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Enthalpy
· The heat (energy) content of a system
· Enthalpy change (H); the heat energy change
measured under conditions of constant
pressure
· H = Hp ­ Hr
· Subtracting the total enthalpy of the reactants
from the total enthalpy of the products…read more

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Enthalpy cont.
· Exothermic reactions release heat energy. H
is always negative
· Endothermic reactions absorb heat energy.
H is always positive
· Standard conditions; 100kPa and a stated
temperature (normally 298K)…read more

Slide 4

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Standard conditions
· Value of enthalpy change is dependant on..
· The temperature and pressure
· The amount of substance used
· The value, in KJ, is always given per mole of
substance…read more

Slide 5

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Enthalpy of formation/combustion
· Enthalpy of formation; the enthalpy change
when one mole of a compound is formed from
its elements in their standard states
· By definition, the standard enthalpy of
formation of an element in its standard state
is zero
· Enthalpy of combustion; the enthalpy change
when one mole of a substance is burned
completely in oxygen under standard
conditions…read more

Slide 6

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Calculating enthalpy change
· Enthalpy changes are measured using a
calorimeter, which insulates the reaction
system from its surroundings
· q=mcT
· Specific heat capacity; the heat energy
required to raise the temperature of 1g of a
substance by 1K
· For pure water, the specific heat capacity (c) is
4.18…read more

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