In the figure, you’ll see that EA is the activation energy—necessary to start the reaction process, while “delta H” is the change in enthalpy in the exothermic reaction. There is a period of time called the transition state, at the top of the activation energy hill, in which there may be intermediates in the reaction process. If the reaction is endothermic, heat will be absorbed and the delta H will be positive. There will still be an activation energy to get the reaction going but the energy of the products is higher than the energy of the reactants. The absorbed energy is stored in the bonds made by the products as potential energy (that could be reversed as the reaction goes in the reverse). In both cases, the activation energy is gotten back as the reactant energy goes to the product energy.
TYPES OF CHEMICAL REACTIONS There are several general kinds of chemical reactions that can occur, which are different in substantive form: •
Combination reactions—this is when two or more reactions form a single product. An example of this is the reaction where 2 sodium (Na) molecules plus chlorine gas (Cl2) react to make sodium chloride (NaCl). Even the burning of coal or solid carbon (C) plus oxygen (O2) goes to make CO2 or carbon dioxide. Technically, there can be more than one product as long as things are combined in the reaction.
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Decomposition reactions—these types of reactions are the opposite of combination reactions. In this type of reaction, a single compound breaks down into two or more simpler substances. An example of this is the breakdown of water into hydrogen gas and oxygen. The breakdown of hydrogen peroxide into oxygen gas and water is a decomposition reaction.
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Single displacement reactions—in this type of reaction, a more active element kicks out a less active element from a compound. An example reaction is Zinc solid plus Copper sulfate goes to copper solid plus zinc sulfate. In this reaction, zinc displaces copper in its reaction with the sulfate ion. Zinc is more reactive than copper and the reverse reaction is not likely to occur. 102
