3.2.1 - enthalpy changes

• A system in a chemical reaction is the atoms and bonds involved in the chemical reaction
• The law of conservation is the amount of energy in an isolated system remains the same. Energy cannot be destroyed or created, it can only be transferred from one form to another.
• Breaking bonds is endothermic. Energy is taken in to break bonds.
• Making bonds is exothermic. Energy is released to make bonds
• An endothermic reaction is a reaction with an overall positive enthalpy change
• an exothermic reaction is a reaction with an overall negative enthalpy change
• The energy profile diagram for an exothermic reaction is

  
• Energy profile diagram for an endothermic reaction is

  
• Activation energy is the minimum energy required for a reaction to take place
• Activation energy arrow on an enthalpy change diagram always points upwards
• In standard state means the state an element / compound exists at in standard conditions
• Enthalpy change of formation is:

  the energy change that takes place when one mole of a compound is formed from its constituent elements under standard conditions.
• What is enthalpy change of formation?

  The energy change that takes place when one mole of a compound is formed from its constituents elements under standard conditions.
• An equation that represents standard enthalpy of formation is

  H2 + 1/2 O2 ———> H2O
• Enthalpy change of combustion is:

  the energy change that takes place when one mole of a substance is completely combusted / reacted with oxygen
• What is enthalpy change of combustion?

  the energy change that takes place when one mole of a substance is completely reacted with oxygen under standard conditions.
• An equation that represents standard enthalpy of combustion

  C3H8 + 5O2 ——> 3CO2 + 4H2O
• Enthalpy change of neutralisation is the energy change that takes place when one mole of water is formed from a neutralisation reaction
• What is enthalpy change of neutralisation?

  energy change that takes place when one mole of water is formed from a neutralisation reaction
• Enthalpy change of a reaction is the energy change associated with a given reaction.
• What is enthalpy change of reaction?

  the energy change associated with a given reaction
• What is average bond enthalpy?

  the average energy required to break one mole of gaseous bonds
• Bond enthalpies are not as accurate as standard enthalpies of formation/combustion as they are a mean for the same bond across different molecules.
• Enthalpy change can be calculated using average bond enthalpies

  Delta H = sum of bond enthalpies of reactants - sum of bond enthalpies of products
• Hess’ law states that if a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route
• When calculating standard enthalpy of a reaction of formation, in a enthalpy cycle diagram:

  the arrows point away from the box and the box has elements written in it
• When calculating standard enthalpy of a combustion reaction in an enthalpy cycle diagram:

  the arrows point towards the box and the box has combustion products written in it
• You can calculate enthalpy change From experimental data using the equation:

  Q = mc ^T

  where m is the mass of the substance being heated, c is the specific heat capacity of that substance, ^T is the change in temperature
• Specific heat capacity is the energy needed to raise 1g of a substance by 1K.
• This diagram shows a simple calorimeter

  
• Advantages of using a bomb calorimeter are:

  minimises heat loss

  pure oxygen used - ensures complete combustion
• Experimental methods for enthalpy determination might not be accurate because

  heat is lost to the surroundings

  not in standard conditions

  water may evaporate
• When calculating enthalpy of reaction, use the moles calculated from the equation, then multiply by any number in front of the substance