energy balances on reactive process

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.

energy balances on reactive process

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