Energetics Flashcards
is breaking bonds endo or exo
endo
define enthalpy
the heat content of a system
what is the equation for enthalpy change
q = mc(change in)T
q = energy
m = mass
c = specific heat capacity
T = temperature
how do you calculate enthalpy change per mole
1 - use q=mcat
2 - calculate moles with m=cv (make sure volume is in dm3)
3 - divide q by moles
ADD NEGATIVE SIGN AS ITS EXO
convert to KJ
how do you calculate the enthalpy of neutralisation
1 - use q=mcat
2 - calculate moles of water produced with n=cv (make sure volume is in dm3)
3 - divide q by moles
ADD NEGATIVE SIGN AS ITS EXO
convert to KJ
what is a standered enthalpy change of reaction
enthalpy change measured under standerd conditions (298K and 100KPA)
what is the symbol for standerd enthalpy change
ΔH° (line through the middle of circle)
what are the units for standerd enthalpy change
KJ mol-1
define the standered enthalpy of combustion
the enthalpy change that occurs when one mole of a substance is completely burnt in oxygen under standerd conditions
define the standerd enthalpy of formation
the enthalpy change that occurs when one mole of a compound is formed from its elements in their standerd states
what is used to calculate enthalpy of combustion
flame calorimertry
why does flame calorimetry cause errors
- incomplete combustion
- evaporation of fuels
- heat loss to the surroundings
how can you increase the accuracy of flame calorimetry
add a draught shield
burn in pure oxygen
when is hesses law exothermic or endothermic
exo - when reactants produce products (bonds made)
endo - when products produce reactants (bonds broke)
what is the equation for hesses law for enthalpy of formation
sum of products energy - sum of reactants energy
In hesses law, which way do the arrows point for enthalpy of formation
up
what is the equation for hesses law for enthalpy of combustion
sum of reactants energy - sum of products energy
In hesses law, which way do the arrows point for enthalpy of combustion
down
what are the 3 areas of hesses law for combustion
elements, products, oxides
define mean bond enthalpy
the energy needed to break 1 mole of covalent bonds, averaged from a range of different products
when is mean bond enthalpy + or -
+ when bonds broken - endo
- when bonds made - exo
how do you calculate mean bond enthalpies
sum of reactants bond enthalpys - sum of products bond enthalpys
