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8/10/2019 Energy Balances on Reactive Process
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8/10/2019 Energy Balances on Reactive Process
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Heat of reaction = amount of heat that must beadded or removed during a chemical reaction.
Enthalpy = amount of heat content used or released
in a system at constant pressure.
Enthalpy is expressed as the change in enthalpy.
The change in enthalpy is related to a change ininternal energy (U) and a change in the volume (V),which is multiplied by the constant pressure of thesystem.
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Heat of reaction can be calculated from heat of
formation data.
Hr= ( Hfof products)( Hfof reactants)
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H andHrxn
= represents the change in the enthalpy;(Hproducts-Hreactants)
a positive value indicates the products have
greater enthalpy, or that it is an endothermic
reaction (heat is required)
a negative value indicates the reactants have
greater enthalpy, or that it is an exothermic
reaction (heat is produced)
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Internal energy of reaction,Ur
Internal energy occur when the reaction takes place
in a closed reactor at constant volume.
U r = H rRT( gasproductvi - gas reactntvi )
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There are two ways to calculate H (enthalpy) : By experimentally(using calorimeter)
By numerically
H= vp Hf(products) - vr Hf
(reactants)
Where:
Vp= stoichiometric coefficient of the product fromthe balanced reaction
Vr= stoichiometric coefficient of the reactant fromthe balanced reaction
Hof = standard enthalpy of formation for thereactants or the products
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EXAMPLE:
Calculate the value of Ho for the reaction:2 S (g)+ 2 OF2 (g) SO2 (g)+ SF4 (g)
When
OF2 (g)+ H2O(l) O2 (g) + 2HF(g) Ho = -277 kJSF4 (g)+ 2 H2O (l) SO2(g) + 4HF(g) Ho = -828 kJS(g)+ O2 (g) SO2 (g) Ho = -297 kJ
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SOLUTION:
( OF2 (g)+ H2O (l) O2 (g)+ 2HF(g) Ho = -277 kJ ) x 2(SF
4 (g)
+ 2 H2
O(l)
SO2 (g)
+ 4HF(g)
Ho = -828 kJ ) FLIP( S(g)+ O2 (g ) SO2 (g) Ho = -297 kJ ) x 2
2 OF2 (g) +2H2O (l) 2 O2 (g) + 4 HF (g)SO2 (g) + 4HF (g) SF4 (g) + 2 H2O (l)
2 S (g) + 2 O2 (g) 2 SO2 (g)
2 S (g)+ 2 OF2 SO2 (g) + SF4 (g)
Ho = -277 kJ x 2 = -554 kJHo = -828 kJ + 828 kJHo = -297 kJ x 2 = -594 kJHo = -320 kJ
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Formation reactions are chemical reactions that form
one mole of substance from its constituent elements
in their standard states.
Heat formation = the measure of energy released or
consumed when one mole of substance in standard
state ( 1atm and 298.15 K ) is formed from its pureelements under the same conditions
H= ( Hf) (products) - ( Hf) (reactants)
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Draw and labela flowchart
Use material
balances and phaseequilibrium
relationship suchas Roult's Law to
determine as manystream componentamounts as flowrate as possible
Choosereference states
for specificenthalphycalculations andprepare and fillin an inlet-outletenthalphy table
Calculate H,subtitute the
calculated valuein the appropriate
form of theenergy balances
question andcomplete the
requiredcalculation
Two methods arecommonly used to
choose reference statesfor enthalphy
calculations and to
calculate specificenthalpies and H.
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Standard heat of combustion of a subtance Hc
isthe heat of combustionof that subtance with oxygen
to yield specified products at standard condition(25Cand 1 atm )
HC = [Hc (reactants) ] [Hc(products)]
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The standard heat of reaction
C2H4(g)+ 2 CI2(g) C2HCI3(l)+H2(g)+HCI(g)is
H r = -420.8kJ/mol. Calculate U r for thisreaction.
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