Chemical Reactions

1. Chemical Reactions 2/3- 2/4

2. Signs of a chemical reaction?

3. Indicators of chemical reactions • Emission of energy • Formation of a gas • Formation of a precipitate • Colorchange

4. What is a chemical reaction composed of? • Contains reactants and products • Formulas must be written correctly with symbols and subscripts 3) Law of conservation of matter requires that coefficients be used to ensure that atoms

5. Describing chemical reaction • The way atoms are joined is changed • Atoms aren’t created or destroyed. • May involve a catalyst • Can be described several ways In a sentence • Solid Copper reacts with chlorine gas to form aqueous copper (II) chloride. In a word equation or formula equation • Copper(s) + chlorine(g) ®copper(II) chloride(aq) • Cu(s) + Cl2(g)® CuCl2(aq)

6. Reaction Energy • All chemical reactions are accompanied by a change in energy.  • Exothermic - reactions that release energy to their surroundings (usually in the form of heat) • ΔH (enthalpy) is negative – energy leaving system • Endothermic - reactions that need to absorb heat from their surroundings to proceed. • ΔH (enthalpy) is positive – energy coming into the system

7. Reaction Energy • Spontaneous Reactions - Reactions that proceed immediately when two substances are mixed together.  Not all reactions proceed spontaneously. • Some require… • Activation Energy – the amount of energy that is required to start a chemical reaction. • Once activation energy is reached the reaction continues until you run out of material to react.  

8. What is a catalyst? • Does not cause a reaction to occur, but speeds up the rate which a reaction occurs • Can be in the form of the following: • Energy- light, heat • Chemicals • Enzymes are biological or protein catalysts.

10. Word and Formula Equations

11. Word Equations • Word Equations: an equation in which the reactants and products in a chemical reaction are represented by words instead of chemical formulas. • The problem with word equations is they do not actually show the number of atoms or molecules of each substance… formulas would have to be written out for this to happen. (Absent? We looked at examples of these in class)

12. Formula Equations • Represents reactants and products of a chemical reaction by their symbols or formulas. • Unbalanced- does not account for law of conservation of matter • Balanced- using coefficients show the representative numbers • See table 2 on page 266 for important symbols

13. Summary of Symbols

14. Word Formula practice Fe(s) + O2(g)® Fe2O3(s) Solid iron reacts with oxygen gas to form solid iron III oxide (rust).

15. Formula Equation Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water. HNO3(aq) + Na2CO3 (s)® NaNO3(aq) + H2O(l)

16. Convert this to a formulaequation Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (II) chloride and hydrogen sulfide gas. Fe2S3(s) + HCl(g)®FeCl2 (s) + H2S(g)

17. A silver spoon tarnishes. The solid silver reacts with sulfur in the air to make solid silver sulfide, the black material we call tarnish and water. Ag (s) + H2S (g) + O2(g)® Ag2S (s) + H2O

18. In-class work • In-class we practiced writing word and formula equations for different chemical reactions. • See chemical reactions sheet

19. Balancing Equations ___ H2(g) + ___ O2(g) ---> ___ H2O(l) 2 2 • What Happened to the Other Oxygen Atom? • This equation is not balanced! Until…

20. CH4 + 2 O2 CO2 + 2 H2O Reactants Products 1 C atom 1 C atom 4 H atoms 4 H atoms 4 O atoms 4 O atoms Timberlake, Chemistry 7th Edition, page 167

21. Reactants  Products + C(s) + O2(g) CO2(g) carbon oxygen carbon dioxide Reactants Product 1 carbon atom 1 carbon atom 2 oxygen atoms 2 oxygen atoms catalyst – speeds up reaction + Pt H2(g) + O2(g) H2O(l) hydrogen oxygen water 2 2 Pt Reactants 4 hydrogen atoms 4 hydrogen atoms 2 oxygen atoms 2 oxygen atoms Reactants Product 2 hydrogen atoms 2 hydrogen atoms 2 oxygen atoms 1 oxygen atoms Reactants Product 2 hydrogen atoms 4 hydrogen atoms 2 oxygen atoms 2 oxygen atoms Un balanced Timberlake, Chemistry 7th Edition, page 164

22. Cl H H H Cl Cl Cl H reactants products reactants products H H Cl Cl Unbalanced and Balanced Equations H Cl Cl Cl H H H2 + Cl2 2 HCl (balanced) (unbalanced) H2 + Cl2 HCl 2 1 2 2 1 2 2 2

23. Types of Reactions • There are millions of reactions • Objectives: • We will learn 6 types. • We will be able to predict the products. • We will be able to predict whether they will happen at all.

24. Synthesis Reactions • Also called combination reactions • 2 elements, or compounds combine to make one compound. • A + B ® AB • Na (s) + Cl2 (g)® NaCl (s) • Ca (s) +O2 (g)® CaO (s) • SO3 (s) + H2O (l)® H2SO4 (s) • We can predict the products if they are two elements. • Mg (s) + N2 (g)® Mg3N2(s)

25. A simulation of the reaction: 2H2 + O2   ® 2H2O  

26. Synthesis Reaction Direct combination reaction (Synthesis) 2 Na + Cl2 2 NaCl Cl Na Na Cl  Cl Cl Na Na General form: A + B  AB element or element or compound compound compound

27. Synthesis Reaction Direct combination reaction (Synthesis) 2 Mg + O2 2 MgO Mg Mg2+ O2- O O2- O Mg Mg2+ General form: A + B  AB element or element or compound compound compound

28. Decomposition Reactions • decompose = fall apart • one compound (reactant) falls apart into two or more elements or compounds. • Usually requires energy • AB ® A + B • NaCl Na + Cl2 • CaCO3 CaO + CO2

29. Decomposition Reactions • Can predict the products if it is a binary compound • Made up of only two elements • Falls apart into its elements • H2O • HgO H2(g) + O2(g) Hg (s) + O2(g)

30. Decomposition Reactions • If the compound has more than two elements you must be given one of the products • The other product will be from the missing pieces • NiCO3(aq) • H2CO3(aq)® CO2(g) + Ni (s) H2(g) + CO2(g)

31. H H H H Decomposition Reaction Decomposition reaction 2 H2O + 2 H2 O2 H O O H + H O O H General form: AB A + B compound two or more elements or compounds

32. Single Replacement • single displacement • One element replaces another • Reactants= an element and a compound. • Products= a different element and a different compound. • A + BC ® AC + B • Based on the activity series (in other words the element replacing must be more active.

33. Element Reactivity Activity Series Li Rb K Ba Ca Na Mg Al Mn Zn Cr Fe Ni Sn Pb H2 Cu Hg Ag Pt Au Halogen Reactivity F2 Cl2 Br2 I2 Foiled again – Aluminum loses to Copper

34. Potassium reacts with Water P O W !

35. Double Replacement • Two things replace each other. • Reactants must be two ionic compounds or acids. • Usually in aqueous solution AB+ CD ®AD + CB ZnS + 2HCl ® ZnCl + H2S AgNO3 + NaCl ® AgCl+ NaNO3

36. Double Replacement Reaction

37. Formation of a solid AgCl AgNO3(aq) + KCl(aq)  KNO3 (aq) + AgCl(s)

38. Single and Double Replacement Reactions Single-replacement reaction Mg + CuSO4 MgSO4 + Cu General form: A + BC  AC + B Double-replacement reaction CaCO3 + 2 HCl  CaCl2 + H2CO3 General form: AB + CD  AD + CB

39. Combustion • A reaction in which a compound (often carbon) reacts with oxygen • CH4 + O2 ®CO2 + H2O • C3H8 + O2® CO2 + H2O • C6H12O6 + O2® CO2 + H2O

40. The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. The chemical equation for this reaction is C + O2 CO2

41. Exploding Flour FLOUR EXPLOSION: Fuel (flour) Ignition (candle) Oxygen (combustion) 6” PVC Pipe As confinement increases, EXPLOSION is greater.

42. Combustion of a Hydrocarbon GENERAL FORMULA: CH + O2 CO2 + H2O Many homes get heat from propane (C3H8) heaters. Write a balanced chemical equation for the complete combustion of propane gas. C3H8(g) + O2(g)  CO2(g) + H2O(g) C3H8(g) + O2(g)  CO2(g) + H2O(g) 5 3 4 + energy

43. Combustion of Hydrocarbon (cont.) Ideal Stoichiometry D C3H8(g) + O2(g)  CO2(g) + H2O(g) 5 3 4 + energy Too ‘rich’ (not enough oxygen – too much fuel) D 3 4 C3H8(g) + O2(g)  3CO(g) + H2O(g) + energy 3 4 C3H8(g) + O2(g)  C (g) + H2O(g) 2 + energy SOOT

44. Combustion of Methane Gas Davis, Metcalfe, Williams, Castka, Modern Chemistry, 1999, page 245

45. + + = Combustion of Methane Gas One methane molecule Two oxygen molecules One carbon dioxide molecule Two water molecules CH4 2 O2 CO2 2H2O 1 carbon + 4 hydrogen 1 carbon + 2oxygen 2 oxygen + 4hydrogen 4oxygen Davis, Metcalfe, Williams, Castka, Modern Chemistry, 1999, page 245

46. Combustion of Glucose Kelter, Carr, Scott, Chemistry A World of Choices 1999, page 130

47. Combustion of Iron • Formation of Rust • Thermite Reaction • underwater welding • Temp. = ~3500oC 4 Fe+O2 2 Fe2O3 Fe2O3 + 2 Al  2 Fe + Al2O3 + 199 kcal

48. Combustion of Copper • Copper burns with a green color • Copper forms a patina (oxide) • green in color • CuO2 • black in color • CuO Statue of Liberty is covered with copper that has oxidized to form copper (II) oxide, CuO2.

49. Acid/Base Reaction • An acid and a base react to form a salt and water. • Always in aqueous solution • Acid (H+) + Base (OH-) →Salt + H2O • NaOH + HCl → NaCl + H2O • NH4OH + H2SO4 → (NH4)2SO4 + H2O

50. Redox How to recognize which type • Look at the reactants • Element(E), Compound(C) • E + E • C • E + C • C + C • Acid + Base • Look at the Products • CO2 + H2O Synthesis Decomposition Single replacement Double replacement Acid/Base reaction Combustion