Chemical Reactions

1. 0 Chemical Reactions Chapter 5 Dr. Victor Vilchiz

2. 0 Solution Components • In order to have a solution we must have at least “TWO” components. • The Solvent which is the compound present in biggest abundance, water in most cases presented in this chapter. • The Solute which is the “impurity” or compound present in the smallest amount. • The resulting mixture is the solution.

3. 0 Dissolving Ionic Compounds • Some compounds dissociate when placed in water. • This does not mean we will get the constituent elements once in water. • For example: when table salt (NaCl) is added to a container with water as soon as the grains of salt touch the water NaCl ceases to exist. In solution, we have the component ions Na+ and Cl-

4. 0 Ions in Aqueous Solution Ionic Theory of Solutions • Many ionic compounds dissociateinto independent ions when dissolved in water • These compounds that “freely” dissociate into independent ions in aqueous solution are called electrolytes. • Their aqueous solutions are capable of conducting an electric current. Figure 4.2 illustrates this.

5. 0 Ions in Aqueous Solution Ionic Theory of Solutions • Electrolytes are substances that dissolve in water to give an electrically conducting solution. • Thus, in general, ionic solids that dissolve in water are electrolytes. • Some molecular compounds, such as acids, also dissociate in aqueous solution and are considered electrolytes.

6. 0 Ions in Aqueous Solution Ionic Theory of Solutions • Molecular compounds that dissolve usually do not dissociate into ions. • These compounds are referred to as nonelectrolytes. They dissolve in water to give a nonconducting solution. Conductivity Test

7. 0 Ions in Aqueous Solution Ionic Theory of Solutions • There are few molecular compounds (acids & alcohols) that upon solvation dissociate into ions, this is due to the weak interaction between the atoms. • The resulting solution is electrically conducting, and so we say that the molecular substance is an electrolyte.

8. 0 Does water conduct? • As we have seen current/energy does not flow in a circuit unless there are “free” ions in solution. • This means that a sample or “pure” water will not conduct electricity or current. • Tap water conducts electricity and current because there are dissolved ions present put there on purpose and some from pipes dissolving.

9. 0 Strong Electrolytes Ionic Theory of Solutions • Electrolytes can be separated into two different categories • Strong electrolytes. • A strong electrolyte is an electrolyte that exists in solution almost entirely as ions. • The solvation process is represented by a one-way arrow in the chemical reaction implying a path of no return.

10. 0 Weak Electrolytes Ionic Theory of Solutions • Weak electrolytes. • A weak electrolyteis an electrolyte that dissolves in water to give a relatively small percentage of ions. • The solvation process is presented by a double sided arrow implying an equilibrium between reactants and products. • Most soluble molecular compounds are either nonelectrolytes or weak electrolytes. Strength Test

11. 0 Why dissociation? • Dissociation takes place because the attractive force between the ions can be overcome by other forces. • The solvent is able to surround ions and provide stronger forces of attraction. Why? • If water has no charge how can it create this attractive forces that compete with coulombic interaction? • This can be explained if we look at the electron distribution in water.

12. 0 Polarity • Electron distribution • If we draw a water molecule representing its true shape we will see that the electrons are not evenly distributed. While there are no real charges the difference in electron density acts as a separation of charges which leads to a pseudo ionic behavior. The Red represents a high density of electrons (-); the blue represents a low density of electrons (+). The separation of charge we see in water is labeled as the polarity of the molecule; the higher the difference in electron density the higher the polarity of the molecule.

13. 0 Polarity • The polarity of a molecule depends mainly on two factors • Shape of the molecule • Composition • It is represented by an arrow with its head pointing towards the negative charge side and a crossed tail on the positive side of the molecule

14. 0 Polarity and Solvation • Molecules that have a separation of charges are called polar molecules • When ionic compounds are added to water, the ions break apart and the water molecules arrange themselves so the negative end (O) points towards the cations and the positive end (H’s) point towards the anions. Representation

15. 0 Insoluble Compounds • No compound is really insoluble. • However, if the amount that dissolves is compared to the starting amount it is found to be insignificant that they are said to be insoluble. • Example • NaCl in H2O @20°C =365g/L • AgCl in H2O @20°C =0.009g/L

16. 0 Chemical Equations Molecular and Ionic Equations • A molecular equation is one in which the reactants and products are written as if they were molecules, even though they may actually exist in solution as ions. • Note that Ca(OH)2, Na2CO3, and NaOH are all soluble compounds but CaCO3 is not.

17. 0 Chemical Equations Molecular and Ionic Equations • An ionic equation, however, represents strong electrolytes as separate independent ions. This is a more accurate representation of the way electrolytes behave in solution. The downward arrow represents a precipitate which will fall to the bottom of the container

18. (strong) (insoluble) (strong) (strong) 0 Chemical Equations Molecular and Ionic Equations • A complete ionic equation is a chemical equation in which strong electrolytes (such as soluble ionic compounds) are written as separate ions in solution. • Complete and net ionic equations

19. 0 Chemical Equations Molecular and Ionic Equations • Complete and net ionic equations. • A net ionic equation is a chemical equation from which the spectator ions have been removed. • A spectator ion is an ion in an ionicequationthat does not take part in the reaction (present on both sides of the arrow in the same state.

20. 0 Chemical Equations Molecular and Ionic Equations • Complete and net ionic equations • Let’s try an example. First, we start with a molecular equation. • Nitric acid, HNO3, and magnesium nitrate, Mg(NO3)2, are both strong electrolytes.

21. 0 Chemical Equations Molecular and Ionic Equations • Complete and net ionic equations • Separating the strong electrolytes into separate ions, we obtain the complete ionic equation. • Note that the nitrate ions did not participate in the reaction. These are spectator ions.

22. 0 Chemical Equations Molecular and Ionic Equations • Complete and net ionic equations • Eliminating the spectator ions results in the net ionic equation. This equation represents the “essential” reaction.

23. 0 Types of Chemical Reactions • Most of the reactions we will study fall into one of the following categories • Precipitation Reactions • Acid-Base Reactions • Oxidation-Reduction Reactions

24. 0 Types of Chemical Reactions Precipitation Reactions • In a precipitation reactionwe start with 2 soluble compounds dissolved in water and when mixed they produce at least one insoluble compound, which precipitates (falls to the bottom). • For example, the reaction of sodium chloride with silver nitrate forms AgCl(s), an insoluble precipitate.

26. 0 Precipitation Reactions • Does this mean that if we mixed two soluble ionic compounds we will always form a precipitate? NO • What is the driving force for precipitation reactions? • While the formation of the solid can be viewed as the driving force, it is the removal of ions from solution that is the true driving force.

27. 0 Types of Chemical Reactions Precipitation Reactions • Solubility rules • Substances vary widely in their solubility, or ability to dissolve, in water. • For example, NaCl is very soluble in water whereas calcium carbonate, CaCO3, is insoluble in water. (see Figure 4.5)

28. 0 Types of Chemical Reactions Precipitation Reactions • Predicting Precipitation Reactions. • To predict whether a precipitate will form, we need to look at potential insoluble products. • Table 4.1 lists eight solubility rules for ionic compounds. These rules apply to the most common ionic compounds.

29. 0 Types of Chemical Reactions Precipitation Reactions • Predicting Precipitation Reactions. • Suppose you mix together solutions of nickel(II) chloride, NiCl2, and sodium phosphate, Na3PO4. • How can you tell if a reaction will occur, and if it does, what products to expect?

30. 0 Types of Chemical Reactions Precipitation Reactions • Predicting Precipitation Reactions. • Precipitation reactions have the form of an “exchange reaction.” • An exchange (or metathesis) reaction is a reaction between compounds that, when written as a molecular equation, appears to involve an exchange of cations and anions.

31. 0 Types of Chemical Reactions Precipitation Reactions • Predicting Precipitation Reactions. • Now that we have predicted potential products, we must balance the equation. 3 2 6 • We must verify that NiCl2 and Na3PO4 are soluble and then check the solubilities of the products.

32. 0 Types of Chemical Reactions Precipitation Reactions • Predicting Precipitation Reactions. • Table 4.1 indicates that our reactants, nickel(II) chloride and sodium phosphate are both soluble. (aq)(aq) (s)(aq) • Looking at the potential products we find that nickel(II) phosphate is not soluble although sodium chloride is.

33. 0 Types of Chemical Reactions Precipitation Reactions • Predicting Precipitation Reactions. • We predict that a reaction occurs because nickel(II) phosphate is insoluble and precipitates from the reaction mixture. • To see the reaction that occurs on the ionic level, we must rewrite the molecular equation as an ionic equation.

34. 0 Types of Chemical Reactions Precipitation Reactions • Predicting Precipitation Reactions. • First write strong electrolytes (the soluble ionic compounds) in the form of ions to obtain the complete ionic equation

35. This equation represents the “essential” reaction. 0 Types of Chemical Reactions Precipitation Reactions • Predicting Precipitation Reactions. • After canceling the spectator ions, you obtain the net ionic equation.

36. 0 Types of Chemical Reactions • Acid-Base Reactions • Acids and bases are some of the most important electrolytes. (see Table 4.2) • They can cause color changes in certain dyes called acid-base indicators. • Household acids and bases. (see Figure 4.7) • Red cabbage juice asan acid-base indicator.(see Figure 4.8)

37. Types of Chemical Reactions Acid-Base Reactions • The Ancient Concept • In ancient times acids and bases had a different meaning. • An acid was defined as a sour substance. • A Base was defined as a substance that was both bitter and slippery

38. Types of Chemical Reactions Acid-Base Reactions • The Arrhenius Concept • The Arrhenius concept definesacids as substances that contain H and produce hydrogen ions, H+, when dissolved in water. • An example is nitric acid, HNO3, a molecular substance that dissolves in water to give H+ and NO3-.

39. Types of Chemical Reactions Acid-Base Reactions • The Arrhenius Concept • The Arrhenius concept defines basesas substances that contain OH and produces hydroxide ions, OH-, when dissolved in water. • An example is sodium hydroxide, NaOH, an ionic substance that dissolves in water to give sodium ions and hydroxide ions. • He really meant contain OH-

40. Types of Chemical Reactions Acid-Base Reactions • The Arrhenius Concept • However, there are substances that we now classify as bases or acids but they do not follow the Arrhenius definition. • For example ammonia, NH3, is a base but it does not contain OH-, • Therefore we need a second definition that can take compounds like ammonia into account.

41. Types of Chemical Reactions Acid-Base Reactions • The Brønsted-Lowry Concept • The Brønsted-Lowry concept of acids and bases avoids the problems of composition inherent in the Arrhenius definitions by basing the definitions on the transfer of protons (H+) instead. • In this view, acid-base reactions are proton-transfer reactions and there must be two reactions taking place at once.

42. Types of Chemical Reactions Acid-Base Reactions • The Brønsted-Lowry Concept • The Brønsted-Lowry concept defines anacidas the species (molecule or ion) that donates a proton (H+) to another species in a proton-transfer reaction. • Abase is defined as the species (molecule or ion) that accepts the proton (H+) in a proton-transfer reaction.

43. H+ Types of Chemical Reactions Acid-Base Reactions • The Brønsted-Lowry Concept In the reaction of ammonia with water, • The H2O molecule is the acid because it donates a proton. The NH3 molecule is a base, because it accepts a proton. • Likewise NH4+ is an acid because it can donate one of the protons, and OH- is a base since it can accept a proton.

44. Types of Chemical Reactions Acid-Base Reactions • The Brønsted-Lowry Concept • The H+(aq) ion due to its small size has a very high positive charge density. • The polarity of the water molecules allowed for the water molecules to be closely associated with the proton making it appear as if the hydrogen ion was bonded to water molecules as it moves. • This “mode of transportation” for the H+ ion is called the hydronium ion.

45. H+ where HNO3 is an acid (proton donor) and H2O is a base (proton acceptor). Types of Chemical Reactions Acid-Base Reactions • The Brønsted-Lowry Concept The dissolution of nitric acid, HNO3, in water is therefore a proton-transfer reaction

46. Water and Acid/Base Rxns Acid-Base Reactions • As we have seen there are cases in which water: • Donates a Proton acting as an acid. • Accepts a Proton acting as a base. • Molecules can act both as acid or base depending on the environment they are in are called amphiprotic.

47. Types of Chemical Reactions Acid-Base Reactions • In summary, the Arrhenius concept is very basic and the Brønsted-Lowry concept was developed to cover cases left out; however, in water they are almost the same. • Arrhenius Concept • acid: proton (H+) donor to the water • base: hydroxide ion (OH-) donor to the water

48. Types of Chemical Reactions Acid-Base Reactions • In summary, any Arrhenius acid/base is also a Brønsted-Lowry acid/base but not the other way around. The Brønsted-Lowry concept acid: proton (H+) donor to anything base: proton (H+) acceptor from anything

49. Acid/Base Concepts Acid-Base Reactions Lewis Acid/Base Concept Bronsted-Lowry Acid/Base Concept Arrhenius Acid/Base Concept

50. Types of Chemical Reactions Acid-Base Reactions • Arrhenius Acids/Bases • Acids can be separated into two subcategories depending on the strength of the acid. • The Strength of the acid is determined by how easily it releases the proton. The easier it is to give the proton away the stronger the acid. • A strong acid is an acid that ionizes completely in water; it is a strong electrolyte.