CHEMISTRY | EXTRA REFERENCE | EBOOK
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Chemical Reactions
Chemical reactions are also known as chemical changes.This refers to the changes in the structure of molecules. Such reactions can result in molecules at taching to each other to form larger molecules, molecules breaking apart to form two or more smaller molecules, or rearrangements of atoms within molecules. Chemical reactions usually involve the making or breaking of chemical bonds. A chemical reaction does not involve or change the nucleus of the atom in any way. Only the electron clouds around the atoms, involved in the reaction, interact. A chemical reaction can result in:
- molecules attaching to each other to form larger molecules
- molecules breaking apart to form two or more smaller molecules
- rearrangements of atoms within molecules
A chemical reaction almost always involves a change in energy, conveniently measured in terms of heat. The energy diference between the "before" and "after" states of a chemical reaction can be calculated theoretically using tables of data (or a computer). For example, consider the reaction CH4 + 2O2 ?
CO2 + 2H2O (combustion of methane in oxygen). By calculating the amounts of energy required to break all the bonds on the left ("before") and right ("af-ter") sides of the equation, we can calculate the energy diference between the reactants and the products. This is referred to as ∆H, where ∆ (Delta) means diference, and H stands for enthalpy, a measure of energy which is equal to the heat transferred at constant pressure. ∆H is usually given in units of kJ (thousands of joules) or in kcal (kilocalories). If ∆H is negative for the reaction, then energy has been released. This type of reaction is referred to as exothermic (literally, outside heat, or throwing of heat). An exothermic reaction is morefavourable and thus more likely to occur. Our example reaction is exothermic, which we already know from everyday experience, since burning gas in air givesof heat. A reaction may have a positive ∆H. This means that, to proceed, the reac- tion requires an input of energy from outside. This type of reaction is called endothermic (literally, inside heat, or absorbing heat).
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Chemistry