Bond enthalpies Flashcards
1
Q
why is bond enthalpy alway spositive
A
- To break a covalent bond - endothermic change…
- requires energy
2
Q
What is bond dissociation enthalpy?
A
- the enthalpy change required to break a covalent bond with all species in the gaseous state..
- . the same amount of energy is given out when a bond is formed
3
Q
Why do compounds have an average value?
A
- Different bonds have different bond enthalpies… This is called the mean bond enthalpy.
4
Q
Suggest why other than incomplete combustion or heat transfer to atmosphere, why the value of enthalpy of combustion may differ?
A
- not done under standard condition
5
Q
For a large temperature rise, why is an uncertainty for rise in temperature ±0.5 degrees celcius adequate?
A
- temperature change is much bigger than the uncertainty
6
Q
why does the Cl-Cl bond enthalpy greater than Br-Br
A
- shorter bonds…
- greater attraction
7
Q
what is bond enthalpy
A
- energy to break one mole of a bond in the gas phase
- averaged over range of molc
8
Q
What do you do work out the bond enthalpy to form bonds?
A
- always negative, always releases energuy.
- .. the negative of the bond enthalpy
9
Q
How do you work out ΔH from the bond enthalpy values
A
- ΔH = bond broken - bonds formed
10
Q
Why does value obtained by student vary from measured value? in calorimetry
A
- heat loss
- water may end up in gaseous state due to temp rise
- reactants may not be in standard states
- data refer to gaseous species, enthalpy of combustion refers to liquid in their standard states
- data do not refer specific compounds
- incomplete combustion
