enthalpy change

Cards (13)

what needs to happen for bonds to be broken?
energy is taken in from the surroundings
what happens when bonds are formed?
energy is given out
what does the overall energy change of the reaction depend on?
how much energy is transferred in these processes
what is the enthalpy change when energy is taken in from the surroundings?
positive
what is the enthalpy change when energy is released?
negative
overall enthalpy change equation
ΔH = energy to break bonds (+ve) + energy to make bonds (-ve)
standard conditions of enthalpy
100 kpa and 298 K
enthalpy of formation (ΔfH) definition
the enthalpy change when one mole of a substance is produced from its elements under standard conditions
enthalpy of combustion (ΔcH) definition
the enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions
different covalent bonds require different amounts of energy to be broken
where can values be found experimentally?
using calorimetry methods
why do the experimental bond enthalpy values often different from the date book values? 
they are not exact and vary in each situation
what does this mean for the data book values?
they are averaged values