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Home » Chemistry Homework Help » Physical Chemistry » Enthalpy, Chemical Reactions
Enthalpy, Chemical Reactions
A chemical reaction changes the enthalpy of a chemical system.

Some elaboration of the equations that we write to describe chemical reactions is necessary when we deal with the energy changes that accompany these reactions. The energy change that accompanies a reaction depends on whether each of the reagents is a solid, liquid or gas. The state of the reagents is usually specified by adding s, l, or g respectively, after the formula. Occasionally a more careful description is necessary. For example, when carbon is involved in a reaction, it is necessary to a state whether it is graphite or diamond. One must be especially careful with water, which can reasonably be involved as a gas or a liquid.

Another characteristic of equations written for use in the energy calculations is the appearance of fractional numbers as coefficients in the equation. If, for example, we were interested in the energy change for the reaction in which 1 mol of water molecules is formed from the elements, we would write

H2(g) + 1/2 O2 (g) H2O(l) ΔH = -285.83 kJ

If the reaction were written with integers to show the formation of 2 mol of water molecules, twice as much energy would be delivered to the thermal surroundings for the amounts implied by the equation. Then we would write

2H2(g) + O2(g) 2H2O(l) ΔH = -571.66 kJ

Combustion reactions are usually written for the combustion of 1 mol of material. For benzene, for example, one would usually write

C6H6(g) + 15/2 O2(g) 6CO2(g) + 3H2O(l) ΔH = -3301.51 kJ

Unless other information is given, the indicated energy changes apply to the other reaction occurring at the standard pressure of 1 bar and a temperature of 25˚C (298.15 K). These conditions are often indicated by a superscript 298, suggesting the temperature value, and a degree sign implying the standard pressure. With these indications we would write, for example

H2(g) + 1/2 O2(g) H2O(l) ΔH˚298 = -285.83 kJ

Indirect determination of enthalpy change in chemical reactions:

Many reactions are not suitable for direct calorimetric study. The internal-energy or enthalpy changes can often be obtained by an indirect method. This indirect procedure was originally suggested by Hess in 1840, and it is often known as Hess’s law of heat summation. We now recognize that it is merely an application of the first law of thermodynamics.

The indirect determination of the enthalpy change can be illustrated with the reaction in which carbon is converted from graphite to diamond, i.e.

C(graphite) C(diamond) ΔH = ?

Although this reaction can be made to occur, it is certainly very unsuitable for any direct calorimetric study.

The combustion of both graphite and diamond can be conveniently studied and these reactions and the enthalpy changes for the combustion are

C(graphite) + O2(g) CO2(g) ΔH˚298 = -393.51 kJ

C(diamond) + O2(g) CO2(g) ΔH˚298 = -395.40 kJ


The enthalpy changes of these reactions clearly differ as a result of the differences enthalpies of graphite and diamond, as shown by writing

-393.51 kJ = HCO2 – HC(graphite) – HO2

-395.40 kJ = HCO2 – HC(diamond) – HO
2

Subtraction of these algebraic equations with cancellation of the enthalpies of CO2 and O2 gives, on rearrangement,

HC(diamond) - – HC(graphite) = -393.51 + 395.40 = +1.89 kJ

For the original graphite-to-diamond reaction, we now can write

C(graphite) C(diamond) ΔH = +1.89 kJ


The same result can be obtained by thinking of the common product, here CO2, as an intermediate for the reaction in which we are interested. Then equations are written for which the original reactants form this intermediate, and the equation for which this intermediate produces the products of the original reaction. Here we have

C(graphite) + O2(g) CO2(g) ΔH = -393.51 kJ

And

CO2(g) C(diamond) + O2(g) ΔH = +395.40 kJ

The net result of the two reactions obtained by an algebraic like addition that gives

C(graphite) C(diamond) ΔH = -393.51 + 395.40 = 1.89 kJ

In an alternative procedure, both reactions are written to show the formation of the common intermediate, or intermediates. Then the equations are subtracted to produce the cancellation that gives the equation for the overall transformation to reactants to products.

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