<text><span class="style10">hemical Reactions (2 of 3)</span><span class="style7"></span><span class="style10">Electron-transfer reactions: oxidation and reduction</span><span class="style7">Magnesium metal (Mg) burns with an incandescent white flame in air because of a vigorous reaction with oxygen, forming magnesium oxide: Mg(s) + O2(g) 2MgO(s)This is an example of the class of reactions known as </span><span class="style26">oxidations</span><span class="style7">, which include all combustion processes such as those occurring when fuels burn in air, as well as the reactions that cause metals to corrode in air. The product in this case is an ionic solid, which we could write more specifically as Mg to the power of 2 +O to the power of 2-. During the reaction, magnesium loses two electrons to form the cation Mg2+; the electrons are accepted by oxygen, which becomes the oxide anion O2- .The transfer of electrons between chemical species is a common process in many chemical reactions, so the term oxidation has come to possess a wider meaning than that implying solely the addition of oxygen atoms to an element or compound. As in the case of the formation of magnesium oxide, oxidation means the loss of electrons by a compound; the opposite process, </span><span class="style26">reduction</span><span class="style7">, implies a gain of electrons. Thus magnesium is said to be oxidized to Mg2+, while oxygen is reduced to O2-; the overall reaction is described as a </span><span class="style26">redox</span><span class="style7"> process. Many metals are extracted from their ores by reduction reactions.If an electric current is passed through an electrolyte such as an aqueous solution of copper(II) chloride (CuCl2), a redox process known as </span><span class="style26">electrolysis</span><span class="style7"> occurs. Positively charged Cu2+ ions are attracted to the negative electrode (the cathode), where they take up two electrons each and are thereby reduced to copper metal, which is deposited on the cathode. At the same time, the negatively charged Cl- ions are attracted to the positive electrode (the anode), where they give up their extra electrons (i.e. are oxidized) to form chlorine gas. Electrolysis is the basis of </span><span class="style26">electroplating</span><span class="style7">, in which a thin layer of metal, such as copper, tin, chromium or silver, is applied as a protective or decorative finish on cheaper or less durable materials. It is also used to purify metals and to extract reactive metals such as aluminum from their ores.In a </span><span class="style26">disproportionation</span><span class="style7"> reaction, a single chemical species is simultaneously oxidized and reduced. For example, copper(I) sulfate (Cu2SO4) dissolves in water to produce copper metal and copper(II) sulfate: Cu2SO4(aq) Cu(s) + CuSO4(aq)Here one Cu+ ion is oxidized to Cu2+, while the second is reduced to Cu metal.</span><span class="style10">Reaction equilibria</span><span class="style7">All the reactions described so far have gone to completion, i.e. a fixed quantity of reactants is converted into a fixed quantity of products. However, in general such a state of affairs is more the exception than the rule. The end of a reaction occurs when there is no further change in the amount of products formed or reactants destroyed: this is the point at which a reaction is said to reach </span><span class="style26">equilibrium</span><span class="style7">. At equilibrium, there may be appreciable amounts of reactants still present. For example, when acetic acid is dissolved in water, it forms a </span><span class="style26">weak acid</span><span class="style7">, because at equilibrium there is only a low concentration of hydronium ions:CH3COOH(l) + H2O(l) <--->CH3CO2-(aq) + H3O + (aq)The equilibrium, indicated by the two headed arrow, lies in favor of the reactants (in contrast to </span><span class="style26">strong acids</span><span class="style7">, such as hydrochloric acid, which are totally dissociated into ions). For the same reason, aqueous ammonia is a </span><span class="style26">weak base</span><span class="style7">, because at equilibrium there is a low concentration of hydroxide ions in the solution. Most of the solution consists of unreacted ammonia and water, in contrast to </span><span class="style26">strong</span><span class="style7"> </span><span class="style26">bases</span><span class="style7">, which consist solely of ions.The position of equilibrium in a chemical reaction (whether it favors reactants or products) depends in a detailed way on the thermodynamic properties of all the species involved.</span></text>
</content>
<content>
<layer>background</layer>
<id>26</id>
<text><span class="style10">n oxidation reaction</span><span class="style7">: magnesium metal burns in air to produce magnesium oxide. This common and important type of reaction includes combustion processes, the reactions that occur when metals corrode in air, and the biochemical reactions by which we obtain energy from food.</span></text>
</content>
<content>
<layer>background</layer>
<id>23</id>
<text>ΓÇó ELEMENTS AND THE PERIODIC TABLEΓÇó CHEMICAL BONDSΓÇó METALSΓÇó NATURAL COMPOUNDS</text>
