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In physics, you must have learned that Energy is the ability to do work. Energy is not only important in Physics but also important in other Science related disciplines. Various forms of energy include heat, light, sound, electrical, potential, kinetic, mechanical, and nuclear energy. It is also important to recall that energy can also be converted from one form to another and examples include:

  • Conversion of Chemical energy into electrical energy- as in battery cells used in radio, torchlight, and clock
  • Conversion of electrical energy into heat energy as in electrical pressing iron

Thermodynamic Terms

Thermodynamics means the flow of heat energy and some important terms that one must acquaint himself with include the following:

  • System: A system is the material or materials(or reactants) under study. It can be in a simple apparatus, such as a test tube, or as complex as a refinery.
  • Surrounding: The surrounding is that portion of matter which is not under study, but can exchange materials with the system.
  • An isolated System: An isolated system does not exchange heat energy and matter with its surrounding. For instance, when a reaction is conducted in a vacuum flask or lagged container, it becomes an isolated system.
  • An Open System: This is a system that exchanges both matter and heat energy with its surrounding, e.g a reaction conducted in a test tube or a beaker is an open system.
  • Closed System: This is a system that exchanges only heat with its surrounding. For instance: a reaction involving the collection of a gas over water is a closed system.

Effects Of Energy In Chemical Reactions

Under the effects of energy in Chemical reactions, there are three topics we would discuss and some of them include heat content/enthalpy.

Heat Content

Every chemical substance has its own energy called the heat content or enthalpy represented by H. The heat content or enthalpy H, of any substance cannot be determined experimentally; Only the enthalpy change that accompanies a physical or chemical change can be determined. Enthalpy change is represented by ΔH. Mathematically, enthalpy change is the difference between the heat content of products ΣH(products) and heat content of reactants ΣH(reactants).

Enthalpy Or Heat Change ΔH

A chemical or physical process is either accompanied by the absorption or evolution of heat. The enthalpy change, ΔH is the heat absorbed or evolved during the course of a chemical reaction or physical process, at constant temperature and pressure. The enthalpy change is usually recognized by a rise or fall in the temperature of the system.

Exothermic & Endothermic Reactions

The two types of energy changes that accompany physical and chemical processes are exothermic and endothermic changes. In an exothermic change, heat is liberated by the reactants to the surrounding which leads to a rise in the temperature of the system, and loss of heat by the reactants to the surrounding.

In an endothermic reaction, heat is absorbed by the reactants from the surrounding. It leads to a fall in the temperature of the system and corresponds to a gain of heat by the reactants from the surrounding.

Signs Of Enthalpy Changes

The sign assigned to the enthalpy change ΔH of a reaction is based on the first law of thermodynamics called law of conservation of energy which states that energy is neither created nor destroyed during a physical or chemical process, but can be converted from one form to another.

ΔH = ΣH(products) – ΣH(reactants)

Case I: The head content of the products is more than that of the reactants i.e

ΣH(products) > ΣH(reactants)

Consider a hypothetical reaction:
A B + C

Given that

ΣH(reactants) = 45.0kJ

ΣH(products) = 70.0kJ

Enthalpy change is given as: ΔH = ΣH(products) – ΣH(reactants)

Substituting the values, ΔH = (70.0 – 40.0)kJ = +30.0kJ

The answer means that when reactant A absorbs 30.0kJ of heat from the surrounding, it decomposes to give B and C.

Factors That Affect Enthalpy Change

The following factors determine the value of ΔH of a reaction:

  • Physical states of reactants and products
  • Quantity or concentration of reactants
  • Temperature
  • Pressure

Types Of Enthalpy Change

There are specific names given to enthalpy changes accompanying certain types of physical and chemical changes:

  • Standard Heat of formation
  • Standard Heat Of Combustion
  • Heat of Neutralization
  • Molar Heat of Vapourization
  • Molar Heat of Fusion
  • Heat OF Hydration
  • Molar Heat Of Solution
  • Heat Of Dilution
  • Heat Of Transformation
  • Heat Of Atomization
  • Heat Of Sublimation
  • Enthalpy Heat of Reaction

Entropy Change

The Entropy s of a system is defined as the degree of randomness or disorderliness of the system. Like H, entropy S, is a state function meaning it cannot be determined experimentally. Entropy is measured in Joules per Kelvin per mole(JK-1mol-1). Its value increases as order decreased or disorder increases.

Mathematically, Entropy Change, ΔS = S(products) – S(reactants)

If the entropy of products S(products) is greater than the entropy of reactants, S(reactants) then the entropy change, ΔS is positive, and vice versa.

Before we conclude, we would talk about Gibbs Free energy which is usually denoted by G. Gibbs free energy is the available energy for doing work. The heat content or enthalpy, H of a chemical substance is composed of two portions. That portion that is available for doing work is called Gibbs free energy. The remaining portion, represented by the quantity q, is used n increasing the entropy S.

Mathematically H = G + q= G + TS

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