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.

Classifying Chemical Reactions 1. Synthesis reactions A + B  AB 2 or more substances (elements or compounds) react to produce a single product.

Classifying Chemical Reactions Synthesis reactions A + B  AB 8 Fe + S 8  8 FeS CaO + H 2 O  Ca(OH) 2 2SO 2 + O 2  2SO 3

Practice Problems 1- Aluminum and sulfur react to form … 2- Water and dinitrogen pentoxide react to produce nitric acid. 3 – Magnesium + oxygen  ?

Classifying Chemical Reactions 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 + O 2  CO 2 2H 2 + O 2  2H 2 O 2SO 2 + O 2  2SO 3 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. C 10 H O 2  10 CO H 2 O

Practice Problems 1- Nitrogen dioxide and oxygen react to produce dinitrogen pentoxide. 2- Ethane (C 2 H 6 ) burns completely in the air. 3 – Iron forms rust (Fe 2 O 3 ) when exposed to air.

Classifying Chemical Reactions 3. Decomposition Reactions AB  A + B One reactant breaks down into 2 or more products. the “opposite of synthesis” often REQUIRE energy to occur

Classifying Chemical Reactions Decomposition Reaction AB  A + B 2 H 2 O  2 H 2 + O 2 NH 4 NO 3  N 2 O + 2H 2 O

Practice Problems 1- 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.

Classifying Chemical Reactions 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)

Classifying Chemical Reactions Single Replacement A + BX  AX + B There are 2 kinds following this general pattern: 1- Mg + 2 H 2 O  Mg(OH) 2 + H 2 (a metal ion replaces a H + ) 2- Cu + 2AgNO 3  2Ag + Cu(NO 3 ) 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 Series Reactivity 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(NO 3 ) 2 ? Ca + LiBr ? Zn + PbCl 2 ? Al + MgSO 4 ?

Classifying Chemical Reactions Single replacement reactions can follow another pattern: X + BY  BX + Y  F 2 + 2NaBr  2NaF + Br 2 (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 + ZnCl 2  ? Fe + CuSO 4  ? Br 2 + MgCl 2  ? F 2 + HCl  ? Fe + Na 3 PO 4  ? Mg + AlCl 3  ?

Classifying Chemical Reactions 5. Double Replacement Reaction AX + BY  AY + BX Ions are exchanged between 2 compounds.

Classifying Chemical Reactions 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.

Classifying Chemical Reactions All double replacement reactions will produce either water, a precipitate, or a gas. Pb(NO 3 ) KI  PbI 2 (s) + 2 KNO 3 (A precipitate is a product.) Ca(OH) 2 + 2HCl  CaCl 2 + 2H 2 O (Water is a product.) KCN + HBr  KBr + HCN (g) (A gas is produced.)

Classifying Chemical Reactions 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(NO 3 ) 3 + H 2 SO 4  NaOH + CuCl 2  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.