l3 energy balance reactive system

8/10/2019 L3 Energy Balance Reactive System

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Energy Balance-

in Reactive System

1


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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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*

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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* 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



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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



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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



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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)



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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



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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

l3 energy balance reactive system

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