by Maryam Hussain
The heat of formation:
The enthalpy of formation, also
known as the heat of formation, of a compound is the heat involved in
its formation from its constituent elements. The enthalpies of the formation
of elements are assumed to be zero. Thus, for the special case of the heat
reaction of a chemical reaction, the heat of formation, and enthalpies of each
of the terms on the left-hand side of the chemical equation are zero, and the
heat of reaction becomes equal to the heat of formation of the given compound.
The standard heat of formation
of a chemical compound is the enthalpy change when one g-mole of a specie at
25°C and 1 atm is formed from its elements in their naturally occurring states.
Heats of formation of compounds are always expressed with reference to a
standard state.
The molal heat of the
formation of a compound represents the heat of reaction, ΔoHf
(kJ/mol), when 1 mole of the compound is formed from the elements in a reaction
& the reaction beginning and ending at 25°C and at a pressure of 1
atmosphere with the reacting elements are in the stable states at these
conditions of temperature and pressure.
The heat of the formation of a
compound is positive when its formation from the elements is accompanied
by an increase in enthalpy of the formed compound. If the heat of the formation
of a compound is positive, it is called an endothermic compound. In the
same way, a compound whose heat of formation is negative is called an exothermic
compound.
The heat of reaction:
The heat of reaction, ΔHr, is the
enthalpy change for a chemical reaction in which stoichiometric quantities of
reactants at temperature T and pressure P react completely (here completely
means 100% conversion of reactants into products) to form products at the same
temperature and pressure.
The standard heat of reaction,
denoted by ΔHro, is defined as the difference between the
enthalpies of the products in their standard states and of the reactants in
their standard states, all at the same temperature; the standard conditions
being 25°C and 1 atm.
Consider the reaction
A + B → R + S
For this reaction, the heat of the reaction may be
calculated as
ΔHr = ΔHfR + ΔHfS - ΔHfA - ΔHfB
The heat of reaction is dependent
not only on the chemical nature of each reacting material and product but also
on their physical states. To define a heat of reaction, it is necessary to
specify completely the nature and physical state (solid, liquid, gas) of
each material involved.
The formula of a substance
appearing in an equation designates not only the nature of the substance but
also the quantity which is involved in the reaction. Thus, H3PO4
indicates 1 mole of phosphoric acid, and 1.5N2 indicates 1.5 moles
of nitrogen.
With an irreversible reaction,
all equations are written with the reactants on the left and the products on
the right side. The value of the heat of reaction accompanying an equation
is the heat of reaction when the reaction proceeds from the left to the right
of the equation as written. If the reaction proceeds in the reverse
direction the heat of the reaction is of opposite sign.
There are two methods of
expressing heating values and these differ in the state selected for the water
present in the system after combustion.
The total heating value or
higher heating value or gross calorific value of a fuel is the heat evolved in
its complete combustion under constant pressure at a temperature of 18°C when
all the water formed and the water originally present as a liquid in the fuel
is condensed to the liquid state.
The net heating value or
lower heating value or gross calorific value is similarly defined except that
the final state of the water in the system after combustion is taken as vapor
at 18°C. The net heating value is obtained from the total heating value by
subtracting the latent heat of vaporization at 18°C of the water formed and
vaporized in the combustion. Recent methods for determining any of these
heating values, operate the system at 1-atmosphere pressure and 25 oC Celsius
temperature.
The relative energies of the
products to the reactants determine whether the chemical reaction is an
exothermic reaction (gives off heat to its surroundings) or an endothermic
reaction (absorbs heat from its surroundings).
In a reaction between molecules, energy
is required to break the reactant chemical bonds, and energy is released when
the product bonds form. If the energy required to break the reactant
chemical bonds is lesser than the energy when the product bonds are formed
& energy is released (as in the water formation reaction), the reaction is
called an exothermic reaction. In an exothermic reaction, the product
molecules, at a given temperature and pressure, have lower internal energies
(and hence lower enthalpies) than the reactant molecules at the same
temperature and pressure.
Note: If a chemical reaction
is highly exothermic, the released heat can lead to unsafe temperature
rises. Diluting the reactants with a carrier gas such as nitrogen is one way of
controlling the resulting temperature increase.
The heat of combustion:
The heat of combustion and the
heating value of a fuel is the same in the definition. The heat of
combustion may be reported as either net heating value (lower heating value,
LHV) or gross heating value (higher heating value, HHV).
To differentiate between the two,
reconsider the chemical equation for methane combustion. Burring of methane
produces two products: CO2 and H2O. The CO2
will remain a gas under all industrial combustion conditions. However, H2O,
and alike compounds, can exist as either a liquid or a vapor, depending on how
much heat is extracted from the process. If so much heat is removed from the
system, while the system is maintained at 25 oC, that the H2O
condenses, then the combustion yields its HHV. If water is released from the
stack as a vapor, then combustion yields the LHV at 25 oC.
The process industry usually uses
the LHV. Boiler and turbine calculations usually use the HHV.
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