Types of Chemical Reactions Types of Chemical Reactions A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. ] Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity. Classically, chemical reactions encompass changes that strictly involve the motion of electrons in the forming and breaking of chemical bonds between atoms, and can often be described by a chemical equation. The general concept of chemical reactions between atoms has been be extended to the nonchemical reactions between entities smaller than atoms, including nuclear reactions, radioactive decays, and reactions between elementary particles as described by quantum field theory. The substance (or substances) initially involved in a chemical reaction are called reactants or reagents. Chemical reactions are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants.
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Reactions often consist of a sequence of individual sub-steps, the so-called elementary reactions, and the information on the precise course of action is part of the reaction mechanism. Chemical reactions are described with chemical equations, which graphically present the starting materials, end products, and sometimes intermediate products and reaction conditions. Different chemical reactions are used in combination in chemical synthesis in order to obtain a desired product. In biochemistry, series of chemical reactions catalyzed by enzymes form metabolic pathways, by which syntheses and decompositions impossible under ordinary conditions are performed within a cell. Writing a Chemical Reaction A chemical reaction is written with the formula of reactants on the left side, and formula of the products formed on the right. An arrow is placed in-between the reactants and the products. Example:1 In a reaction between Hydrochloric acid (HCl) and sodium hydroxide (NaOH), the chemical equation is written as: HCl + NaOH → H2O + NaCl An arrow separates the reactants from the products. The arrow is usually pointed in the direction in which it is proceeding. If a reaction is reversible, the middle arrow is shown as double arrows: (→ ←).
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Since each chemical reaction follows law of mass action, the moles of a particular element will be same throughout. If 2 moles of chlorine are reacting, 2 moles of chlorine would be present, in any combined form in the products. Example:2 In the reaction between nitrogen and hydrogen to form ammonia, 2 moles of N2 react with 3 moles of H2 to form 2 moles of NH3. This is shown in the form of equation as: 2 N2 + 3H2 ⇌ 2NH3 So, every chemical equation has to be solved or balanced for the moles of reactants reacting and moles of products formed. Limitation of a Chemical Equation There are certain limitation of chemical equations as follows : 1. Chemical equation does not clarify the state of the substances. So (s) for solid, (l) for liquid, (g) for gas and (vap) for vapor may be added. 2. The reaction may or may not be conclude but the equation does not reveal it. 3. Chemical equation does not give any information regarding the speed of the reaction. 4. Chemical equation does not give the concentration of the substances and in some cases, the terms like diluted and concentrated are used.
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