Energetics

• Enthalpy change is heat energy change.
• The standard conditions are 100kPa and 298K.
• In a reaction, bonds are both broken and made. For a bond to break, energy must be taken in. For a bond to be formed, energy must be given out.
• When energy is taken in, the enthalpy change is positive.
• When energy is released, the enthalpy change is negative.
• To calculate the overall enthalpy change, you must add the energy requite to break bonds (+), with the energy required to make bonds (-).
• Endothermic energy level diagram

  
• Exothermic energy level diagram

  
• The enthalpy change when quantities of a substances in standard states react completely under standard conditions, is known as the enthalpy change of reaction.
• The enthalpy change when one mole of a substance is produced from its elements under standard conditions, is known as the enthalpy change of formation.
• The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions, is know as the enthalpy change of combustion.
• The enthalpy change when solutions of an acid and alkali react together under standard conditions to produce one mole of water, is known as the enthalpy change of neutralization.
• The enthalpy change when one mole of gaseous atoms is formed from an element under standard conditions, is known as the enthalpy change of atomisation.
• The calorimetry is an experimental method for finding the enthalpy change by measuring temperature change over time.
• ΔH = Hf(products) - Hf(reactants)
• There can be many sources of error when finding the enthalpy change using a calorimetry. Heat loss can occur due to conduction or convection, or there can be human errors when the temperature was inaccurately measured. The heat loss can be reduced by using a lid, and insulating the outsides.
• Hess's Law states that the overall enthalpy change for a reaction is the same, regardless of the route taken.
• Enthalpy change of Formation: The arrows point upwards from the central product C as both A and B are formed from the elements at C.

  
• Enthalpy change of Combustion: The arrows point towards the central product, since A and B burn to form the products at C.

  
• The energy required to break one mole of the stated bond in a gaseous state, under standard conditions, is known as bond enthalpy.