Inorganic Chemistry

D

Displacement Reactions

Displacement reactions cause elements to displace each other from a compound. For example, magnesium (Mg) displaces titanium (Ti) in the following reaction: 2Mg + TiCl₄ → Ti + 2MgCl₂. Manufacturers use this particular displacement reaction to extract titanium, valued for its strength and light weight by the aerospace and other industries, from the compound titanium tetrachloride (TiCl₄).

E

Exchange Reactions

Exchange reactions are driven by compounds wanting to exchange ions in order to form more stable products, namely acids or salts. For example, titanium tetrachloride (TiCl₄) reacts violently with water as TiCl₄ exchanges one titanium ion (Ti⁺⁴) for every four hydrogen ions (H⁺) in the water: TiCl_4(g)+ 2H₂O_(g) → 4HCl_(g) + TiO_2(s). This reaction produces hydrochloric acid (HCl) and titanium dioxide (TiO₂). Titanium dioxide typically occurs in mineral deposits as an impure black substance. Chlorinating titanium oxides from mineral compounds produces TiCl₄. Because titanium dioxide is needed as the major pigment in white paint, manufacturers use the above exchange reaction to isolate titanium dioxide from TiCl₄.

VII

Factors Influencing Reactions

Chemical reactions occur when certain physical and chemical factors make conditions energetically favorable for the reactants to combine into products (see Thermodynamics). Some factors, such as the potential energy (stored energy) associated with the reactants, can trigger a spontaneous chemical reaction. If the products have a higher level of entropy (disorder among the particles) than the reactants, this difference can also initiate a chemical reaction. External factors, such as heat or the presence of a catalyst (a substance that increases reaction rate without being chemically changed), can trigger or increase the rate of a reaction (see Catalysis).

A

Exothermic and Endothermic Reactions

Chemical reactions can occur spontaneously if the reactants possess more potential energy (stored energy) than the products. This type of reaction occurs spontaneously because of the downhill energy path (from more potential energy to less). These reactions are called exothermic (heat-producing) reactions, because potential energy is converted to heat as the reactions proceed. Conversely, endothermic (heat-absorbing) reactions do not occur spontaneously because of the uphill energy path that exists. The products of endothermic reactions contain more potential energy than the reactants. As a result, energy must be added to trigger an endothermic reaction.

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B

Entropy

Entropy is the tendency for matter to become disordered. Nature requires the input of energy to maintain an ordered state—a bedroom will become messy if not periodically cleaned; a car will eventually fall into disrepair if not regularly serviced. Entropy is an important force in chemistry. If other factors influencing a reaction are held equal, a chemical reaction will proceed spontaneously if the products have higher entropy (are more disordered) than the reactants. This law explains why ozone (O₃) gas can spontaneously decompose into molecular oxygen (O₂): 2O_3(g) → 3O_2(g). This reaction occurs because the molecular order is diminished, resulting in a higher level of entropy.

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