1. Chemical Reactions Chapter 8

2. Chemical Reactions Chemical reactions represent the formation of a new substance (remember chemical changes). We know a chemical change has occurred if: Give some examples--- Some can be found on pages 261-262.

3. Chemical Equations A chemical equation is a representation of a chemical reaction. The starting material(s) are the reactants and they react to form products. Reactants  Products We can use words to describe the reaction or use the formulas of the substances involved. YOU MUST KNOW THE CORRECT FORMULAS TO WRITE THE CORRECT EQUATION

4. Chemical Equations For example: methane combines with oxygen to produce carbon dioxide and water. Methane + oxygen  carbon dioxide + water CH 4 + O 2  CO 2 + H 2 O This is sometimes called a skeleton equation We have written the correct formulas for the substances involved. Note oxygen gas is diatomic-what are the other diatomic molecules?

5. Chemical Equations If we know the physical state of the reactants and products we can add them to our equation. s-l-g-aq are commonly used symbols. Since methane is a gas we can rewrite our skeleton equation as: CH 4(g) + O 2(g)  CO 2(g) + H 2 O (l)

6. Chemical Equations Equations must conform to the Law of Conservation of Mass which we already know (?)-In writing equations the atoms of the reactants must be balanced by the atoms of the products. Lets look at our equation again CH 4 + O 2  CO 2 + H 2 O On the reactant side there are: 4-H,1C and 2O

7. CH 4 + O 2  CO 2 + H 2 O On the product side 2H,1C and 3O. Things are not in balance! We know the formulas are correct so we can’t change them but we can use coefficients before the formula. Consider the H-4 on the left and 2 on the right. So CH 4 + O 2  CO 2 + 2H 2 O Now we need to balance the O’s-4 on the right and 2 on the left, so

8. CH 4 + 2O 2  CO 2 + 2H 2 O Now things are balanced, so our final equation would be: CH 4(g) + 2O 2(g)  CO 2(g) + 2H 2 O (l)

9. Chemical equations Some other examples: Iron(III) oxide reacts with hydrogen gas to produce metallic iron and water.

10. Another example Ammonia reacts with oxygen gas to produce nitrogen monoxide gas and water vapor.

11. Types of Reactions 1. Combustion reactions occur when substances react with oxygen and usually produce a flame in the process. We will restrict our discussion for reactions of oxygen with hydrocarbons (compounds containing carbon and hydrogen only). The previous reaction of methane fits this description. Combustion of hydrocarbons always produce carbon dioxide and water.

12. An example: Combustion of butane C 4 H 10 Hint: when you have an even number of atoms on one side of the equation and an odd number on the other-multiply everything by 2

13. Reaction Types Combination or synthesis reactions: You do not have to predict products of these reactions, but you must be able to identify them and be able to balance them!

14. 14 Combination or Synthesis Reactions Combination reactions occur when two or more substances combine to form a compound. There are three basic types of combination reactions. 1.Two elements react to form a new compound 2.An element and a compound react to form one new compound 3.Two compounds react to form one compound

15. 15 Combination Reactions 1.Element + Element  Compound A.Metal + Nonmetal  Binary Ionic Compound

16. 16 Combination Reactions 1.Element + Element  Compound A.Metal + Nonmetal  Binary Ionic Compound

17. 17 Combination Reactions 1.Element + Element  Compound B.Nonmetal + Nonmetal  Covalent Binary Compound

18. 18 Combination Reactions 2.Compound + Element  Compound

19. 19 Combination Reactions 3.Compound + Compound  Compound – gaseous ammonia and hydrogen chloride – lithium oxide and sulfur dioxide

20. 20 Decomposition Reactions Decomposition reactions occur when one compound decomposes to form: 1.Two elements 2.One or more elements and one or more compounds 3.Two or more compounds

21. 21 Decomposition Reactions 1.Compound  Element + Element – decomposition of dinitrogen oxide decomposition of calcium chloride  decomposition of silver halides

22. 22 Decomposition Reactions 2.Compound  One Element + Compound(s) – decomposition of hydrogen peroxide

23. 23 Decomposition Reactions 3.Compound  Compound + Compound – decomposition of ammonium hydrogen carbonate

24. Disassociation Reactions These are not true chemical reactions (no new substance is produced), but they are important to understand. When an ionic compound dissolves in water it does so by breaking up or dissociating into its component ions. So…. NaCl(aq)  Na + (aq) + Cl - (aq)

25. Some other examples: CaBr 2  Mg 3 (PO 4 ) 2  Al 2 (SO 4 ) 3 

26. 26 Displacement Reactions Displacement reactions Displacement reactions occur when one element displaces another element from a compound. – These are reactions in which the more active metal displaces the less active metal or hydrogen from a compound in aqueous solution. – Activity series is given on page 281. The most active metals are at the top and will displace those lower on the list.

27. 27 Displacement Reactions 1.[More Active Metal + Salt of Less Active Metal]  [Less Active Metal + Salt of More Active Metal] – Molecular or formula unit equation

28. Note that the silver nitrate and copper(I) nitrate are in solution, so we can write an ionic equation by showing these compounds in their dissociated form: Did any compound or ion remain the same during this reaction? An ion that does not undergo a change is called a spectator ion.

29. These ions can be eliminated from the equation producing a net ionic equation.

30. 30 Displacement Reactions 2.[Active Metal + Nonoxidizing Acid]  [Hydrogen + Salt of Acid] – Common method for preparing hydrogen in the laboratory. Molecular or formula unit equation

31. 31 Displacement Reactions Total ionic equation You do it! Net ionic equation You do it!

32. 32 Displacement Reactions The following metals are active enough to displace hydrogen – K, Ca, Na, Mg, Al, Zn, Fe, Sn, & Pb Non-metals can displace less active non-metals, we will just consider the halogens (?). The activity series for halogens is simple-its their order on the periodic table. Fluorine is the most active, iodine the least.

33. 33 Displacement Reactions 3.[Active Nonmetal + Salt of Less Active Nonmetal]  [Less Active Nonmetal + Salt of More Active Nonmetal] Molecular equation Total ionic equation

34. 34 Displacement Reactions Net ionic equation

35. 35 Double-Displacement or Metathesis Reactions Metathesis reactions Metathesis reactions occur when two ionic aqueous solutions are mixed and the ions switch partners. AX + BY  AY + BX Metathesis reactions remove ions from solution in two ways: 1.form predominantly unionized molecules like H 2 O 2.form an insoluble solid Ion removal is the driving force of metathesis reactions.

36. 36 Metathesis Reactions 1.Acid-Base (neutralization) Reactions – Formation of the nonelectrolyte H 2 O – acid + base  salt + water – a nonelectrolyte is a compound that does not produce ions-With the exception of organic acids and some other organic compounds, all other molecular compounds are nonelectrolytes.

37. 37 Metathesis Reactions Molecular equation Total ionic equation Net ionic equation

38. 38 Metathesis Reactions 2.Precipitation reactions 2.Precipitation reactions are metathesis reactions in which an insoluble compound is formed. – The solid precipitates out of the solution much like rain or snow precipitates out of the air.

39. 39 Metathesis Reactions Precipitation Reactions Molecular or formula unit equation Total ionic reaction

40. 40 Metathesis Reactions Net ionic reaction

41. 41 Metathesis Reactions Molecular or formula unit equation Total ionic reaction

42. 42 Metathesis Reactions Net ionic reaction