  Arises when there is an overall difference in polarity across the molecule, due to the arrangement of polar bonds and the geometry of the molecule
• Polar molecules


  • CO2 - the C=O bonds are polar but the molecule is linear so the dipoles cancel out
  • H2O - the O-H bonds are polar and the geometry is bent so the molecule is polar
• Induced dipole


  Can form when the electron orbitals around a molecule are influenced by the distributions of electrons on another particle
• Intermolecular forces


  • Van der Waals forces (weakest)
  • Permanent dipole
  • Hydrogen bonding (strongest)
• Van der Waals forces

  Act as an induced dipole between molecules, the strength depends on the Mr of the molecule and its shape
• Alkane chain length

  As chain length increases, Mr increases, resulting in stronger intermolecular forces and higher boiling point
• Alkane branching


  Branching weakens van der Waals forces as the chains are less able to pack tightly together, resulting in lower boiling point
• Group 7 elements


  The strength of van der Waals forces increases down the group, resulting in higher melting and boiling points
• Permanent dipole intermolecular forces

  Act between molecules with polar bonds, the δ+ and δ- regions attract each other forming a lattice-like structure
• Hydrogen bonding


  The strongest type of intermolecular force, acts between hydrogen and the three most electronegative atoms: nitrogen, oxygen and fluorine
• Hydrogen bonding


  Causes much higher melting and boiling points compared to similar-sized molecules without hydrogen bonding
• Water has unusually high melting and boiling points for the size of the molecule due to hydrogen bonding
• The hydrogen bonds in water hold the molecules in a rigid structure with air gaps, resulting in ice having a lower density than liquid water
• Alcohols


  Have much higher boiling points than alkanes with similar Mr due to hydrogen bonding between the oxygen lone pair and hydrogen atom
• Alcohols and water are good solvents for compounds that can form hydrogen bonds, but poor solvents for polar molecules that cannot form hydrogen bonds
• Activation energy


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


  Shows the enthalpy changes, activation energy is the difference between the reactants and the top of the hump
• For a reaction to occur successfully, the collisions must have energy greater than or equal to the activation energy and the reacting particles' orientation must be correct
• Conditions of a reaction can be altered to impact the collisions of the particles and change the rate of reaction
• Activation energy


  Energy required for a reaction to take place between two colliding reactant particles
• Enthalpy profile diagram


  Diagram showing enthalpy changes
• Activation energy (Eₐ)


  Difference between the reactants and the top of the hump on an enthalpy profile diagram
• Rates of Reaction


  1. Reactant particles collide

  2. Collisions must have energy greater than or equal to activation energy

  3. Reacting particles' orientation must be correct
• Activation energy


  Minimum amount of energy required for two particles to react
• Changing reaction conditions


  Impacts the collisions of the particles and can be altered to give the particles more energy
• Changing reaction conditions


  Increases the likelihood of a collision occurring with sufficient energy to react, leading to a greater rate of reaction
• Rate of reaction


  Equal to the gradient of the concentration-time graph curve at a given point
• Finding rate of reaction from concentration-time graph

  1. Draw a tangent to the curve at the given time

  2. Calculate the gradient of the tangent
• Initial rate of reaction

  Found by drawing a tangent to the curve when time = 0
• Units of rate


  mol dm⁻³s⁻¹

  mol dm^-3 s^-1
• Suitable physical quantities to measure to find rate of reaction

  • Concentration
  • Gas volume
  • Mass
• Increasing concentration of a species

  Increases the rate of reaction
• Increasing pressure of a gas

  Increases the rate of reaction
• Increasing temperature


  Increases the rate of reaction
• Catalyst


  Substance that increases the rate of reaction without being used up