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Thermochemistry is a branch of chemistry that deals with the interrelation of heat with chemical reaction or physical change of state. Generally, either physical and chemical reactions occur through the release or the absorption of heat from the surrounding. The surrounding can include the entire universe or within a a system.

The law of conservation of energy explains that every substance stores a certain amount of energy, which cannot be destroyed or created. Also the "enthalpy change of the first chemical equation equals the sum of the enthalpy changes of the other chemical equations if a chemical equation can be written as the sum of several other chemical equations". This is called Hess' Law . [1]

Important Terms

Endothermic and Exothermic process

In thermochemical calculations, the direction of flow the heat is given with the point view of the system. An endothermic process is the system which the substances gains heat while the surrounding cool down. An Exothermic process is the system which the substance lose its heat while the surrounding heat up. For example, while exercising heat will flow out from the body and goes into the surrounding, which is an exothermic process.

This is the example of exothermic process. The sun gains heat from itself, and then flow out the heat to its surround. Thus, we get the sunshine.


During the reaction among the substances, heat often occurs, and throughout the heat, the energy transfers between a system and its surroundings. Heat is not conserved,which means it can be destroyed or created by the surrounding temperature. One of the major unit of the measuring heat is calories, which requires to raise the temperature of one gram of water from 14.5ºC. Also, heat uses the unit of joule in the SI system. The heat captivity explains raising the temperature of a defined amount of pure substance by the temperature scale of Celsius or Kelvin. [2]


The terms of Temperature is the system of the measuring of how hot and cold the substance is. There are several scales of measuring temperature; the Kelvin scale, the Celsius (ºC) and Fahrenheit (ºF). The Celsius and Fahrenheit scales are two of the scales which measure the standard temperature such as weather and the temperature of water. The United States uses the Fahrenheit scale as the main measuring scale for the weather temperatures. Otherwise, the Kelvin scale is used for measuring the absolute temperatures, as an example, comparing the temperature at the volume and pressure of an ideal gas conclude to zero. Temperature has an intensive property which does not matter of volume of substance. For example, you have a cup of water which has 20ºC, and then, you doubled it and measured the water temperature again. The water temperature would not be changed unless the surrounding temperature changed.[3]


You can determine how much of work occurred during the reaction from the product of the force used to move an object multiply by the distance the object is moved.


There are two kinds of energy systems; enthalpy and internal energy system. The enthalpy is defined as the sum of the total amount of the energy system and the product of the pressure if the gas in the system multiply by the volume of the product. You can get the change in the enthalpy of the system from the enthalpy of the final state minus the initial state of the system. [4]

  • Enthalpy ( H) is the amount of heat content.

-Heat content is accounted for by a change in "heat flow" or enthalpy of the reaction system.
1. Endothermic reaction: H > 0
-Heat is absorbed into the substance in this reaction.
2. Exothermic reaction: H < 0
- Heat is released from the substance in this reaction.


A calorimetric analysis is one of the method of examining the energy exchange between the reaction and its surrounding. A calorimeter measure the energy exchange between the system and its surrounding when they reacts. The chemical or the physical reaction occurs within the confines of the calorimeter.
Equation: The Qsurr = Qcal + Qcontents


this if the machine which measure the energy of the substance. This is a huge size of calorimeter.
This is a Liquid Argon Calorimeter. The calorimeter is containing the argon in a liquid state at -185ºC which is surrounded by a cryostat. Flowing Protons and electrons through the lead plates, some of the energy is transformed into the particles and spread in the form of a shower, so revealing their presence.