Chemical Reactions

1. Chemical Reactions 9.2: Classifying Chemical Reactions

2. Objectives • Classify chemical reactions. • Identify the characteristics of different classes of chemical reactions.

3. 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.

4. 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.

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

6. Classifying Chemical Reactions • Synthesis reactions • A + B  AB • 8 Fe + S8 8 FeS • CaO + H2O  Ca(OH)2 • 2SO2 + O2 2SO3

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

8. 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.

9. 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

10. 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

11. Practice Problems • - Nitrogen dioxide and oxygen react to produce dinitrogen pentoxide. • - Ethane (C2H6) burns completely in the air. 3 – Iron forms rust (Fe2O3) when exposed to air.

12. 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

13. Classifying Chemical Reactions • Decomposition Reaction • AB  A + B • 2 H2O  2 H2 + O2 • NH4NO3 N2O + 2H2O

14. Practice Problems • - Aluminum oxide decomposes when electricity is passed through it. • - Heating sodium hydrogen carbonate produces sodium carbonate, carbon dioxide, and water. • - Nickel (II) hydroxide decomposes to produce nickel (II) oxide and water.

15. Classifying Chemical Reactions 4. Single Replacement • A + BX  AX + B or • X + BY  BX + Y • Atoms ofone element replace the atoms of another element that is in a compound (Look for an element reacting with a compound)

16. Classifying Chemical Reactions • Single Replacement • A + BX  AX + B • There are2 kinds following this general pattern: • - Mg + 2 H2O  Mg(OH)2 + H2 (a metal ion replaces a H+) • - Cu + 2AgNO3 2Ag + Cu(NO3)2 (a metal ion replaces another metal ion)

17. 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.

18. 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

19. Will these reactions occur? • Ag + Cu(NO3)2 ? • Ca + LiBr ? • Zn + PbCl2 ? • Al + MgSO4 ?

20. 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!)

21. Practice Problems • K + ZnCl2  ? • Fe + CuSO4 ? • Br2 + MgCl2  ? • F2 + HCl  ? • Fe + Na3PO4  ? • Mg + AlCl3  ?

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

23. 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.

24. Classifying Chemical Reactions • 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.)

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

26. Practice Problems • Predict the products: • Al(NO3)3 + H2SO4 • BaCl2 + K2CO3  • Write the equations. • Lithium iodide and silver nitrate react to produce solid silver iodide and lithium nitrate. • Sodium oxalate and lead (II) nitrate react to produce solid lead (II) oxalate and sodium nitrate.