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You know a chemical reaction takes place if one or more of these occur:
Color changes - Different combinations of molecules reflect light differently. A color change indicates a change in molecules.
Heat content changes - In all chemical reactions, the heat content of the reactants and the heat content of the products is never the same. Sometimes the difference is great and can be easily detected. At other times, the difference is slight and more difficult to detect.
Gas produced - Whenever a gaseous product forms in a liquid solution, bubbles can be seen. A colorless gas produced in a reaction of solids is much harder to detect.
Precipitate forms - Precipitates are insoluble products formed by a reaction taking place in a liquid solution. This insoluble product will eventually settle to the bottom, but might immediately appear by turning the clear solution cloudy.
Most chemical reactions can be placed into one of basic types:
1. Decomposition Reactions – реакции разложения
A compound breaks into parts.
compound → element + element
2H2O → 2H2 + O2
Some decomposition complications with heat:
Some acids, when heated, decompose into an acidic oxide and H2O.
H2SO3 → SO2 + H2O
Metallic hydroxides, when heated, decompose into a metallic oxide and H2O.
Ca(OH)2 → CaO + H2O
Metallic carbonates, when heated, decompose into a metallic oxide and CO2.
Li2CO3 → Li2O + CO2
Metallic chlorates, when heated, decompose into metallic chlorides and O2.
2KClO3 → 2KCl + 3O2
2. Synthesis Reactions – реакции присоединения (синтеза)
Elements are joined together.
element + element → compound
2H2 + O2 → 2H2O
Compounds are joined together
compound + compound → compound
6CO2 + 6H2O → C6H12O6 + 6O2
3. Single Displacement Reactions – реакции единичного замещения
A single element replaces an element in a compound.
element + compound → element + compound
Zn + 2HCl → H2 + ZnCl2
4. Double Displacement Reactions – реакции двойного замещения
An element from each of two compounds switch places.
compound + compound → compound + compound
H2SO4 + 2NaOH → Na2SO4 + 2H2O
5. Combustion Reactions – реакции горения
A hydrocarbon (a compound containing only carbon and hydrogen) combines with oxygen. The products of combustion are always carbon dioxide and water.
hydrocarbon + oxygen → carbon dioxide + water
CH4 + 2O2 → CO2 + 2H2O
When metallic substances combine with oxygen, the result is an oxidation-reduction reaction.
The rusting of iron - 4Fe + 3O2 → 2Fe2O3
Chemical reactions can be classified in other ways as well:
Neutralization Reactions – реакция нейтрализации - Special types of double displacement reactions that involve the reaction between an acid and base to form a salt and water.
acid + base → salt + water
A suspension of solid magnesium hydroxide in water is widely used as an antacid to neutralize excess stomach acid:
Mg(OH)2 (s) + 2HCl (aq) → MgCl2 (aq) + 2H2O
Oxidation-Reduction Reactions – окислительно-восстановительные реакции
Any reaction in which elements experience a change in oxidation number.
one atom gains e-minus and another atom looses e-minus
S + O2 → SO2
In the reaction above, sulfur and oxygen both have an oxidation number of zero before the reaction. After the reaction, sulfur is +4 and oxygen is −2.
Precipitation Reactions – реакции осаждения.
Aqueous reactions that involve the formation of a precipitate (solid).
soluble compound + soluble compound → insoluble compound
2KI (aq) + Pb(NO3)2 (aq) → 2KNO3 (aq) + PbI2 (s) ↓
The physical state symbol (aq) says the reaction is taking place in a water solution. The physical state symbol (s) says the lead (II) iodide is a solid - therefore insoluble in the solution.
Hydrolysis (гидролиз) is a reaction involving the breaking of a bond in a molecule using water.
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