module 3: enthalpy changes Flashcards
breaking bonds
endothermic
forming bonds
exothermic (releases energy)
enthalpy
total energy of a system
standard conditions
pressure (gases) 101 kPa (1atm)
concentration (solutions) 1moldm-3
stated temperature 25 ° (298K)
exothermic
temperature of surroundings increases
chemical energy to thermal energy
-ve enthalpy change
standard enthalpy change of reaction definition
molar quantities as stated in a chemical equation react together under standard conditions
standard enthalpy change of reaction equation
ΔrHꝊ= ΣEꝊ(bonds broken) - ΣEꝊ(bonds formed
standard enthalpy change of formation equation
ΔrHꝊ= ΣΔfHꝊ(products)- ΣΔfHꝊ(reactants)-
standard enthalpy change of combustion equation
ΔrHꝊ=ΣΔcHꝊ(reactants)-ΣΔcHꝊ(products)
standard enthalpy change of combustion
1 mole of a substance completely burned in O2 under standard conditions
standard enthalpy change of formation
1 mole of substance formed from its constituent elements in their standard states at s.t.p
bond enthalpy
average energy required to break 1 mole of covalent bonds in gaseous molecules
Hess’ Law
the enthalpy change is independent of the path taken, provided the initial and final conditions are the same
use known enthalpy changes, enthalpy change cannot be measured directly
q= mcΔT
q= heat transferred to or from mass of water J
c= specific heat capacity of water (4.18 KJmol-1)
ΔrH=q/n
measured in KJmol-1
Hess’ cycle calculations
step 1: energy put into break bonds forming separate gaseous atoms
step 2: gaseous atoms combine to form bonds and the energy released is equal and opposite to that of the breaking bonds
step 1- step 2
standard enthalpy change of neutralisation
acid and base in standard states react to form 1 mole of H20 under standard conditions
sources of error when determining enthalpy change of reaction
heat loss to surroundings
methods for reducing heat loss
lid
insulation
draught shields
sources of error when determining enthalpy change of combustion
heat loss
incomplete combustion
evaporation from wick
non-standard conditions
Ea activation energy
energy barrier
minimum energy required for reaction to take place
small Ea
rapid reaction, energy readily available from surroundings
limitations of average bond enthalpy
average value obtained from many molecules, actual value may be slightly different
weak acids
some energy used to dissociate
less exothermic
is zero enthalpy possible?
no!
hess cycle with formation
intermediary step = elements
hess cycle of combustion
intermediary step = H2O + CO2
providing O2 isn’t reactant
calorimetry
method of measuring the change in energy of a system
analysing heat exchange between the system and its surroundings
bond enthalpies are always
positive
Hess’ cycle calculations
pick species that only appear once
you can reverse reaction and change sign
remember to multiply through
