The more general term for rusting and other similar processes is corrosion. Other terms that are commonly used in descriptions of chemical changes are burn, rot, explode, decompose, and ferment. Chemical properties are very useful in identifying substances. However, unlike physical properties, chemical properties can only be observed as the substance is in the process of being changed into a different substance.
A chemical change is also called a chemical reaction. A chemical reaction is a process that occurs when one or more substances are changed into one or more new substances.
Zinc Zn is a silver gray element that can be ground into a powder. If zinc is mixed at room temperature with powdered sulfur S , a bright yellow element, the result will simply be a mixture of zinc and sulfur.
No chemical reaction occurs. The iron II ions react with them to form green rust:. That isn't the end of the story. These iron ions are responsible for the formation of the reddish deposit that gradually eats holes in auto bodies and metal roofing worldwide. They combine with the extra hydroxide ions to form iron III hydroxide:. This compound dehydrates to become Fe 2 O 3. H 2 O , which is the chemical formula for rust. If you're interested in writing a balanced equation for the entire process, you need only know the initial reactants and the products of the reaction.
In a balanced equation, the same number of oxygen, hydrogen and iron atoms must appear on both sides of the equation. Balance the number of hydrogen atoms by multiplying the number of water molecules by 6 and the number of hydroxide molecules by 4.
A physical change is a change to a sample of matter in which some properties of the material change, but the identity of the matter does not. Physical changes can further be classified as reversible or irreversible.
The melted ice cube may be refrozen, so melting is a reversible physical change. Physical changes that involve a change of state are all reversible. Other changes of state include vaporization liquid to gas , freezing liquid to solid , and condensation gas to liquid. Dissolving is also a reversible physical change. When salt is dissolved into water, the salt is said to have entered the aqueous state.
The salt may be regained by boiling off the water, leaving the salt behind. When a piece of wood is ground into sawdust, that change is irreversible since the sawdust could not be reconstituted into the same piece of wood that it was before. Cutting the grass or pulverizing a rock would be other irreversible physical changes.
Firewood also represents an irreversible physical change since the pieces cannot be put back together to form the tree. As the rust forms on the surface of the iron, it flakes off to expose more iron, which will continue to rust. Rust is clearly a substance that is different from iron. This is an example of an electrochemical reaction and corrosion. Two distinct electrochemical reactions occur:. There is anodic dissolution or oxidation of iron going into aqueous water solution:.
Cathodic reduction of oxygen that is dissolved into water also occurs:. The iron ion and the hydroxide ion react to form iron hydroxide:. The iron oxide reacts with oxygen to yield red rust, Fe 2 O 3. Because of the electrochemical nature of the reaction, dissolved electrolytes in water aid the reaction. Rust occurs more quickly in saltwater than in pure water, for example.
Keep in mind oxygen gas O 2 is not the only source of oxygen in air or water. Carbon dioxide CO 2 also contains oxygen. Carbon dioxide and water react to form weak carbonic acid. Carbonic acid is a better electrolyte than pure water.
As the acid attacks the iron, water breaks into hydrogen and oxygen. Free oxygen and dissolved iron form iron oxide, releasing electrons, which can flow to another part of the metal.
Once rusting starts, it continues to corrode the metal. Rust is brittle, fragile, progressive, and weakens iron and steel. To protect iron and its alloys from rust, the surface needs to be separated from air and water.
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