L3 Energy Balance Reactive System
Transcript of L3 Energy Balance Reactive System
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Energy Balance-
in Reactive System
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LEARNING OBJECTIVES
By the end of this topic, you should be able to:
Highlight previous lectures
Heat of Formation
Heat of Combustion Performed energy balance for reactive system
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ENERGY BALANCES ON REACTIVE PROCESSES
Previous lecture
Heat of reaction
r = product - reactant
EXO: r (T, P): -ve and ENDO: r (T, P): +ve
r nearly independent of pressure r (T)
Value depends on the stoich. Equations
Value of heat of reaction dep. on the phases of
reactants and products
Standard heat of reaction or - reactants and
products at a specified reference temperature and
pressure
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ENERGY BALANCES ON REACTIVE PROCESSES
*
Hesss Law
* General statement of Hesss Law
If the stoich. Equation of for rxn 1 can be obtained byalgebraic operations on stoich. of reaction 2, 3 .. Then
the heat of reaction or 1 can be obtained by performing
the same operations on the heats of reactions or 2,
or 3
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ENERGY BALANCES ON REACTIVE PROCESSES
* Standards heat of formation of = enthalpy changes
when 1 mole of compound formed at a reference
temperature and pressure (in our text 25oC and 1 atm)
* Heat of reaction from heat of formation:
reactants
fi
o
iproducts
fi
o
ifi
o
iir
o
HHHH
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* Standards heat of formation: example to find heat of
reaction from heats of formation
* Heats of Combustion
* Energy Balances on Reactive Process
- General procedures
*
Today lecture
ENERGY BALANCES ON REACTIVE PROCESSES
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EXAMPLE 1
* Determination of heats of reaction from heats of formation
Determine the standard heat of reaction for the following reaction:
Na2CO3 (s) + 2Na2S (s) + 4SO2(g) 3 Na2S2O3 (s) + CO2 (g)
Solution
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* Heats of Combustion
* Standard heat of combustion oC , heat of combustion of substance with
oxygen to yield CO2(g) and H2O(l); both reactants and products are at
25oC and 1 atm.
* Table B.1 lists out the standard heats of combustion
* Tabulated data based on the assumption:
* all carbon forms CO2(g)
* all hydrogen forms H2O(g)
* all sulfur forms SO2(g)
* all nitrogen forms NO2(g)
* Example from Table B.1
ENERGY BALANCES ON REACTIVE PROCESSES
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Heat of combustion for glycerol = ?
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* Heats of Combustion (cont.)
* Can be used to determine heat of reaction - when involve only
combustible substances and combustion products
* To determine the heat of reaction, use the tabulated data for heats
of combustion
products
ici
reactants
iciic
i
ir )H()H()H(Hoooo
ENERGY BALANCES ON REACTIVE PROCESSES
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* Calculation of a Heat of Reaction from Heats of Combustion
Calculate the standard heat of reaction for the dehydrogenation of ethane:
C2H6 C2H4 + H2
SOLUTION From Table B.1
* To determine the heat of reaction, use the tabulated data for heats of combustion
(oc )C2H6 = -1559.9 kJ/mol
(oc )C2H4 = -1411.0 kJ/mol
(oc )H2 = -285.84 kJ/mol
EXAMPLE 2
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kJ/mol136.93H
kJ/mol285.84)(1410.99)(1559.9H
)H()H()H(H
r
r
prod ucts
ici
reactants
iciic
i
ir
o
o
oooo
EXAMPLE 2
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One principal application, determine heat of formation forcombustible substance whose formation reaction do not occur
naturally:
5 C (s) +6 H2 (g) C5H12(l) : o
f= ?
* (of)C5H12(l) can be determine by using the heat of combustion
(experimental) of carbon, hydrogen and pentane:-
(of ) C5H12 (l) = 5 (o
c ) C (s) + 6 (oc ) H2 - (
oc ) C5H12 (l)
ENERGY BALANCES ON REACTIVE PROCESSES
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Procedures
Generally it is the same as the procedures used in solving the energy
balances for the non-reactive system:
- FLOWCHART!
- MATERIAL BALANCES TO DETERMINE STREAM COMPONENTS
- REFERENCE STATES
- INLET-OUTLET ENTHALPY TABLE
- CALCULATE H
- SOLVE THE ENERGY BALANCES BY INCORPORATING THE HEAT
OF REACTION INTO THE EQUATION
ENERGY BALANCES ON REACTIVE PROCESSES
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ENERGY BALANCE EQUATION
Generally, the energy balance equation for any process unit is
given as:
Input + Generation - Output - Consumption = Accumulation
Enters
through
system
boundaries
Produced
within
system only
for reactive
system (+) or
(-)
Leaves
through
system
boundaries
Consumed
within
system only
for reactive
system
Buildup within
system only for
transient operation
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CONCLUSION
You have learn:
Calculate heat of reaction using heat of formation and heat of
combustion.
Energy balance Equation for reactive system