Wednesday, December 28, 2011

Chemical Reactions - Chemistry HW 11/4/10

Combination / Synthesis
In a synthesis reaction, two or more simple substances combine to form a more complex substance. Two or more reactants yielding one product is another way to identify a synthesis reaction. These reactions come in the general form of:
A + B ---> AB
example : 

combination of iron and sulfur to form iron (II) sulfide: 8 Fe + S8 ---> 8 FeS 

            simple hydrogen gas combined with simple oxygen gas can produce a more complex substance-----water!

EX. 2Mg(s)  +  O2(g)  →    2MgO(s)
2.      Nonmetal  +  oxygen  →    nonmetallic oxide
EX. C(s)  +  O2(g)  →    CO2(g)
3.      Metal oxide  +  water  →    metallic hydroxide
EX. MgO(s)  +   H2O(l)  →    Mg(OH)2(s)
4.      Nonmetallic oxide  +  water  →    acid
EX. CO2(g)  +  H2O(l)  →    ; H2CO3(aq)
5.      Metal + nonmetal  →    salt
EX. 2 Na(s)  +  Cl2(g)  →    2NaCl(s)
6.      A few nonmetals combine with each other.
EX. 2P(s)  +  3Cl2(g)  →    2PCl3(g)

Decomposition / Analysis Reaction
     A decomposition reaction is the opposite of a synthesis reaction.  In a decomposition reaction, a more complex substance breaks down into its more simple parts. One reactant yields 2 or more products. Compounds that are unstable will decompose quickly without outside assistance.
         These reactions come in the general form:         AB ---> A + B

a.Thermal Decomposition – It is the most common type of decomposition caused by a rise in temperature
EX. CaCO3(s)  →    CaO(s)  +  CO2(g)
1.      Most metallic hydroxides, when heated, decompose into metallic oxides and water.
EX. Ca(OH)2(s)  →    CaO(s)  +  H2O(g)
2.      Metallic chlorates, when heated, decompose into metallic chlorides and oxygen.
EX. 2KClO3(s)  →    2KCl(s)  +  3O2(g)
3.      Some acids, when heated, decompose into nonmetallic oxides and water.
EX. H2SO4  →    H2O(l)  +  SO3(g)
4.      Some oxides, when heated, decompose.
EX. 2HgO(s)  →    2Hg(l)  +  O2(g)

b. Photo-chemical Decompositions – Chemical decompositions that occur with light as a factor
c. Electrolytic Decomposition – It is a reaction where electricity is used to decompose compounds. They are frequently used in the industrial manufacture or purification of many elements and compounds. 
                        EX. 2H2O(l)  →    2H2(g)  +  O2(g)
EX. 2NaCl(l)  →    2Na(s)  +  Cl2(g)

Substitution Reaction / Single displacement / Single Replacement /
In a single replacement reaction, a single uncombined element replaces another in a compound. A more reactive element takes the place of another element in a compound and sets the less active one free. Two reactants yield two products. These reactions come in the general form of:
A + BC ---> AC + B or  AX  +  Y  →    AY + X

examples :      Mg + 2 H2O ---> Mg(OH)2 + H2
2 Na(s) + 2 HCl(aq) → 2 NaCl(aq) + H2(g)
Fe(s)  +  CuSO4(aq)  →    FeSO4(aq)  +  Cu(s)
2Na(s)  +  2H2O(l)  →    2NaOH(aq)  +  H2(g)
Cl2(g)  +  2NaBr(aq)  →    2NaCl(aq)  +  Br2(l)

Double Replacement /Double displacement  / Metathesis / Ionic
In a double replacement reaction, the anions and cations of two different molecules switch places, forming two entirely different compounds. These reactions are in the general form:
AB + CD ---> AD + CB

·         Two reactants yield two products. The reaction will produce at least one of the following:
1.      a precipitate
EX. BaCl2(aq)  +  Na2 SO4(aq)  →    2NaCl(aq)  +  BaSO4(s)
1.      a gas
EX. HCl(aq)  +  FeS(s)  →    FeCl2(aq)  +  H2S(g)
2.      water or some other non-ionized substance.

Example:     Pb(NO3)2 + 2 KI ---> PbI2 + 2 KNO3
CaCO3(s)  +  HCl(aq)  →    CaCl2(aq)  +  CO2(g)  +  H2O(l)

Acid-base Neutralization 
     This is a special kind of double displacement reaction that takes place when an acid and base react with each other. The H+ ion in the acid reacts with the OH- ion in the base to form water and an ionic salt.

Example: reaction of hydrobromic acid (HBr) with sodium hydroxide: 
HBr + NaOH ---> NaBr + H2O
HA + BOH ---> H2O + BA
·         Hydrochloric acid and sodium hydroxide ---> sodium chloride and water
HCl + NaOH ---> NaCl + H2O

·         Sulphuric acid and potassium hydroxide ---> potassium sulphate and water
H2SO4 + 2KOH ---> K2SO4 + 2H2O

·         Nitric acid and calcium hydroxide ---> calcium nitrate and water
2HNO3 + Ca(OH)2 ---> Ca(NO3)2 + 2H2O

·         Ethanoic acid and sodium hydroxide ---> sodium ethanoate and water
(Acetic acid and sodium hydroxide ---> sodium acetate and water , using the older names)
CH3COOH + NaOH ---> NaOOCCH3 + H2O

Endothermic Reactions
      Chemical reactions in which energy is absorbed are endothermic. Energy is required for the reaction to occur. The energy absorbed is often heat energy or electrical energy. Adding electrical energy to metal oxides can separate them into the pure metal and oxygen. Adding electrical energy to sodium chloride can cause the table salt to break into its original sodium and chlorine parts.

Exothermic Reactions
     Chemical reactions in which energy is released are exothermic. The energy that is released was originally stored in the chemical bonds of the reactants. Often the heat given off causes the product(s) to feel hot. Any reaction that involves combustion (burning) is an exothermic chemical reaction.

Oxidation-Reduction or Redox Reaction
      Redox is an abbreviation of reduction/oxidation reactions. In a redox reaction, the oxidation numbers of atoms are changed. Redox reactions may involve the transfer of electrons between chemical substances. One substance gains oxygen (or loses hydrogen) and another loses oxygen (or gains hydrogen).

example: 2 S2O32−(aq) + I2(aq) --> S4O62−(aq) + 2 I(aq)
·         Iron and oxygen ---> iron oxide, where the iron gets oxidised
4Fe + 3O2 ---> 2Fe2O3

Combustion / burning
 A combustion reaction is a type of redox reaction in which a combustible material combines with an oxidizer to form oxidized products and generate heat.  Usually in A combustion reaction is when oxygen combines with another compound to form water and carbon dioxide. These reactions are exothermic, meaning they produce heat. The term combustion is usually used for only large-scale oxidation of whole molecules.
Combustion is similar to a decomposition reaction, except that oxygen and heat are required for it to occur. If there is not enough oxygen, the reaction may not occur. Sometimes, with limited oxygen, the reaction will occur, but it produces carbon monoxide (CO) or even soot. In that case, it is called incomplete combustion.
Example: burning of napthalene:
C10H8 + 12 O2 ---> 10 CO2 + 4 H2O

Disproportionation - a redox reaction in which one reactant forming two distinct products varying in oxidation state.

2 Sn2+(aq) → Sn(s) + Sn4+(aq)

Organic reactions encompass a wide assortment of reactions involving compounds which have carbon as the main element in their molecular structure. The reactions in which an organic compound may take part are largely defined by its functional groups

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