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Module 4: Acid-Base Equilibria in Aqueous Solution

Chemistry 123. Module 4: Acid-Base Equilibria in Aqueous Solution. Click Here. Notes On This Presentation. This presentation is completely interactive In order for this presentation to work you MUST follow the indicated tabs on each slide

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Module 4: Acid-Base Equilibria in Aqueous Solution

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  1. Chemistry 123 Module 4: Acid-Base Equilibria in Aqueous Solution Click Here

  2. Notes On This Presentation • This presentation is completely interactive • In order for this presentation to work you MUST follow the indicated tabs on each slide • Answer the question on a separate piece of paper and compare with the slides • This presentation’s purpose is diagnose what you haven’t understood or what you haven’t attempted • Follow it properly and you will succeed! Next Slide

  3. Section 4.1 Acids, Bases and Conjugate Acid-Base Pairs When discussing Chemical Equilibria, a popular subtopic is the Chemical Equilibria involving Acid-Base Reactions. This topic can explore everything from the acid-base reactions occurring in the human body to the acid base reactions of occurring in water. In the following section, we will talk about the fundamental concepts required for this topic, Acids and Bases. Next Slide

  4. Section 4.1 BRONSTED-LOWRY According to Johannes Nicolaus Brønsted and Thomas Martin Lowry an ACID is a substance that DONATES a proton (H⁺ ion) NH₄⁺ → NH₃ + H⁺ And a BASE is a substance that ACCEPTS a proton (H⁺ ion) C₂H₅OH + H⁺ → C₂H₅OH₂⁺ Thus an Acid-Base Reaction is the reaction in which a proton is transferred from the Acid to the Base NH₄⁺ + C₂H₅OH ↔ NH₃ + C₂H₅OH₂⁺ Acids, Bases and Conjugate Acid-Base Pairs The first two reactions are reversible, which is why the last reaction is reversible, just easier to consider the forward reaction when considering what is an acid and base. Next Slide

  5. Section 4.1 ACID-BASE PAIRS An Acid-Base Pair (often referred to as a conjugate acid-base pair) is an acid and base pair that differ by only one proton. From the example from the previous slide, the Conjugate Acid-Base Pairs would be: NH₄⁺ + C₂H₅OH ↔ NH₃ + C₂H₅OH₂⁺ ACID-BASE PAIR: NH₄⁺ and NH₃ ACID-BASE PAIR: C₂H₅OH and C₂H₅OH₂⁺ If consider reactions in one direction, we can categorize the acid base products as conjugate acids or conjugates bases to there respected acid or base reactant. CH3COOH + NH3 → NH₄⁺ + CH3COO− Acids, Bases and Conjugate Acid-Base Pairs Conjugate Base Conjugate Acid Next Slide

  6. Section 4.1 LEWIS When dealing with Acid-Base reactions you will find that it will not always be the case that a Proton (H⁺ ion) is being transferred. For example, how would you explain that following reaction that exhibits acid-base properties: This was resolved when Gilbert N. Lewis proposed his definition of Acids and Bases. An ACID is an atom, ion, or molecule that can accept a pair of electrons to form a covalent bond. A BASE is an atom, ion, or molecule that can donate a pair of electrons to form a covalent bond. Acids, Bases and Conjugate Acid-Base Pairs Next Slide

  7. Section 4.1 LEWIS So your probably wondering, what is an electron pair acceptor or donor? Consider the following Lewis Reaction and there respected Orbital Structures: ↗ + Acids, Bases and Conjugate Acid-Base Pairs H F This electron pair has the ability to DONATE its electrons to the following molecule which will gladly ACCEPT this pair of electrons H N B F F N H F B F H H H F LEWIS ACID LEWIS BASE Next Slide

  8. Section 4.1 QUESTION SECTION QUESTION 1 A Bronsted-Lowry __________ is a proton acceptor and a Lewis __________ is an electron pair acceptor, while a Bronsted-Lowry __________ is a proton donor and a Lewis __________ is an electron pair donor. Acids, Bases and Conjugate Acid-Base Pairs Answer

  9. Section 4.1 QUESTION SECTION QUESTION 1 ANSWER A Bronsted-Lowry BASE is a proton acceptor and a Lewis ACID is an electron pair acceptor, while a Bronsted-Lowry ACID is a proton donor and a Lewis BASE is an electron pair donor. BASE ACID ACID BASE Acids, Bases and Conjugate Acid-Base Pairs Answer

  10. Section 4.1 QUESTION SECTION QUESTION 2 What is the acid, base, conjugate acid and conjugate base for the following reactions: Acids, Bases and Conjugate Acid-Base Pairs Answer

  11. Section 4.1 QUESTION SECTION QUESTION 2 ANSWER What is the acid, base, conjugate acid and conjugate base for the following reactions: Acids, Bases and Conjugate Acid-Base Pairs Base Acid Conjugate Base Conjugate Acid Answer

  12. Section 4.2 Self-Ionization of Water and the Ion Product of Water A popular substance in all areas of science is water because it exhibits properties that are very interesting to study. In chemistry, we can look at the equilibrium reaction that occurs within water and distinguish fundamental relationships between its reactants and products. Next Slide

  13. Section 4.2 SELF-IONIZATION OF WATER The equilibrium that exists within water is due to the self-ionization reaction between two water molecule to produce the ions H₃O⁺ and OH⁻. This can then be studied using the law of equilibrium and used to determine the equilibrium constant for the reaction. This equilibrium constant, as with all other constants, are dependent on TEMPERATURE and thus varies with different temperatures. The given value is at 25⁰C. Self-Ionization of Water and the Ion Product of Water This expression is often referred to as the ION PRODUCT OF WATER Next Slide

  14. Section 4.2 QUESTION SECTION QUESTION 3 Calculate the [H₃O⁺] and [OH⁻] for the ion product of water at 25⁰C. Self-Ionization of Water and the Ion Product of Water Next Slide

  15. Section 4.2 QUESTION SECTION QUESTION 3 ANSWER Calculate the [H₃O⁺] and [OH⁻] for the ion product of water at 25⁰C. Thus the [H₃O⁺] = [OH⁻] = 1 x 10⁻⁷ Self-Ionization of Water and the Ion Product of Water Next Slide

  16. Section 4.3 The pH Scale In chemistry, pH is the measure of the acidity or basicity of an aqueous solution. Thus the pH scale would be a standard in which the acidity and basicity of different aqueous substances could be compared. This scale expands an enormous range and exhibits exponential differences differences between substances. Next Slide

  17. Section 4.3 pH and pOH When comparing concentrations, chemists often use the pH or pOH scale depending on what's being compared (i.e. hydronium ions or hydroxide ions), this is because the concentrations of these ions in solution differ by exponential values, thus we determine a logarithmic value for all concentrations so that we can compare these values in a smaller range. This logarithmic value is an APPROXIMATION for the pH/pOH of a system because by definition it should represent the “activity” of the ions present in solution, which is roughly the same for dilute solutions. It is important to remember that these values aren’t increasing/decreasing in a linear fashion, it is the logarithmic function that allows these values to be represented in such a way. Basic The pH Scale Acidic Next Slide

  18. Section 4.3 RELATIONSHIPS If we relate the pH and pOH equations with the ion product of water we can achieve a new equation that will make it easy when converting between both scales. The pH Scale This is a common rule of logarithms Remember this relationship from Module 3 Next Slide

  19. Section 4.3 QUESTION SECTION QUESTION 4 What is the pH of a solution whose [H+] is 2.75 x 10-4 M? What is the pOH of a solution whose [H+] is 2.75 x 10-4 M? Is this solution acidic or basic? The pH Scale Next Slide

  20. Section 4.3 QUESTION SECTION QUESTION 4 ANSWER What is the pH of a solution whose [H+] is 2.75 x 10-4 M? What is the pOH of a solution whose [H+] is 2.75 x 10-4 M? Is this solution acidic or basic? Because the pH of the solution is below 7, the solution is ACIDIC. The pH Scale Next Slide

  21. Section 4.3 QUESTION SECTION QUESTION 5 What is the [H+] of a solution whose [OH-] is 9.31 x 10-2 M? Is this solution Acidic or Basic? The pH Scale Next Slide

  22. Section 4.3 QUESTION SECTION QUESTION 5 ANSWER What is the [H+] of a solution whose [OH-] is 9.31 x 10-2 M? Is this solution Acidic or Basic? This solution is BASIC because its pH value is greater than 7. The pH Scale Next Slide

  23. Section 4.3 QUESTION SECTION QUESTION 6 What is the pH in a 500 mL solution if there is 6 mols of HCL? Is this a good approximation for the pH of the solution? The pH Scale Next Slide

  24. Section 4.3 QUESTION SECTION QUESTION 6 ANSWER What is the pH in a 500 mL solution if there is 6 mols of HCL? Is this a good approximation for the pH of the solution? NO, this isn’t a good approximation. First off, in solutions of high concentrations of HCl it will not dissociate 100% leaving some HCl still in contact in solution, which would increase our found pH. Another point is that, as stated in the notes, pH should be calculated as the activity or effective concentrations of hydrogen ions when solutions are not dilute, which in this case, is not a dilute solution. HCl is a strong acid and dissociates 100% into ion form. The pH Scale Next Slide

  25. Section 4.4 Ionization of Acids and Bases in Water Because water can act as both a base and an acid, chemists use it as a test substance to determine, when mixed with others substances, if water acts as a base or acid, in return, allowing chemists to determine the seconds substance ability to be an acid or a base. Substance like water that can act as both a base or acid are called AMPHIPROTIC. Next Slide

  26. Section 4.4 WHEN IN WATER If your wondering why this reaction doesn’t revert back to the original reactants, it is because of a lot of factors that our chemistry minds aren’t ready for, but they including size of the molecule, electronegativity, and number of atoms and electrons within a molecule. In the end, these molecules are more favorable than any other possible combinations of the exchanged proton. When in water, an ACID will cause water to act as a BASE, inducing the formation of H₃O⁺ ions. EXAMPLE + → + Ionization of Acids and Bases in Water Water provides an electron pair for the H⁺ ion + H - H Cl O O Cl H H H H ACID BASE BASE ACID Next Slide

  27. Section 4.4 WHEN IN WATER When in water, a BASE will cause water to act as an ACID, inducing the formation of OH⁻ ions. EXAMPLE + ↘ + As said in the previous slide, this rearrangement of protons is the most favorable than any other possible rearrangement. Ionization of Acids and Bases in Water Ammonia provides an electron pair for the H⁺ ion + - H O N O H H N H H H H ACID H H BASE H BASE ACID Next Slide

  28. Section 4.4 RELATIONSHIPS When we add either an acid or base to water we can deduce simple inequalities between [H₃O⁺] and [OH⁻]. Because of these inequalities, it is easy to determine if the solution is Acidic, Basic or Neutral When we add BASE to water, we know that water acts as an ACID and induces the production of OH⁻ ions, but we know that water will still participate in its self-ionization reaction, thus we can say: [OH⁻] > [H₃O⁺] When we add ACID to water, we know that water acts as a BASE and induces the production of H₃O⁺ ions, but we know that water will still participate in its self-ionization reaction, thus we can say: [H₃O⁺] > [OH⁻] Ionization of Acids and Bases in Water In general, if a solution satisfies this inequality, the solution is BASIC In general, if a solution satisfies this inequality, the solution is ACIDIC Next Slide

  29. Section 4.4 ACID/BASE IONIZATION CONSTANTS When dealing with acid/base solutions, it is very useful to determine the strength of the acid/base solution (i.e. the solutions ability to lose (ACID) or accept (BASE) a proton). An easy way to determine the strength of both an ACID and BASE is to look at their respected Equilibrium constant which is often referred to as the IONIZATION CONSTANT. Ionization of Acids and Bases in Water These are the general Chemical Equations for Acid/Base Reactions. If we determine the reaction Quotient at equilibrium, we can determine the Ionization Constant and make assumptions about the strength of the Acid/Base in terms of the ionization constant. Next Slide

  30. Section 4.4 ACID/BASE IONIZATION CONSTANTS ANSWER If it is a strong Acid, we know that the concentration of the hydronium ion and the conjugate base would be much larger than that of the Acid, thus the value would be very large. In general: Ka >> 1 QUESTION What would the value of the ionization constant be if it were strong acid? Ionization of Acids and Bases in Water ANSWER If it is a weak Base, we know that the concentration of the hydroxide ion and the conjugate acid would be much smaller than that of the Base, thus the value would be very small. In general: Kb << 1 QUESTION What would the value of the ionization constant be if it were weak Base? Next Slide

  31. Section 4.4 COMMON STRONG ACIDS AND BASES Ionization of Acids and Bases in Water Next Slide

  32. Section 4.4 PERCENT IONIZATION The percent ionization of an acid/base solution is defined as the degree of ionization multiplied by 100%, where the degree of ionization is the amount of acid that ionized over the initial amount of acid. Ionization of Acids and Bases in Water Next Slide

  33. Section 4.4 QUESTION SECTION QUESTION 7 TRUE/FALSE 1) An acid with Kₐ = 10² is considered a Strong Acid. 2) The ionization of strong acids are more than 99%. 3) HI, HF, HBr and HCl are binary acids which are all considered Strong Acids. 4) H₂SO₄ is the strongest of Strong Acids because of its ability to give up two protons per molecule where both donations are above 99% ionization. 5) Acids protonate water molecules when dissolved in water, making the conjugate of water an ACID. Ionization of Acids and Bases in Water Next Slide

  34. Section 4.4 QUESTION SECTION QUESTION 7 ANSWER 1) TRUE, but more specifically, when Kₐ ≥ 10² it is a Strong Acid and the ionization of the Acid is 99% but, the initial concentration of the acid must be less than or equal to 1 mol/L 2) TRUE 3) FALSE, HF is not a Strong Acid, will discuss in later section. 4) FALSE, HSO₄⁻ , the second available proton donor does not qualify as a Strong Acid. 5) TRUE Ionization of Acids and Bases in Water Next Slide

  35. Section 4.4 QUESTION SECTION QUESTION 8 What is the [H₃O⁺], [OH⁻], pH and pOH for a solution that contains 5 mols of a Strong Acid Salt mixed with 10L of water? Ionization of Acids and Bases in Water Next Slide

  36. Section 4.4 QUESTION SECTION QUESTION 8 ANSWER First off, the equation for an acid reaction with water is: Ionization of Acids and Bases in Water Next Slide

  37. Section 4.4 QUESTION SECTION QUESTION 9 If it was understood that any solution that adds an acid or base with Ka or Kb value < 10⁻¹⁶ that the effect on pH and pOH would be inexistent, Then determine the pH and pOH of a 2 liter solution that contains .5 mols of a strong acid, .5 mols of a strong base, .5 mols of an acid with Ka = 10⁻³, .5 mols of a base with Kb = 10⁻⁵, 1 mol of a base with Kb = 10⁻²². Also, assume that these acids and bases have no effect on the other and there reactions hit an equilibrium dependent on each of there own reactants and products. Ionization of Acids and Bases in Water Next Slide

  38. Section 4.4 QUESTION SECTION QUESTION 9 ANSWER First off, we know right away that the concentration of the hydronium and hydroxide ions of the strong acid and base is: And right away, because there concentrations are the same, we know that they will form an equilibrium where an insignificant amount of moles of each would be present, thus leaving only moles of water. So we can immediately cross out these values of Acid and Base. We can also eliminate the base with low Kb value because of the known fact that its Kb is smaller than a Kb that would have an effect on pH. Ionization of Acids and Bases in Water Next Slide

  39. Section 4.4 QUESTION SECTION QUESTION 9 ANSWER So we must only consider the acid and base whose Ka and Kb is in the appropriate range. First the Acid, we find: Second the Base, we find: Ionization of Acids and Bases in Water Next Slide

  40. Section 4.4 QUESTION SECTION QUESTION 10 State the Strong Acids and Strong Bases!!! Ionization of Acids and Bases in Water Next Slide

  41. Section 4.4 QUESTION SECTION QUESTION 10 ANSWER Ionization of Acids and Bases in Water Next Slide

  42. Section 4.4 QUESTION SECTION QUESTION 11 What is the degree of ionization of a .5M acid whose Ka = 4 x 10⁻³? What is the degree of ionization of a .5M acid whose Ka = 4 x 10⁶? Ionization of Acids and Bases in Water Next Slide

  43. Section 4.4 QUESTION SECTION QUESTION 11 ANSWER What is the degree of ionization of a .5M acid whose Ka = 4 x 10⁻³? Thus, the degree of ionization is .0427/.5 =.0854 What is the degree of ionization of a .5M acid whose Ka = 4 x 10⁶? Since Ka >> 1, we know that the percent ionization is roughly 100%, and thus the degree of ionization is 1. Ionization of Acids and Bases in Water Next Slide

  44. Section 4.5 Molecular Structure and Acid Strength When dealing with Acids, it is often proper to compare their respected acid strengths by utilizing different properties of an acid like its Molecular Structure, in this section we will discuss these two topics and others that will allow us to rank and compare acids. Next Slide

  45. Section 4.5 ACID STRENGTH Molecular Structure and Acid Strength By definition, ACID STRENGTH is an acids ability or tendency to lose a proton. An Strong Acid than would have a large acid strength because its tendency to lose a proton is very high. In general, acid strength is determined by two factors BOND LENGTH and ELECTRONEGATIVITY. Next Slide

  46. Section 4.5 BOND LENGTH BOND LENGTH, or ATOMIC RADIUS can help deduce the strength of an acid. The reason this is due to the spatial arrangement of electrons that will exist around the atom. If the Atomic Radius (Bong length) increases, we know that the electrons will have more space to move, and thus the ability for the molecule to want to hold on to a proton is much less, allowing for the molecule to readily ionize. If we think of a molecule whose electrons are concentrated in a small area (i.e. smaller bond length) the molecule will more readily hold onto that proton because of the strong negative charge. Molecular Structure and Acid Strength Next Slide

  47. Section 4.5 ELECTRONEGATIVITY ELECTRONEGATIVITY plays a role in the ability for an acid to be strong because if we consider an atom with a high electronegativity, we know that the atom will have a higher tendency to attract electrons, which in result will cause a stronger attraction between the proton. This stronger attraction will want to prevent the proton from leaving. In general, the stronger the electronegativity, the weaker the acid strength. Molecular Structure and Acid Strength COURSE NOTES STATES The electron withdrawing character of a molecule will increase the acid strength. This is also true, because this implies that as electrons have the tendency to leave the acid strength will increase. Which is a correct statement. Next Slide

  48. Section 4.5 BINARY ACIDS A BINARY ACID, is an acid composed of only hydrogen atoms and one other atom. These types of acids are good to compare acid strength because they have similar properties because of this restriction on their chemical formula. In general: As the POLARITY of a molecule increases (i.e. the ability for one molecule to have a stronger pull than the other) the atomic strength increases. As the BOND LENGTH of a molecule increases, the atomic strength increases. We can generalize these relations on the periodic table. Molecular Structure and Acid Strength Next Slide

  49. Section 4.5 BINARY ACIDS In this direction, we will assume that the increase in POLARITY will deduce the strength of the acid. Molecular Structure and Acid Strength In this direction, we will assume that the increase in BOND LENGTH will deduce the strength of the acid Next Slide

  50. Section 4.5 BINARY ACIDS Yet another method at determining the strength of an acid has to do with the HETEROLYTIC BOND DISSOCIATION ENERGY. The heterolytic bond dissociation energy is the energy produced by the cleavage of a chemical bond in a molecule producing a cation and an anion. In general we will say that if it is easier to dissociate the acid into its ions, than the Stronger the Acid. Molecular Structure and Acid Strength Next Slide

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