3.2.1 Energetics Definitions

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Energetics Definitions
Enthalpy ­ The heat content of a system measured at a constant pressure. H = (enthalpy of
products) ­ (enthalpy of reactants)
Standard enthalpy ­ Heat content of a system measured at 100kPa and 298K (Standard
conditions) . Cannot be measured directly so must measure the enthalpy change.
Standard enthalpy of reaction ­ The enthalpy change for the number of moles of reactants as
show in the reaction equation, measured at standard conditions with the substances in their
standard states. The solutions should have concentration 1.00moldm-3
Standard enthalpy of formation ­ The enthalpy change when one mole of a compound is made
from its constituent elements with reactants and products in their standard states, measured at
standard conditions.
Standard enthalpy of combustion ­ The enthalpy change when one mole of a compound is
completely burned in oxygen with all compounds in their standard states measured at standard
Standard enthalpy of neutralisation ­ The enthalpy change per mole of water formed when an
acid is neutralised by an alkali under standard conditions.
Standard enthalpy of atomisation - The enthalpy change when the atoms in one mole of the
substance are separated completely. (i.e. to make it go from solid to gas)
Hess' law ­ The total enthalpy change of a reaction is independent of the route taken as long as
the initial and final conditions are the same.
Activation energy- The minimum energy the reactants require to react. It is always positive
(endothermic) as energy is absorbed to break bonds.
Bond dissociation enthalpy ­ The enthalpy change when one mole of covalent bonds is
dissociated into gaseous atoms. Always positive (endothermic) as energy is taken in by the system.
Mean bond dissociation enthalpy ­ The value of bond dissociation enthalpy averaged across
many compounds containing that bond. Only approximately correct.
An acid neutralised by an alkali is always an exothermic reaction (negative enthalpy change)
H+ + OH--> H2O
This reaction happens with all strong acids so they all have similar enthalpies of neutralisation.
However with weak acids where they do not fully dissociate so many different molecules and ions
are left in solution for different acids, giving different enthalpies of neutralisation.
Weak acids have less exothermic EON because energy is needed to break the O-H bond when the
acid does not fully dissociate.

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Thermometric titration ­ Finding maximum temp in acid + alkali reaction
Hydration of anhydrous copper sulphate
Cannot measure this equation easily
4+ -> CuSo
So use 2 enthalpy changes
CuSO aq -> CuSO
4+ 4 (aq)
Also make CuSO4.
H +aq -> CuSO
2O 4
The draw both arrows down to CuSO4 the
a Hess' cycle.…read more


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