9.2 enthalpy changes Flashcards
bond breaking is
endothermic
- you need energy to do it
- it has a positive enthalpy change value
bond formation is
exothermic
- energy is released
- the enthalpy chabege value is negative
define bond dissociation enthalpy
the amount of energy needed to break bond per mole in its gaseus state
define and explain average bond enthalpies
- they’re not exact because the value is calculated average from a range of values that differ due to the shape of the compound they come from
- def: the energy needed to break one mole of bonds in the gas phase averaged over many different compounds
To record enthalpy change you need two recordings
- the number of moles your reacting
- the change in temprature
Enthalpy changes equation for value in Joules
q=mc^T
units for the q=mc^T
q=heat lost or gained in J **
m=mass of water in a calorimeter or solution in the container in grams. the mass is measured not of the reactant but of the TOTAL mass of material that will record the heat change.
c=specific heat capacity of water 4.18Jg-1K-1**
ΔT = Tfinal – T initial (this will give the correct sign as exothermic reaction ΔH is –ve and endothermic reactions ΔH is +ve.
is the The thermometer part of the system?
The thermometer is not part of the system, but contained within the
SURROUNDINGS
system, surroundings, universe
system = contents of the flask
universe=suroundings and system
Temperature is measured in kelvin but the fixed points are……
are equal (ice melting, water
boiling) so a 1K rise = 1oC
this means temperature CHANGE is the same in both K or oC.
If 50cm3 of solution is added then the mass will be
50g
how do you calculate enthalpy change in KJ/mol
ΔH = mcΔT/no moles
same as ΔH = q/
kJ mol-1
comment on thre accuracy for the enthalpy change of combustion
likley to be less exothermic than expected because
* heat loss to the suroundings mainly air surounding the flame
* incomeplte combustion of methanol eg.black soot on beaker
* evaporation of methanol from the wick. try weigh imediatley after extinguishing
liley to be more exothermic because
- non standard conditions