energetics

• enthalpy change is the heat energy transferred in a reaction at constant temperature (units: kj mol-1)
• STANDARD CONDITIONS:

  • 100KPa (1atm)
  • 298K (25c)
  • substance in their standard states
• exothermic reactions increase in temperature, enthalpy change is negative
• endothermic reactions decrease in temp, enthalpy change is positive
• exothermic examples:

  • combustion
  • oxidation
• endothermic examples:

  • thermal decomposition
  • photosythesis
• bond enthalpies are specific depending on the atoms attached on either side of the bond
• bond enthalpy is the energy needed to break a bond
• breaking a bond is endothermic, forming a bond is exothermic
• mean bond enthalpy is the average energy needed to break a certain type of bond, OVER a range of compounds
• enthalpy change = bonds broken - bonds formed
• STANDARD ENTHALPY CHANGE OF FORMATION:

  • enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
• STANDARD ENTHALPY CHANGE OF COMBUSTION:

  • enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions, with all reacts and products in their standard states
• STANDARD ENTHALPY CHANGE OF REACTION:

  • enthalpy change when a reaction occurs in the molar quantities show in the chemical equation, under standard conditions, with all reactants and products in their standard states
• to find enthalpy change you need to know the no. of moles reacting, the temperature change and how much stuff you're heating
• calorimetry experiments are used to measure the amount of energy released when a substance is heated
• SIMPLE EQUIPMENT FOR ΔH:

  • polystyrene beaker to reduce the amount of heat lost or gained through the sides

  
• EQUIPMENT TO MEASURE ΔH OF COMBUSTION:

  • you will lose heat to the surroundings based on the insulation
  • incomplete combustion may take place = less energy given out

  
• ACCURATE TEMPERATURE CHANGE:

  1. record temp at regular intervals, including few minds before
  2. plot a graph (2 lines of best fit)
  3. extend both lines so they pass the point of when the reactants were added
  4. distance between two lines = TEMP CHANGE

  
• ENTHALPY CHANGE:

  • followed by the equation ΔH°c = q/n

  
• ENTHALPY OF NEUTRALISATION: one mole of water is formed by the reaction of an acid & alkali

  1. add a known volume of acid to an insulated container, measure temp
  2. add a known volume of alkali
  3. record temp at intervals
  4. stir solution = evenly heated
  5. use accurate temp change method
• hess's law stated that the total enthalpy change for a reaction is independent of the route taken
•  ΔHf :

  • value for elements is 0, there is no change in enthalpy
• ΔH°c :

  • oxygen doesn't have an enthalpy change of combustion as you can't burn 1 mole of oxygen in oxygen