Hess’s law of thermochemistry is one of the most important laws of chemistry for you to understand. The function of this first law is to determine the magnitude of the enthalpy change for a reaction.
Thermochemistry is the study of chemical reactions and the energy changes involved in them. To study thermochemistry, it is necessary to define “system” and “environment”.
The system is everything that is the focus of attention. While the environment is everything that is not included in the system.
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Hess’s Law of Thermochemistry
In thermochemistry there is Hess’s law which is useful for determining the magnitude of the enthalpy change or reaction. The value of the enthalpy change is expressed as a state function (∆H).
According to Hess’s law, the change in a chemical reaction has the same value, even if the steps taken to produce the reaction are different. This is because the enthalpy change is a state function.
Hess’s law emerged from an experiment that Germain Hess conducted in 1840. The formula for this is:
“The amount of heat that a chemical reaction requires or releases does not depend on the course of the reaction, but according to the initial and final states.”
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Factors Affecting Hess’s Law
There are several factors that affect this one chemical law, namely:
- The amount of reacting substance in the calorimeter will determine the heat.
- Temperature or temperature, the higher the temperature, the more the enthalpy change will increase.
- The type of reaction that comes out, whether endothermic or exothermic can affect the enthalpy.
- The solvent, if it has a high boiling point, affects the reactions that occur in the calorimeter.
- The nature of the substance, including whether or not the substance reacts easily will affect the speed at which the reaction takes place. This happens because between the opposite ions there is an attractive force.
- Concentration, in Hess’s law of thermochemistry, the greater the concentration, the greater the likelihood of a reaction occurring.
- Changes in pressure have little effect on the solubility of a liquid or solid in a liquid solvent, but the solubility of gases will always increase.
- Catalyst, this substance is added to the reaction to increase the speed of the reaction.
- Heat, is the heat that affects Hess’s law.
- Stirring and shaking, the more often and quickly, the greater the enthalpy value, both in direction 1 and direction 2.
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Application of Hess’s Law
In practice, we can combine several reactions that have seen enthalpies to obtain other enthalpies.
The reverse of a reaction will result in a change without enthalpy. That is, if the enthalpy goes exothermic it will be opposite to the endothermic reaction.
The sign of the enthalpy of an endothermic reaction will be opposite to the sign of the enthalpy of an exothermic reaction.
In addition, another application of Hess’s law of thermochemistry is to look at the product of the reaction with a number. The results seen later will remain with the same number. (R10/HR-Online)
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