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Enthalpy of Formation of Carbon Dioxide

May 6, 2013

Enthalpy is a term used in thermodynamics.

It is also termed as enthalpy change of fusion.

It is equal to the sum of the internal energy of the thermodynamic system plus the product of its volume multiplied by the pressure exerted on it by its surroundings.

H = E + PV

where H is enthalpy,E is internal energy,P is pressure and V is volume.

Thus it can be seen that enthalpy is dependent on pressure,temperature and factors of surroundings too.

Consider a closed thermodynamic system which is undergoing change.

The law of conservation of energy, the change in internal energy is equal to the heat transferred to the system minus the work done by the system.

dE =HeatTransferred – Work done.

In other words enthalpy can be defined as sum of the capacity to do non-mechanical work and the capacity to release heat. From the above discussion it can be deduced that it is impossible to measure enthalpy directly however enthalpy can be measured indirectly as its change dH.

Enthalpy of formation of CO2

Standard enthalpy of formation is also called as standard heat of formation.

It is measure of change of enthalpy when one mole of the substance is formed from its constituients under STP.

The standard temperature is 25 degree Celcius and the standard pressure is 1 bar.

It is represented by ΔHfO.

Let us consider formation of carbon dioxide.

When benzene is burnt in air, carbon dioxide is formed.

The balanced reaction is written as under.

2 C6H6(l) + 15 O2(g) → 12 CO2(g) + 6 H2O(l) .

The enthalpy of above reaction is  is -6535.1 kJ/mol.

12 xStandard enthalpy of formation of carbon dioxide + 6 x enthalpy of formation of water-[2 x enthalpy of formation of benzene -15 x enthalpy of formation of oxygen]= enthalpy of above reaction.

But we know that enthalpy of formation ofelements is zero.

So enthalpy of formation of oxygen=0

12 x Standard enthalpy of formation of carbon dioxide=enthalpy of above reaction-6 x enthalpy of formation of water + 2 x enthalpy of formation of benzene

12ΔHfC02=ΔHreaction –6ΔHfwater +2ΔHfbenzene

= -6535.1+1714.98 + 98.12

12CO2 = -4721.9
CO2 = -393.5 kJ/mol

So this is an exothermic reaction accompanied by release of energy.

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