Module 3.2.1- Enthalpy changes

• enthalpy change


  H(products) - H(reactants)

  
• Exothermic change


  from system TO surroundings

  Delta H is negative

  
• Endothermic change


  from surroundings TO system

  Delta H is positive

  
• Enthalpy profile diagram - exothermic reactions

  
• Enthalpy profile diagram- endothermic reactions

  
• Activation energy


  the minimum amount of energy required to start a chemical reaction
• Standard conditions

  100 kPa 298 K 1 mol dm^-3 Standard state
• Standard enthalpy change of reaction


  The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states.
• Standard enthalpy change of formation


  - The enthalpy change that takes place - when one mole of a compound is formed - from its elements in their standard states - under standard conditions.
• Standard enthalpy change of combustion


  - The enthalpy change that takes place when one mole of a substance reacts completely with oxygen - under standard conditions- with all reactants and products being in their standard states.
• Standard enthalpy change of neutralisation


  - The enthalpy change when an acid and alkali react together to form one mole of water- under standard conditions
• Calculating energy change

  q=mc delta T

  
• after working out "q" in q= mc delta T what do you need to do generally

  Divide the answer by 1000 to get your answer in kJ
• delta H =


  -q/moles moles in enthalpy change of combustion is the hydrocarbon burnt

  
• Why was less heat transferred to the water in the experiment

  - heat loss to surroundings

  - incomplete combustion

  - non-standard conditions

  - evaporation of alcohol
• Determination of an enthalpy change of neutralisation

  
• Average bond enthalpy


  The energy required to break one mole of a specified type of bond in a gaseous molecule
• Making bonds is...

  ...exothermic
• Breaking bonds is...

  ...endothermic
• Calculating enthalpy changes from average bond enthalpies


  sigma(bond enthalpies in reactants) - sigma(bond enthalpies in products)

  
• Hess's Law


  the overall enthalpy change in a reaction is equal to the sum of enthalpy changes for the individual steps in the process
• Hess Cycle: Enthalpy change of formation


  A= C- B

  
• Hess Cycle: Enthalpy change of combustion


  A= B- C