1.4 energetics Flashcards
what is enthalpy change?
heat energy transferred in a reaction at constant pressure.
overall effect of bond making/ breaking
units of enthalpy change
KJmol-1
what are standard conditions?
100KP (1 atm) pressure and 298K/25c
exothermic reactions
give energy to surroundings
temp up
products less energy than reactant
- enthalpy change
exo reaction examples
oxidation, respiration, neutralisation
endothermic reactions
take in energy from surroundings
temp down
products more energy than reactant
+ enthalpy change
endo reaction examples
photosynthesis, decomposition, sodium carbonate+ ethanoic acid
what are bond enthalpies?
energy needed to break a bond
atoms in molecules held by covalent bonds so take energy to break and release to make
is bond breaking exo or endo?
endo
is bond making exo or endo?
exo
what is a mean bond enthalpy?
average energy needed to break certain type of bond over range of compounds
why do we have MEAN bond enthalpies?
because energy needed depends on where bond is
eg c-c bond needs diff amount of energy in methane than ethane
How do you calculate enthalpy change?
total energy absorbed-total energy released
use average bond enthalpies to work these out
what are the different types of enthalpy change?
standard enthalpy change of formation
standard enthalpy change of combustion
standard enthalpy change of reaction
what is standard enthalpy change of formation?
1 mol of a compound formed from elements in standard states and conditions