1. Chemical Reactions Chapter 5 Dr. Victor Vilchiz

2. Types of Chemical Reactions Acid-Base Reactions Neutralization Reactions Neutralization Reactions In a reaction involving HCN(aq), a weak acid, and KOH(aq), a strong base, the product is KCN, a strong electrolyte Referring to Tables 4.1, 4.2 and 4.3, we obtain this net ionic equation: H+H+ Note the proton transfer.

3. Types of Chemical Reactions Acid-Base Reactions Acid-Base Reactions with Gas Formation Acid-Base Reactions with Gas Formation Carbonates react with acids to form CO 2, carbon dioxide gas. Sulfites react with acids to form SO 2, sulfur dioxide gas.

4. Gas Production in Neutralization Reactions The previous two reactions are overall reactions of the actual molecular events. The previous two reactions are overall reactions of the actual molecular events.

5. Types of Chemical Reactions Acid-Base Reactions Acid-Base Reactions with Gas Formation Acid-Base Reactions with Gas Formation The Driving Force of Neutralization reactions like that in precipitation reactions is the removal of ions from solution in this case to form water. The Driving Force of Neutralization reactions like that in precipitation reactions is the removal of ions from solution in this case to form water. Sulfides react with acids to form H 2 S, hydrogen sulfide gas.

6. Working with Solutions The majority of chemical reactions discussed so far occur in aqueous solution. The majority of chemical reactions discussed so far occur in aqueous solution. When you run reactions in liquid solutions, it is convenient to dispense the amounts of reactants by measuring out volumes of reactant solutions and not mass.

7. Solution Stoichiometry Molarity is the measurement of the concentration of a chemical in solution. Molarity is the measurement of the concentration of a chemical in solution. –The unit of molarity is the Molar (M). Example: Calculate the molarity of a solution made by dissolving 12.94g of Ca(OH) 2 in enough water to make 1.23L of solution. (see Figure 4.19)

8. Molarity Example: how many grams of ammonium nitrate are in a 172.7mL sample of 1.21M NH 4 NO 3 solution?

9. Diluting Solutions When diluting a solution the number of moles is constant. When diluting a solution the number of moles is constant. (Molarity)(Volume) =moles M 1 xV 1 = n = M 2 xV 2 So, as water is added, increasing the final volume, V 2, the final molarity, M 2, decreases. M 2 =M 1 xV 1 /V 2

10. Acid Base Titrations A titration is a laboratory technique used to determine the concentration of a solution sample from the volume of a known concentration solution required to complete a given reaction. A titration is a laboratory technique used to determine the concentration of a solution sample from the volume of a known concentration solution required to complete a given reaction. Titrations are usually used to determine the concentration of acids or bases. Titrations are usually used to determine the concentration of acids or bases.

11. Acid/Base Indicators Most acids and bases as well as the resulting salt solution are colorless. Most acids and bases as well as the resulting salt solution are colorless. –In order to determine when the reaction is complete, we must use chemical indicators. Chemical Indicators in the case of acid/base reactions are weak acids that have the property of changing colors when going from basic to acidic solutions or vice-versa. Chemical Indicators in the case of acid/base reactions are weak acids that have the property of changing colors when going from basic to acidic solutions or vice-versa. Chemical Indicators Chemical Indicators The most used acid/base indicator is phenolphthalein. The most used acid/base indicator is phenolphthalein.

12. Indicators The job of the indicator is to signal to you the point when you are done with the experiment. The point when the color changes is defined as the end point. The job of the indicator is to signal to you the point when you are done with the experiment. The point when the color changes is defined as the end point. The equivalence point is not the same as the end point, ideally it should be but those occasions are rare. The equivalence point is when the amount of titrant added is exactly the amount needed to “neutralize” the analyte in the flask. The equivalence point is not the same as the end point, ideally it should be but those occasions are rare. The equivalence point is when the amount of titrant added is exactly the amount needed to “neutralize” the analyte in the flask.

13. Types of Chemical Reactions Oxidation-Reduction (RedOx) Reactions Oxidation-Reduction (RedOx) Reactions RedOx reactions are by far the most important type of reactions. RedOx reactions involve the transfer of electrons from one species to another. Oxidation is defined as the loss of electrons. Reduction is defined as the gain of electrons. Oxidation and reduction always occur simultaneously, since the electrons lost in the Oxidation must go somewhere.

14. Types of Chemical Reactions Oxidation-Reduction Reactions Oxidation-Reduction Reactions The reaction of an iron nail with a solution of copper(II) sulfate, CuSO 4, is an oxidation- reduction reaction. (see Figure 4.11)(see Figure 4.11) The molecular equation for this reaction is:

15. Types of Chemical Reactions Oxidation-Reduction Reactions Oxidation-Reduction Reactions The net ionic equation shows the reaction of iron metal with Cu 2+ (aq) to produce iron(II) ion and copper metal. Loss of 2 e -1 oxidation Gain of 2 e -1 reduction

17. RedOx Reactions The species that is reduced itself causes another species to be oxidized and is therefore known as the oxidizing agent. The species that is reduced itself causes another species to be oxidized and is therefore known as the oxidizing agent. Similarly, the species that are oxidized causes another to be reduced and is therefore known as the reducing agent. Similarly, the species that are oxidized causes another to be reduced and is therefore known as the reducing agent. –Note: An element is reduced/oxidized the compound containing that element is the oxidizing/reducing agent. Note: An element is reduced/oxidized the compound containing that element is the oxidizing/reducing agent.Note: An element is reduced/oxidized the compound containing that element is the oxidizing/reducing agent.

18. Types of Chemical Reactions Oxidation Numbers Oxidation Numbers The concept of oxidation numbers is a simple way of keeping track of electrons in a reaction. The oxidation number (or oxidation state) of an atom in a substance is the actual charge of the atom if it exists as a monatomic ion. Alternatively, it is hypothetical charge assigned to the atom in the substance by simple rules. Oxidation-Reduction Reactions

19. Oxidation Number Rules

20. Oxidation Numbers It is possible to predict the upper and lower limits of main group elements… It is possible to predict the upper and lower limits of main group elements… –The upper limit is equal to the group number –The lower limit is the group number-8 Keep in mind couple things like… Oxygen will never have an ON=+6 and Fluorine will never have an ON=+7 Keep in mind couple things like… Oxygen will never have an ON=+6 and Fluorine will never have an ON=+7

21. Types of Chemical Reactions Describing Oxidation-Reduction Reactions Describing Oxidation-Reduction Reactions Oxidation-Reduction Reactions Look again at the reaction of iron with copper(II) sulfate. The Iron losses 2 electrons so it is oxidized and at the same time it is the reducing agent. The Copper gains 2 electrons and so it is reduced and at the same time it is the oxidizing agent.

22. Writing RedOx reactions There are two ways to deal with RedOx reactions (balancing purposes): There are two ways to deal with RedOx reactions (balancing purposes): –Treat them as any other reaction –We can write this reaction in terms of two half-reactions. Driving force for these type of reactions is the exchange of electrons. Driving force for these type of reactions is the exchange of electrons.

23. Types of Chemical Reactions Describing Oxidation-Reduction Reactions Describing Oxidation-Reduction Reactions Oxidation-Reduction Reactions A half-reaction is one of the two parts of an oxidation-reduction reaction. One involves the loss of electrons (oxidation) and the other involves the gain of electrons (reduction). oxidation half-reaction reduction half-reaction

24. Types of Chemical Reactions Types of Oxidation-Reduction Reactions Types of Oxidation-Reduction Reactions Oxidation-Reduction Reactions Most of the oxidation-reduction reactions fall into one of the following simple categories: Combination Reactions Decomposition Reactions Displacement Reactions Combustion Reactions

25. Types of Chemical Reactions Combination Reactions Combination Reactions Oxidation-Reduction Reactions. A combination reaction is a reaction in which two substances, usually two elements, combine to form a third substance. Sodium and chlorine combine in a fiery reaction. (see Figure)(see Figure)

26. Types of Chemical Reactions Combination Reactions Combination Reactions Oxidation-Reduction Reactions Other combination reactions involve compounds as reactants.

27. Types of Chemical Reactions Decomposition Reactions Decomposition Reactions Oxidation-Reduction Reactions A decomposition reaction is a reaction in which a single compound reacts to give two or more substances.

28. Types of Chemical Reactions Displacement Reactions Displacement Reactions Oxidation-Reduction Reactions A displacement reaction (also called a single- replacement reaction) is a reaction in which an element reacts with a compound, displacing an element from it.

29. Types of Chemical Reactions Combustion Reactions Combustion Reactions Oxidation-Reduction Reactions A combustion reaction is a reaction in which a substance reacts with oxygen, usually with the rapid release of heat to produce a flame.

30. Types of Chemical Reactions Balancing Simple Oxidation-Reduction Reactions Balancing Simple Oxidation-Reduction Reactions Oxidation-Reduction Reactions At first glance, the equation representing the reaction of zinc metal with silver(I) ions might appear to be balanced. However, a balanced equation must have a charge balance as well as a mass balance.

31. Types of Chemical Reactions Balancing Simple RedOx Reactions Balancing Simple RedOx Reactions Oxidation-Reduction Reactions As mentioned before we can split the reaction into two half-cells before balancing. You will learn this method in Chem 102 when you cover chapter 20. We will balance RedOx reactions using the Oxidation Number method.

32. Types of Chemical Reactions Balancing Simple RedOx Reactions Balancing Simple RedOx Reactions –There are some steps to follow to balance these reactions. Assign ON to ALL elements in the reaction Assign ON to ALL elements in the reaction Identify the species that are oxidized/reduced Identify the species that are oxidized/reduced Compute the number of e - s lost in OX and gained in RED draw lines between the two pairs including the number of e - s lost/gained Compute the number of e - s lost in OX and gained in RED draw lines between the two pairs including the number of e - s lost/gained Multiply one or both reactions so that both numbers match, use these factors as balancing coefficients Multiply one or both reactions so that both numbers match, use these factors as balancing coefficients Balance any other species that were not involved in the electron exchange. Balance any other species that were not involved in the electron exchange. Oxidation-Reduction Reactions

33. Quantitative Analysis Analytical chemistry deals with the determination of composition of materials- that is, the analysis of materials. Analytical chemistry deals with the determination of composition of materials- that is, the analysis of materials. Quantitative analysis involves the determination of the amount of a substance or species present in a sample of material.

34. Quantitative Analysis Gravimetric analysis is a type of quantitative analysis in which the amount of a species in a material is determined by converting the species into a product that can be isolated and weighed. Gravimetric analysis is a type of quantitative analysis in which the amount of a species in a material is determined by converting the species into a product that can be isolated and weighed. Precipitation reactions are often used in gravimetric analysis. The precipitate from these reactions is then filtered, dried, and weighed. Gravimetric Analysis

35. Quantitative Analysis Consider the problem of determining the amount of lead in a sample of drinking water. Consider the problem of determining the amount of lead in a sample of drinking water. Gravimetric Analysis Adding sodium sulfate (Na 2 SO 4 ) to the sample will precipitate lead(II) sulfate. The PbSO 4 can then be filtered, dried, and weighed. This figure shows a similar procedure.figure

36. Quantitative Analysis Suppose a 1.00 L sample of polluted water was analyzed for lead(II) ion, Pb 2+, by adding an excess of sodium sulfate to it. The mass of lead(II) sulfate that precipitated was 229.8 mg. What is the mass of lead in a liter of the water? Express the answer as mg of lead per liter of solution. Suppose a 1.00 L sample of polluted water was analyzed for lead(II) ion, Pb 2+, by adding an excess of sodium sulfate to it. The mass of lead(II) sulfate that precipitated was 229.8 mg. What is the mass of lead in a liter of the water? Express the answer as mg of lead per liter of solution. Gravimetric Analysis

37. Quantitative Analysis Gravimetric Analysis First we must obtain the mass percentage of lead in lead(II) sulfate, by dividing the molar mass of lead by the molar mass of PbSO 4, then multiplying by 100. First we must obtain the mass percentage of lead in lead(II) sulfate, by dividing the molar mass of lead by the molar mass of PbSO 4, then multiplying by 100. Then, calculate the amount of lead in the PbSO 4 precipitated.

38. Quantitative Analysis Volumetric Analysis An important method for determining the amount of a particular substance is based on measuring the volume of the reactant solution. An important method for determining the amount of a particular substance is based on measuring the volume of the reactant solution. Titration is a procedure for determining the amount of substance A by adding a carefully measured volume of a solution with known concentration of B until the reaction of A and B is just complete. Volumetric analysis is a method of analysis based on titration.

39. Chemical Reactions Reactions often involve ions in aqueous solution. Many of these compounds are electrolytes. Reactions often involve ions in aqueous solution. Many of these compounds are electrolytes. We can represent these reactions as molecular equations, complete ionic equations (with strong electrolytes represented as ions), or net ionic equations (where spectator ions have been canceled). Most reactions are either precipitation reactions, acid- base reactions, or oxidation-reduction reactions. Acid-base reactions are proton-transfer reactions. Oxidation-reduction reactions involve a transfer of electrons from one species to another. Summary

40. Chemical Reactions Oxidation-reduction reactions are the most important type of reactions. Oxidation-reduction reactions are the most important type of reactions. Oxidation-reduction reactions usually fall into the following categories: combination reactions, decomposition reactions, displacement reactions, and combustion reactions. Oxidation-reduction reactions usually fall into the following categories: combination reactions, decomposition reactions, displacement reactions, and combustion reactions. Molarity is defined as the number of moles of solute per liter of solution. Knowing the molarity allows you to calculate the amount of solute in a given volume of solution. Quantitative analysis involves the determination of the amount of a species in a material. Summary

41. Chemical Reactions In gravimetric analysis, you determine the amount of a species by converting it to a product you can weigh. Summary In volumetric analysis, you determine the amount of a species by titration.

42. Operational Skills Using solubility rules. Writing net ionic equations. Deciding whether precipitation occurs. Classifying acids and bases as weak or strong. Writing an equation for a neutralization. Writing an equation for a reaction with gas formation. Assigning oxidation numbers. Balancing simple oxidation-reduction reactions. Calculating molarity from mass and volume. Using molarity as a conversion factor. Diluting a solution. Determining the amount of a substance by gravimetric analysis. Calculating the volume of reactant solution needed. Calculating the quantity of a substance by titration.

43. Dissociation Return to Lecture

44. Electrolytic Solutions Negatively charged ions move towards the positive electrode and positively charged ions moves towards the negative electrode Return to Lecture

45. Conductivity Since there are no “free” ions on non-electrolytic solutions there is no flow of energy and the bulb does not lit. The “free” ions on electrolytic solutions completer the circuit and allow energy to flow lighting the bulb. Return to Lecture

46. Weak vs Strong Electrolytes While both types of electrolytes conduct electricity the amount of “free” ions manifests itself by the brightness of the light emitted by the bulb. Return to Lecture

47. Water Solvation of Ions Return to Lecture

48. Precipitation Mixing KI (aq) and Pb(NO 3 ) 2 (aq) leading to precipitation of PbI 2 Return to Lecture

49. Figure 4.5: Limestone Formations. Photo ©Corbis. Return to Lecture

50. Return to Lecture

52. Figure 4.7: Household acids and bases. Photo courtesy of American Color. Return to Lecture

53. Figure 4.8: Preparation of red cabbage juice as an acid-base indicator. Photo courtesy of James Scherer. Return to Lecture

54. Neutralization Reaction Neutralization reaction between Acetic Acid (Vinegar) and Baking Soda (Sodium Bicarbonate).

55. Making a Solution a) Measure the mass of the compound to dissolve, b)make sure all solid makes it into the volumetric flask using the solvent, then dilute to the bottom of the neck, and c) add the last amount of solvent carefully making sure not to go past the volumetric mark. a)b) c) Return to Lecture

56. Titration Titration of an acid with a base. a) the flask contains acid and a few drops of phenolphthalein, which is colorless in acidic conditions, b) the endpoint has been reached (notice the faint pink color of the solution), finally in c), the solution is well beyond the end point since more base was added. a) b) c) Return to Lecture

57. Electron Exchange Electrons from the iron nail are transferred to the copper in solution, the solid copper plates the nail. Notice the color change in the solution, this is due to the lower amount of copper ions in solution. Return to Lecture

58. RedOx Summary Return to Lecture

59. Figure 4.13: A representation of an oxidation reduction reaction. Return to Lecture

60. Figure 4.14: Oxidation reduction reaction of mercury (III) oxide into its elements. Photo courtesy of James Scherer. Return to Lecture

61. Figure 4.15: Oxidation reduction reaction of zinc metal and hydrochloric acid. Photo courtesy of American Color. Return to Lecture

62. Figure 4.16: Oxidation reduction reaction of iron wool and oxygen. Photo courtesy of James Scherer. Return to Lecture

63. Calcium/Chlorine Reaction Return to Lecture

64. Gravimetric Analysis A solution of potassium chromate is poured down a stirring rod into a solution containing an unknown amount of barium ion to precipitate Barium Chromate which is then filtered dried and weighed. Return to Lecture