Energetics

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  • Created on: 30-03-16 18:57
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Energetics
Endothermic ­ take in heat
Exothermic ­ give out heat
Enthalpy change is heat energy change measured under conditions of constant pressure.
Standard enthalpy changes refer to standard conditions (ie. 100kPa) and stated
°
temperature (eg H298 )
Standard Enthalpy of Combustion, C H°
= enthalpy change when 1 mole of substance
completely burned in O 2 under standard conditions with all reactants and products in
standard states
°
Standard Enthalpy of Formation,
fH = Enthalpy change when 1 mole of compound is
formed from its elements in their standard states under standard conditions.
Calorimetry
Heat change, q, in reaction is given by equation q = mcT. m is mass of substance hat has
temp change, T and specific heat capacity, c.
Measuring enthalpy change:
Need to know No. of moles of stuff that's reacting, change in temp and how much stuff
you're heating.
If reaction takes place in solution...
Using calorimetry to find out how much heat is given out by combustion by measuring temp.
Change. To find enthalpy of combustion of flammable liquid, burn in calorimeter as shown below.
As fuel burns it heats water. Can work out heat energy absorbed by water if you know mass of
water, temp change and SHC of water.
Ideally all heat given out by fuel would be absorbed by water but in practice some
heat is always lost so it's hard to get accurate result.
A bomb calorimeter is more accurate but works on same principle.

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Hess's Law
Total enthalpy change for reaction is independent to route taken. Total enthalpy change for one
route is the same as another.
Good to find enthalpy changes that you can't find directly by doing experiment.
Using Enthalpies of Formation
Need to know fH° for all reactants and product that are compounds.
fH° for elements = 0. All
measured at 298K.…read more

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Using Enthalpies of Combustion
Use enthalpy changes of combustion instead of enthalpy changes of formation.
Bond Enthalpies
Atoms in molecules held by strong covalent bonds. Takes energy to break them (endothermic so
H is +ve) and energy is given out when new bonds formed (exothermic so H is ­ve). Bond
enthalpy is energy needed to break a bond.…read more

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Enthalpy change of reaction = total energy absorbed ­ total energy released
To calculate overall enthalpy change for reaction calculate total energy needed to break all
bonds in reactants.
To find total energy released by reaction calculate total energy needed to form all new
bonds in products.
The overall enthalpy change is found by subtracting total energy released from total
energy absorbed.…read more

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