Earth Science: Chemistry – Inorganic Chemistry – Material in Flux
Chemical processes are everywhere. Engines transform gasoline into exhaust gases. Iron chains become rust. In an oven, a raw roast is transformed.
In our bodies, our food is transformed into new materials. Industrially, large amounts of new substances are created by controlling chemical reactions.
Earth Science: Chemistry – Inorganic Chemistry – New Substances are Formed
Chemical reactions are one way we form new substances, both willingly and unwillingly. The reactivity of different compounds can create surprising and useful combinations, or damage and breakdown vital componentry.
New substances are produced when chemical reactions occur. All of these chemical transformations involve either an increase or a decrease of energy. As an example, when 0.141 ounces (4 g) of zinc powder is mixed with 0.0755 ounces (2 g) of sulfur in a dry flask and the resulting mixture is touched with a red hot wire, flames and white smoke appear.
After cooling, a grayish white mixture remains.
This reaction releases heat and is called exothermic. In endothermic reactions, however, energy is removed from the environment. The amount of energy needed to start a reaction (in this case from the red-hot wire) is called the activation energy. The reaction between zinc (Zn) and sulfur (S) can be described by the equation: Zn+S => ZnS. The start materials, or reactants, are to the left of the arrow. To the right of the arrow is the final product.
While the zinc is reacting with the sulfur, it is also reacting with oxygen (O2) from the air, creating zinc oxide: 2Zn+O2=> 2ZnO. The coefficient two denotes the number of zinc atoms. The subscript is used to denote an oxygen molecule consisting of two oxygen atoms and a zinc oxide molecule that has two oxygen atoms. It is also possible to measure the result of this reaction in “moles.” A mole consists of six times 1023 particles.
The equation shows that two moles of zinc atoms react with one mole of oxygen molecules to produce two moles of zinc oxide molecules.
Redox reactions
Originally the term oxidation was used only to describe a reaction in which a substance combines with oxygen. Today every particle that transfers electrons to a reaction partner is said to be oxidized.
For example, zinc is oxidized during a reaction with sulfur. At the same time, every particle that takes up electrons is said to be reduced. This is an oxidation-reduction, or redox, reaction.
Acid-base reactions
Acids dissolve metals and taste sour, while bases feel slippery and taste bitter. Acids turn litmus paper red, and bases turn it blue. Acids release protons (H+) to other ions or molecules, and bases accept protons. If an acid reacts with a base, its protons are accepted by the base.
The reaction between hydrochloric acid (HCI) and ammonia (NH3) is an example of this kind of reaction, described by the equation HCI+NH3=> CI + NH4. The superscript indicates that the chlorine becomes an ion with a single negative charge, and that the ammonia becomes an ion with a single positive charge.
PREVENTING RUST
When iron and steel come in to contact with oxygen in a wet environment, a chemical reaction takes place. The metal is oxidized and an iron oxide compound called rust develops.
Metals require protection against this reaction, passively with protective layers of paint or some noble metal or actively with metallic coating of a non-noble substance (zinc, aluminium, or magnesium alloy). Active corrosion protection is commonly used in shipbuilding where the coating dissolves, functioning as a sacrificial anode while the iron acts as a cathode.