3.1.4 Energetics, A Level Chemistry, AQA

• What is enthalpy change?

  Heat energy transferred in a reaction at constant pressure
• What are standard conditions for enthalpy change?

  100kPa and stated temperature (normally 25 degrees celcius)
• What values of deltaH do exothermic reactions have?

  Negative (less than 0)
• What values of deltaH do endothermic reactions have?

  Positive (more than 0)
• What do exothermic reactions do with energy?

  Give out energy
• What do endothermic reactions do with energy?

  Absorb energy
• Are oxidation reactions endo or exothermic?

  Exothermic
• Is combustion endo or exothermic?

  Exothermic
• Are thermal decomposition reactions endo or exothermic?

  Endothermic
• Is photosynthesis endo or exothermic?

  Endothermic
• What happens to reactant bonds?

  They are broken
• What happens to product bonds?

  They are formed
• Which bonds are broken: reactant or product bonds?

  Reactant
• Which bonds are formed: reactant or product bonds?

  Product
• Is energy taken in or released when you break bonds?

  Taken in
• Is bond breaking endo or exothermic?

  Endothermic
• What values of deltaH does bond breaking have?

  Positive (more than 0)
• How does bond breaking energy change as the bond strength increases?

  It takes more energy to break
• Is energy taken in or released when you make bonds?

  Released
• Is bond making endo or exothermic?

  Exothermic
• What values of deltaH does bond making have?

  Negative (less than 0)
• How does bond making energy change as the bond strength increases?

  They release more energy when formed
• If you need more energy to break bonds than is released when bonds are made, what values of deltaH do you get?

  Positive (more than 0)
• If you need less energy to break bonds than is released when bonds are made, what values of deltaH do you get?

  Negative (less than 0)
• What is bond enthalpy?

  The energy required to break bonds
• What is the mean bond enthalpy?

  The average energy needed to break a certain type of bond, over a range of compounds
• Why are mean bond enthalpies not exact?

  They are an average of energies needed to break the bond over a range of compounds
• Why may a mean bond enthalpy in a data book be different from the mean bond enthalpy in a specific molecule?

  The mean bond enthalpy in the data book is the average for a bigger range of molecules than just the specific molecule.
• Why are mean bond enthalpies always positive?

  Breaking bonds is always endothermic
• Is energy absorbed or given out when bonds are broken?

  Absorbed
• Is energy absorbed or given out during bond formation?

  Given out
• How do you calculate the enthalpy change of a reaction (in terms of energy)?

  Total energy absorbed - total energy released
• How do you calculate the enthalpy change of a reaction (in terms of bonds)?

  Breaking bonds - Making bonds
• Why aren't enthalpy changes for reactions, calculated using mean bond enthalpies, not exact?

  Mean bond enthalpies are averages over a range of molecule and hence aren't totally accurate for certain reactions
• How do enthalpy changes calculated from mean bond enthalpies compare to those calculated using Hess's Law (in terms of accuracy).

  Using mean bond enthalpies makes your result slightly less accurate than using Hess's Law
• What is the standard enthalpy of formation?

  The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
• What is the standard enthalpy of combustion?

  The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions
• What do you use to find out how much heat is given out by a reaction?

  Calorimetry
• What experiment can you use to find enthalpy changes?

  Calorimetry
• How do you perform a calorimetry experiment for the combustion of a flammable liquid?

  You burn the flammable liquid in a calorimeter so it heats the water. You can work out the heat energy that has been absorbed by the water.