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. __ **Combustion:** __ Combustion reactions always involve molecular oxygen O 2. Anytime anything burns (in the usual sense), it is a combustion reaction. Combustion reactions are almost always exothermic (i.e., they give off heat). For example when wood burns, it must do so in the presence of O 2 and heat is produced. Wood as well as many common items that combust are organic (i.e., they are made up of carbon, hydrogen and oxygen). When organic molecules combust the reaction products are carbon dioxide and water (as well as heat).



__ **Condensation:** __ Condensation is a chemical process by which 2 molecules are joined together to make a larger, more complex, molecule, with the loss of water. It is the basis for the synthesis of all the important biological macromolecules (carbohydrates, proteins, lipids, nucleic acids) from their simpler sub-units. A ccording to the IUPAC Gold Book website, a condensation reaction is, "A (usually stepwise) reaction in which two or more reactants (or remote reactive sites within the same molecular entity) yield a single main product with accompanying formation of water or of some other small molecule, e.g. ammonia, ethanol, acetic acid, hydrogen sulfide." This reaction is used for things such as
 * The formation of sugar
 * The formation of an ester functional group
 * The formation of an amide functional group
 * Making proteins from amino acids
 * Making synthetic materials (such as Nylon)

It is important not to get condensation and hydrolysis muddled up, as they are in fact opposite processes! Condensation is so called because the product is drawn together from two other substances, in effect getting smaller by losing water. It does not give off water to condense and run down the window!

In all cases of condensation, molecules with projecting -H atoms are linked to other molecules with projecting -OH groups, producing H 2 O, ( H.OH ) also known as water, which then moves away from the original molecules.



A-H + B-OH --> A-B + H 2 O

 Acid-Base Reaction  An acid-base reaction is type of double displacement reaction that occurs between an acid and a base. The H+ ion in the acid reacts with the OH- ion in the base to form water and an ionic salt: <span style="font-family: Arial,Helvetica,sans-serif;"> HA + BOH → H <span style="font-family: Arial,Helvetica,sans-serif; vertical-align: sub;">2 <span style="font-family: Arial,Helvetica,sans-serif;">O + BA <span style="font-family: Arial,Helvetica,sans-serif;"> The reaction between hydrobromic acid (HBr) and sodium hydroxide is an example of an acid-base reaction: <span style="font-family: Arial,Helvetica,sans-serif;"> HBr + NaOH → NaBr + H <span style="font-family: Arial,Helvetica,sans-serif; vertical-align: sub;">2 <span style="font-family: Arial,Helvetica,sans-serif;">O

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> Oxidation-Reduction or Redox Reaction

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> In a redox reaction the oxidation numbers of atoms are changed. Redox reactions may involve the transfer of electrons between chemical species. The reaction that occurs when In which I <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%; vertical-align: sub;">2 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> is reduced to I- and S <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%; vertical-align: sub;">2 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">O <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%; vertical-align: sub;">3 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%; vertical-align: super;">2- <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> (thiosulfate anion) is oxidized to S <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%; vertical-align: sub;">4 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">O <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%; vertical-align: sub;">6 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%; vertical-align: super;">2- <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> provides an example of a redox reaction: <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> 2 S <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%; vertical-align: sub;">2 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">O <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%; vertical-align: sub;">3 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%; vertical-align: super;">2− <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">(aq) + I <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%; vertical-align: sub;">2 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">(aq) → S <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%; vertical-align: sub;">4 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">O <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%; vertical-align: sub;">6 <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%; vertical-align: super;">2− <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">(aq) + 2 I <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%; vertical-align: super;">− <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">(aq)