(OCR B) A-LEVEL Chemistry

• If a central atom has 4 bonding pairs and no lone pairs, it's shape is tetrahedral and it's bond angle is 109.5°.
• If a central atom has 3 bonding pairs and 1 lone pair, it's shape is pyramidal and it's bond angle is 107°.
• If a reaction is exothermic, then the enthalpy change is negative and energy is given out to the surroundings.
• If a reaction is endothermic, then the enthalpy change is positive and energy is taken in from the surroundings.
• Lattice enthalpy becomes more negative when ionic charge increases or ionic radii decreases.
• The standard enthalpy change for a reaction is the enthalpy change when molar quantities of reactants, as stated in the equation, react together under standard conditions.
• The standard conditions used in chemistry are :

  • Temperature : 298 K
  • Pressure : 1 atm
  • Concentration : 1 mol dm-3
• The standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from it's elements in standard states under standard conditions.
• A catalyst increases the rate of a chemical reaction by providing an alternative pathway with a lower activation enthalpy.