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State Hess Law: Explain The Usefulness Of Hess Law In Thermochemistry

Referencing Styles : Harvard | Pages : 1

The law of summation of constant heat is also known as the law of Hess’s or Hess’ law. The law is about the physical chemistry. The law was named after and introduced by the famous scientist Germain hess. Germain Hess was born in the country Switzerland. Hess was mainly a Russian chemist. His expertise was expanded to the field of Physics too. The publication of the law was first came out in the year 1840. The law explains that the change of total enthalpy during the full course of a chemical reaction does not depend on the steps of the reaction. The change of total enthalpy during a reaction done in several steps is equal to the enthalpy change of a reaction that is done by using only one-step. After studying the Hess’s law, it can be understood that the law is an extended version of the energy conservation principles. The law can also be expressed by the help of the first law of thermodynamics. Chemical processes that include enthalpy, which is completely independent, form the path that the initial state take to reach the final state. Sometimes the enthalpy is refereed as the state function. In numerous reaction, with the help of the calorimetry, the change in the enthalpy can be determined. Some reactions have the same temperature values in initial states and in the final states including the pressures and the values are asserted for those particular processes. The condition of the reactions can vary in every reaction. The total energy required for a chemical reaction to be processed can be determined by the Hess’s law. The whole process can be separated in to steps that are synthetic thus, the characterization becomes very easy.

Several different routes can be used for a chemical change to take place. The total change of the overall enthalpy is same regardless the steps involved or the routes used in the chemical reaction. The initial state and the final state of the reaction should be same. In case of some reactions where the change of enthalpy cannot be calculated or measured directly, with the help of the Hess’s law the calculation can be done directly and easily. In the law defined the Germain hess, the enthalpy change is defined the term ΔH. Every reaction has some chemical equations involved in it and after performing the basic operations of algebra, the change of enthalpy can be measured. Values that were previously determined is used while determining the values during formation of enthalpies. The entropy involved in a chemical reaction has an important role while determine the spontaneity of a reaction. If we know the enthalpy change of the whole equation then it can be compared with the summation of enthalpy change of several small reactions. The enthalpy change in the small equations need to be known to compare the enthalpy change. After the enthalpy has been measured, if the value of the enthalpy is in negative then it can be concluded that the particular reaction is spontaneous and the particular reaction can be expressed as an exothermic reaction. Similarly, if the value of the enthalpy change is in positive, then it can be concluded that the selected reaction is not spontaneous and the reaction can be defined as the endothermic reaction. The additive nature of the enthalpy is stated and explained by the Hess law.

The following section is the detailed discussion of an example of Hess law.

Let us discuss a simple scenario to explain the law. Assume a person has to go to place which is 3 kilometers away. There are several ways to go there. 1) The person can travel directly 3 kilometers and reach the place, 2) The person can travel 1 kilometer then wait for sometimes and then again travel rest of the 2 kilometers, 3) The person can travel 2 kilometers and then later travel the rest 1 kilometer, or finally 4) the person can take a break at every kilometer and reach the destination. Hence, no matter how the person is reaching the destination, the total distance traveled by the person is same.  Now let us look at an example reaction through which the conservation of the total enthalpy change can be explained in details.

Carbon can produce the element carbon dioxide in two ways, which are different. One of the method is with the help of combustion. Excess amount of oxygen combusts with the carbon and in result 393.5kj amount of heat get released form per carbon mole. The equation of the reaction is –

C(s) + O2 (g) → C1O2 (g) ΔH = -393.5 kJ

From the above reaction we can see that carbon and oxygen is reacting together to form carbon dioxide along with releasing some heat. This whole procedure can be done in two steps or stages. In the first step or stage of the process, we use the required carbon to react with the oxygen, which is provided in limited amount. When the carbon reacts with the limited amount of the oxygen, the result of the reaction is the production of carbon monoxide. The equation of the reaction is-

C(s) + ½O2 (g) → CO (g) ΔH = -110.5 kJ

The above reaction produces carbon monoxide and the symbol of the same is CO. The reaction also release -110.5kj amount of heat thus the process is spontaneous. The next stage of the reaction consists of carbon monoxide and the additional amount of oxygen. Here the reaction is being completed with the help of combustion. The equation the second stage of the reaction is-

CO(g) +1/2O2 (g)  → CO2(g)     ΔH = -283.0 kJ

It can be interpreted from the above reaction that with the presence of oxygen, carbon monoxide produces carbon dioxide along with release of some heat. The amount of the heat is -283.0 kJ. When we add these two values of the heat, we can find the total release of heat or enthalpy change (ΔH). The sum is {-110.5 kJ + (-283.0 kJ)} = -393.5 kJ. Hence, from the addition it is clear that the total change of enthalpy in the two reactions is exactly same when the reaction was done in only one-step.


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