Heat in Chemistry

While working on a few chemistry problems I came across mild difficulty in finding the sign of heat. Heat is the transfer of energy that results from the difference in temperature between a system and its surroundings. At a molecular level, heat is the transfer of energy that makes use of or stimulates disorderly molecular motion in the surroundings. For instance, when a hydrocarbon fuel burns, the energy released in the reaction stimulates the surrounding atoms and molecules into more vigorous random motion, and we refer to this escape of energy as heat. Heat is not stored: Heat is energy in transit. So how do we solve for heat?

 

Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the system‘s energy exchange with itssurroundings. Thermochemistry is useful in predicting reactant and product quantities throughout the course of a given reaction. In combination with entropy determinations, it is also used to predict whether a reaction is spontaneous or non-spontaneous, favorable or unfavorable.

Endothermic reactions absorb heat and exothermic reactions release heat. The measurement of quantities of energy transferred as heat is called calorimetry. Such a measurement is commonly made by observing the rise in temperature caused by the process being studied and interpreting that rise in terms of the heat produced. Calorimetry is used to measure the changes in internal energy and enthalpy that accompany chemical reactions.

The sign conventions for heat, work, and internal energy are summarized in the figure below. The internal energy and temperature of a system decrease (deltaE < 0) when the system either loses heat or does work on its surroundings. Conversely, the internal energy and temperature increase (deltaE > 0) when the system gains heat from its surroundings or when the surroundings do work on the system.

diagram

Our first law of thermodynamics says Esys=q +w

Esys stands for E system which is the total Energy for the system. Q stands for heat inputted or released and W for work done on the system.

The system is usually defined as the chemical reaction and the boundary is the container in which the reaction is run. In the course of the reaction, heat is either given off or absorbed by the system. Furthermore, the system either does work on its surroundings or has work done on it by its surroundings. Either of these interactions can affect the internal energy of the system. Heat is found on the most basic step by examining the above formula given a specific amount of work and Esystem. The sign of heat corresponds to that of the equation. One can also find ways to make a problem using the equation above more complicated when we tackle problems with constant volume and pressure where we might have to use enthalpy (H).

The amount of heat generated by a reaction can be quantified. The quantity that specifies the amount of heat is known as the “enthalpy” . If is positive then the reaction is endothermic and if it is negative the reaction is exothermic. For example, consider the combustion of methane:

This equation tells us that the combustion of methane is exothermic and releases 883 kilojoules of energy (in the form of heat) for every mole of methane (CH4). Given this information we can determine how much heat will evolve if we burn a certain mass ofmethane: For example: Suppose we combust 20 g of methane, How much heat will evolve (in kJ)? The procedure is the usual stoichiometry:

Read more at: http://www.chem.tamu.edu/class/majors/tutorialnotefiles/thermo.htm

So we found what heat is and how to find heat quantitatively. Enjoy!

Nnana Amakiri

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