Make your own free website on

FCS Physics and Chemistry

Types of Reactions
Regents Physics
Regents Chemistry
Physics Downloads
Chemistry Downloads
Chemistry Lab
Physics Lab
Weekly Calendar
Helpful Links
Class Photos
Four basic types of chemical reactions:
A. Synthesis (composition):

  • two or more elements or compounds may combine to form a more complex compound.

  • Basic form: A  +  X  →    AX

Examples of synthesis reactions:

  1. Metal  +  oxygen  →   metal oxide

    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)

B. Decomposition:

  • A single compound breaks down into its component parts or simpler compounds.

  • Basic form: AX  →   A  +  X
Examples of decomposition reactions:

  1. Metallic carbonates, when heated, form metallic oxides and CO2(g).

    EX. CaCO3(s)  →    CaO(s)  +  CO2(g)

  2. Most metallic hydroxides, when heated, decompose into metallic oxides and water.

    EX. Ca(OH)2(s)  →    CaO(s)  +  H2O(g)

  3. Metallic chlorates, when heated, decompose into metallic chlorides and oxygen.

    EX. 2KClO3(s)  →    2KCl(s)  +  3O2(g)

  4. Some acids, when heated, decompose into nonmetallic oxides and water.

    EX. H2SO4  →    H2O(l)  +  SO3(g)

  5. Some oxides, when heated, decompose.

    EX. 2HgO(s)  →    2Hg(l)  +  O2(g)

  6. Some decomposition reactions are produced by electricity.

    EX. 2H2O(l)  →    2H2(g)  +  O2(g)

    EX. 2NaCl(l)  →    2Na(s)  +  Cl2(g)

C. Single Replacement:

  • a more active element takes the place of another element in a compound and sets the less active one free.

  • Basic form: A  +  BX  →    AX  +  B  or  AX  +  Y  →    AY + X
Examples of replacement reactions:

  1. Replacement of a metal in a compound by a more active metal.

    EX. Fe(s)  +  CuSO4(aq)  →    FeSO4(aq)  +  Cu(s)

  2. Replacement of hydrogen in water by an active metal.

    EX. 2Na(s)  +  2H2O(l)  →    2NaOH(aq)  +  H2(g)

    EX. Mg(s)  +  H2O(g)  →    MgO(s)  +  H2(g)

  3. Replacement of hydrogen in acids by active metals.

    EX. Zn(s)  +  2HCl(aq)  →    ZnCl2(aq)  +  H2(g)

  4. Replacement of nonmetals by more active nonmetals.

    EX. Cl2(g)  +  2NaBr(aq)  →    2NaCl(aq)  +  Br2(l)

NOTE: Refer to TABLE J, ACTIVITY SERIES, IN YOUR REGENTS REFERENCE TABLES, to predict products of replacement reactions. If the free element is above the element to be replaced in the compound, then the reaction will occurr. If it is below, then no reaction occurs.

D. Double Replacement:

  • occurrs between ions in aqueous solution. A reaction will occurr when a pair of ions come together to produce at least one of the following:

    1. a precipitate

    2. a gas

    3. water or some other non-ionized substance.

  • Basic form: AX  +  BY  →    AY  +  BX
Examples of double replacement reactions:

  1. Formation of precipitate.

    EX. NaCl (aq)  +  AgNO3(aq)  →   NaNO3(aq)  +  AgCl(s)

    EX. BaCl2(aq)  +  Na2 SO4(aq)  →    2NaCl(aq)  +  BaSO4(s)

  2. Formation of a gas.

    EX. HCl(aq)  +  FeS(s)  →    FeCl2(aq)  +  H2S(g)

  3. Formation of water. (If the reaction is between an acid and a base it is called a neutralization reaction.)

    EX. HCl(aq)  +  NaOH(aq)  →    NaCl(aq)  +  H2O(l)

  4. Formation of a product which decomposes.

    EX. CaCO3(s)  +  HCl(aq)  →    CaCl2(aq)  +  CO2(g)  +  H2O(l)

NOTE: See TABLE F (SOLUBILITY GUIDELINES) IN YOUR REGENTS REFERENCE TABLES to decide whether a product of an double replacement reaction is insoluble in water and will thus form a precipitate. If a compound is soluble in water then it should be shown as being in aqueous solution, or left as separate ions. It is, in fact, often more desirable to show only those ions that are actually taking part in the actual reaction. Equations of this type are called net ionic equations. 

Click HERE for Practice Problems!

Finding Missing Info in Chemical Equations
We can find missing reactants / products and missing mass in chemical reactions because we know the following:
1. The Law of Conservation of Mass
2. Four Types of Reactions
3. Rules for Balancing Equations
For Example, we come across this situation:
 2Na  +  2H2O  -->  X  +  2NaOH
What does the X stand for?
To Solve...
  1. We notice that it is a single replacement reaction that involves hydrogen
  2. We look at Table J in the Reference Tables and see that sodium is above hydrogen on the activity series
  3. We also know that when hydrogen is bumped out of a compound it makes hydrogen gas
  4. Finally, we notice that 2 hydrogen atoms are missing on the right side of the we must be correct!

What if we come across this situation?
If 103.0 g of potassium chlorate are decomposed to form 62.7 g of potassium chloride and oxygen gas according to the following:
2KClO3  -->  2KCl  +  3O2  , how many grams of oxygen are formed>
This is easy to solve, since we know about the Law of Conservation of the mass on the left must equal the mass on the right!
2KClO3  -->  2KCl  +  3O2 
103.0g  -->  62.7g  +  mass of O2
Mass O =  40.3g

Forestville Central School
4 Academy Street
Forestville, NY 14062