2.7 Thermochemistry

• when bonds are broken energy is taken in, but what happens when bonds are formed

  energy is given out
• when enthalpy is positive what happens the the energy

  taken in from surroundings
• when energy is released enthalpy is

  negative
• what is the enthalpy change equation

  energy to break bonds+energy to make bonds

  (+ve) + (-ve)
• is the enthalpy change is positive/negative then the reaction is

  exothermic-negative

  endothermic-positive
• what are the enthalpy standard conditions

  100kPa

  298K
• what is enthalpy formation

  the enthalpy change when one mole of substance is made from its own elements under standard conditions
• what is enthalpy combustion

  the enthalpy change when one mole of substance is burned completely in oxygen under standard conditions
• Hess' law

  in overall enthalpy change for the reaction is the same no matter the route taken
• which directions do the arrow point for formation

  outwards

  A and B formed from C
• which direction do the aoows point for combustion

  inwards

  A and B burn to form C
• what is an average bond enthalpy

  the energy required to break one mole of the stated bond in a gaseous state, under standard conditions
• how do you find overall bond enthalpy

  the sum of bonds broken and formed are compared to tell you whether the reaction is endo/exo thermic
• what does each letter stand for q=mcΔT

  q=energy change

  m=mass

  c=specific heat capacity

  ΔT=temperature change
• what does q=mcΔT help you calculate

  energy in or out of a substance of known mass
• what is specific heat capacity

  the energy required to raise 1g of a substance by 1K without change of state
• what is the equation for enthalpy change per mole

  ΔH=q/moles
• why is ΔH found using colorimetry not 100% accurate

  energy loss through convection and conduction

  inaccurate measuring of temperature