Chemical Reactions 9.2: Classifying Chemical Reactions

Chemical Reactions

9.2: Classifying Chemical Reactions

Objectives

• Classify chemical reactions.• Identify the characteristics of

different classes of chemical reactions.

Other symbols to know:• A double arrow indicates that a reaction is

reversible.• A downwards-pointing arrow may be written after

a formula. It indicates that the substance is a precipitate (a solid).

• An upwards-pointing arrow may be written after a formula. It indicates that the substance is a gas.

• Pt A chemical formula written above an arrow in an equation indicates that that substance is a catalyst. A catalyst is a substance that will increase the rate of a reaction but not be consumed in the reaction.

• Recall that a triangle ( ) above the arrow indicates that heat must be added for the reaction to occur.

Classifying Chemical Reactions

• Chemists classify chemical reactions in order to organize the many reactions that occur daily in our world.

• Recognizing the patterns in chemical reaction allows for the prediction of products that will form.


1. Synthesis reactions

A + B AB

• 2 or more substances (elements or compounds) react to produce a single product.


• Synthesis reactions• A + B AB

• 8 Fe + S8 8 FeS

• CaO + H2O Ca(OH)2

• 2SO2 + O2 2SO3

Practice Problems

1 - Aluminum and sulfur react to form …

2 - Water and dinitrogen pentoxide react to produce nitric acid.

3 – Magnesium + oxygen ?


2. Combustion Reactions• Any reaction in which oxygen

combines with a substance and releases energy in the form of heat and light.

Combustion Reactions

• C + O2 CO2

• 2H2 + O2 2H2O

• 2SO2 + O2 2SO3

• Can be classified as a synthesis reactions

• Can also be classified as combustion reactions

Combustion of Hydrocarbons

• All hydrocarbons burn in oxygen. If the combustion is complete, the reaction will always yield carbon dioxide and water.

• C10H8 + 12 O2 10 CO2 + 4 H2O

Practice Problems

1 - Nitrogen dioxide and oxygen react to produce dinitrogen pentoxide.

2 - Ethane (C2H6) burns completely in the air.

3 – Iron forms rust (Fe2O3) when exposed to air.


3. Decomposition Reactions

• AB A + B

• One reactant breaks down into 2 or more products.• the “opposite of synthesis”• often REQUIRE energy to occur


• Decomposition Reaction• AB A + B

• 2 H2O 2 H2 + O2

• NH4NO3 N2O + 2H2O

Practice Problems1 - Aluminum oxide decomposes when

electricity is passed through it.2 - Heating sodium hydrogen carbonate

produces sodium carbonate, carbon dioxide, and water.

3 - Nickel (II) hydroxide decomposes to produce nickel (II) oxide and water.


4. Single Replacement

• A + BX AX + B or• X + BY BX + Y

• Atoms of one element replace the atoms of another element that is in a compound (Look for an element reacting with a compound)


• Single Replacement• A + BX AX + B

• There are 2 kinds following this general pattern:

1 - Mg + 2 H2O Mg(OH)2 + H2

(a metal ion replaces a H+)2 - Cu + 2AgNO3 2Ag + Cu(NO3)2

(a metal ion replaces another metal ion)

A metal ion will not always replace a H ion or another metal ion. It will only replace the H or metal ion IF it has a higher reactivity than those elements.

A Reactivity SeriesReactivity is defined as the ability to react with another substance. A reactivity series ranks metals by reactivity. (See pg. 293 in textbook, Fig. 13.)

Hydrogen

Will these reactions occur?

• Ag + Cu(NO3)2 ?

• Ca + LiBr ?

• Zn + PbCl2 ?

• Al + MgSO4 ?

Classifying Chemical Reactions• Single replacement reactions can follow

another pattern: X + BY BX + Y

F2 + 2NaBr 2NaF + Br2

(a halogen replaces another halogen)

*Reactivity of Halogens decreases from top to bottom.

(Fluorine will replace bromine but bromine will not replace fluorine!)

Practice Problems

• K + ZnCl2 ?

• Fe + CuSO4 ?

• Br2 + MgCl2 ?

• F2 + HCl ?

• Fe + Na3PO4 ?

• Mg + AlCl3 ?


5. Double Replacement Reaction

• AX + BY AY + BX

• Ions are exchanged between 2 compounds.


5. Double Replacement Reaction

• AX + BY AY + BX

To correctly predict the products of these reactions, pair up “the outsides” and “the insides.” Don’t forget to “criss-cross” the charges between each pair, in order to write the correct formula.


• All double replacement reactions will produce either water, a precipitate, or a gas.• Pb(NO3)2 + 2 KI PbI2 (s) + 2 KNO3

(A precipitate is a product.)

• Ca(OH)2 + 2HCl CaCl2 + 2H2O

(Water is a product.)• KCN + HBr KBr + HCN (g)

(A gas is produced.)


• Another important point to note is that the components of double-replacement reactions are usually always dissolved in water.

Practice Problems• Predict the products:

• Al(NO3)3 + H2SO4

• NaOH + CuCl2

• Write the equations.• Lithium iodide and silver nitrate react to

produce solid silver iodide and lithium nitrate.

• Sodium hydroxide and phosphoric acid react to produce water and sodium phosphate.

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Chemical Reactions 9.2: Classifying Chemical Reactions