5.2-5.3 Hess' Law + Bond enthalpies
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Hess’ Law
Total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as initial and final conditions remain the same
What is Hess’ Law used for?
Can be used to calculate enthalpy changes for reactions that cannot be determined experimentally
What does enthalpy of formation values tell us about stability?
Values are useful in indicating stability of compounds relative to elements. The more negative the value, greater the stability
How can enthalpy change be calculated using enthalpy of formation?
It is simply enthalpy of formation of products - enthalpy of formation of reactants
How can enthalpy change be calculated using enthalpy of combustion?
It is enthalpy of combustion of reactants - enthalpy of combustion of products
What would Hess Law equations look like?
Draw up an equation for each reactant and product that results from the formation of elements and cancel out similar species. Remember if switching the equations to cancel, change the sign of the enthalpy value as well and add all values up
Bond enthalpy
Energy required to break one mole of chemical bonds in gaseous state under standard conditions
Is O2–> 2O exothermic or endothermic?
It is endothermic as bonds are being formed
Average bond enthalpy
Energy change when one mole of bonds are broken averaged for the same bond in similar compounds eg. H-H bonds may have different values in different compounds
Complete the statement: O2 and O3 are _____ of oxygen
Allotropes
What does ozone help do?
It helps protect and sustain life on Earth’s surface and in lower atmosphere by absorbing harmful UV radiation
Which UV ray is absorbed the most (order from most to least)?
UVC>UVB>UVA
Does O2 or O3 have more bond energy?
O2 has more as O2 only has double bonds as compared to triple so bond strength is more
What does O2 break up into?
2 gaseous O radicals
What does O2 and O radical react to form?
Gaseous O3
What is O2 and O3 dissociated by?
O2 is dissociated by UVC (shorter wavelength) and O3 by UVB as bonds are weaker in O3 so it requires UV of lower energy
Why is Earth protected by the oxygen reactions?
As it occurs in continuous cycle, the only radiation that reaches earth is UVA which is less harmful
Outline the order of oxygen reactions
• Double O bond is disrupted by UVC to form O radicals • O radical + O2 form O3 • O3 is broken by UVB to form O2 and O radicals again