Exothermic and Endothermic Reactions

1.0 - Introduction

A chemical reaction is defined as "a process in which one or more substances, the reactants, are converted to one or more different substances (the products), through the formation and breaking of bonds. A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products" (Britannica, 2013).

The rate of a chemical reaction is dependent on the amount and rate of collisions in the reactants. A higher concentration of the reactants leads to an increasing reaction rate and a lower concentration of reactants leads to a slower reaction rate (About Chemistry, 2013). The temperature also affects the rate of collisions; as stated in the particle theory, the higher the temperature the faster the particles move thus creating more collisions (Sciencelearn, 2008). The rate of a chemical reaction is also dependent on another particle theory rule; pressure. When there is more pressure, particles have less space to move around, increasing the density. A greater density of particles increases the rate of collisions, thus quickening the reaction rate.

Chemical reactions can be classified into two basic types; endothermic and exothermic. The classification of an endothermic or exothermic reaction depends on energy processes going into the chemical reaction. An exothermic reaction occurs when energy is released and an endothermic reaction occurs when energy is stored. Energy can be classified into two basic types; heat & light. The process of heat energy is associated with the temperature change of the substance. If heat energy is released (exothermic) then the reaction's temperature will increase, if heat energy is stored (endothermic) then the reaction's temperature will decrease. If light energy is released (exothermic), the reaction will glow, if light energy is stored (endothermic), the reaction will not show any signs of radiance (Wyzant, 2009).

Examples of exothermic and endothermic chemical reactions are seen in everyday life. Airbags are stimulated by sensors that detect sudden movement in an automobile. The sensor heats NaN3 (sodium azide) located in the airbag and it decomposes (at 300oC). The decomposition of NaN3 (sodium azide) produces an inflation of nitrogen gas. The chemical reaction between NH4Cl (ammonium chloride) and H2O (water) produces an endothermic reaction. The NH4Cl (ammonium chloride) absorbs the heat from the water and cools it (Science Quest 9, 2012).

Chemical reactions are expressed through utilizing chemical equations for the all the products involved, separating the reactants from the products by an arrow and a majority of chemical reactions are reversible (Science Online, 2010). In a chemical equation, the state of a substance is usually shown beside the chemical formula. There are four states of a substance; liquid (l), gas (g), aqueous (aq) and solid (s).

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