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Module 4: Acid-Base Equilibria in Aqueous Solution Click Here Slide 2 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 presentations purpose is diagnose what you havent understood or what you havent attempted Follow it properly and you will succeed! Next Slide Slide 3 Acids, Bases and Conjugate Acid-Base Pairs Next Slide 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. Slide 4 Acids, Bases and Conjugate Acid-Base Pairs Next Slide According to Johannes Nicolaus Brnsted and Thomas Martin Lowry an ACID is a substance that DONATES a proton (H ion) NH NH + H And a BASE is a substance that ACCEPTS a proton (H ion) C HOH + H CHOH Thus an Acid-Base Reaction is the reaction in which a proton is transferred from the Acid to the Base NH + C HOH NH + CHOH 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. Slide 5 Acids, Bases and Conjugate Acid-Base Pairs Next Slide 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: NH + C H OH NH + C H OH ACID-BASE PAIR: NH and NH ACID-BASE PAIR: C H OH and C H OH 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. CH 3 COOH + NH 3 NH + CH 3 COO Conjugate Base Conjugate Acid Slide 6 Acids, Bases and Conjugate Acid-Base Pairs Next Slide 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. Slide 7 Acids, Bases and Conjugate Acid-Base Pairs Next Slide So your probably wondering, what is an electron pair acceptor or donor? Consider the following Lewis Reaction and there respected Orbital Structures: + N H H H B F F F This electron pair has the ability to DONATE its electrons to the following molecule which will gladly ACCEPT this pair of electrons N H H H F F F B LEWIS ACID LEWIS BASE Slide 8 Acids, Bases and Conjugate Acid-Base Pairs Answer 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. Slide 9 Acids, Bases and Conjugate Acid-Base Pairs Answer 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 BASE Slide 10 Acids, Bases and Conjugate Acid-Base Pairs Answer QUESTION 2 What is the acid, base, conjugate acid and conjugate base for the following reactions: Slide 11 Acids, Bases and Conjugate Acid-Base Pairs Answer QUESTION 2 ANSWER What is the acid, base, conjugate acid and conjugate base for the following reactions: Base Acid Conjugate Base Conjugate Acid Slide 12 Self-Ionization of Water and the Ion Product of Water Next Slide 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. Slide 13 Self-Ionization of Water and the Ion Product of Water Next Slide The equilibrium that exists within water is due to the self-ionization reaction between two water molecule to produce the ions H O 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 25 C. This expression is often referred to as the ION PRODUCT OF WATER Slide 14 Self-Ionization of Water and the Ion Product of Water Next Slide QUESTION 3 Calculate the [H O ] and [OH ] for the ion product of water at 25 C. Slide 15 Self-Ionization of Water and the Ion Product of Water Next Slide QUESTION 3 ANSWER Calculate the [H O ] and [OH ] for the ion product of water at 25 C. Thus the [H O ] = [OH ] = 1 x 10 Slide 16 The pH Scale Next Slide 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. Slide 17 The pH Scale Next Slide 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 arent increasing/decreasing in a linear fashion, it is the logarithmic function that allows these values to be represented in such a way. Basic Acidic Slide 18 The pH Scale Next Slide 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. This is a common rule of logarithms Remember this relationship from Module 3 Slide 19 The pH Scale Next Slide 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? Slide 20 The pH Scale Next Slide 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. Slide 21 The pH Scale Next Slide QUESTION 5 What is the [H + ] of a solution whose [OH - ] is 9.31 x 10 -2 M? Is this solution Acidic or Basic? Slide 22 The pH Scale Next Slide 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. Slide 23 The pH Scale Next Slide 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? Slide 24 The pH Scale Next Slide 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 isnt 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. Slide 25 Ionization of Acids and Bases in Water Next Slide 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. Slide 26 Ionization of Acids and Bases in Water Next Slide When in water, an ACID will cause water to act as a BASE, inducing the formation of H O ions. EXAMPLE + + Cl H H H ACID O BASEACIDBASE Cl O H H H Water provides an electron pair for the H ion If your wondering why this reaction doesnt revert back to the original reactants, it is because of a lot of factors that our chemistry minds arent 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. - + Slide 27 Ionization of Acids and Bases in Water Next Slide When in water, a BASE will cause water to act as an ACID, inducing the formation of OH ions. EXAMPLE + H H ACID O BASE ACID O H Ammonia provides an electron pair for the H ion As said in the previous slide, this rearrangement of protons is the most favorable than any other possible rearrangement. - N H H H BASE N H H H H + Slide 28 Ionization of Acids and Bases in Water Next Slide When we add either an acid or base to water we can deduce simple inequalities between [H O] 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] > [H O] When we add ACID to water, we know that water acts as a BASE and induces the production of H O ions, but we know that water will still participate in its self-ionization reaction, thus we can say: [H O] > [OH] In general, if a solution satisfies this inequality, the solution is BASIC In general, if a solution satisfies this inequality, the solution is ACIDIC Slide 29 Ionization of Acids and Bases in Water Next Slide 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. 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. Slide 30 Ionization of Acids and Bases in Water Next Slide QUESTION What would the value of the ionization constant be if it were strong acid? 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 weak Base? 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 Ionization of Acids and Bases in Water Next Slide QUESTION 11 ANSWER What is the degree of ionization of a.5M acid whose Ka = 4 x 10 ? Thus, the degree of ionization is.0427/.5 =.0854 What is the degree of ionization of a.5M acid whose Ka = 4 x 10 ? Since Ka >> 1, we know that the percent ionization is roughly 100%, and thus the degree of ionization is 1. Slide 44 Molecular Structure and Acid Strength Next Slide 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. Slide 45 Molecular Structure and Acid Strength Next Slide 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. Slide 46 Molecular Structure and Acid Strength Next Slide 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. Slide 47 Molecular Structure and Acid Strength Next Slide 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. 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. Slide 48 Molecular Structure and Acid Strength Next Slide 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. Slide 49 Molecular Structure and Acid Strength Next Slide In this direction, we will assume that the increase in BOND LENGTH will deduce the strength of the acid In this direction, we will assume that the increase in POLARITY will deduce the strength of the acid. Slide 50 Molecular Structure and Acid Strength Next Slide 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. Slide 51 Molecular Structure and Acid Strength Next Slide An OXO-ACID, is an acid that contains an oxygen attached to a hydrogen. In general, when deducing which acid is stronger, you are going to look for the number of electronegative elements in the molecules. A molecule that has more of the same electronegative element will deduce a Weaker Acid because there is more negative force pulling the hydrogen towards the rest of the molecule. It is also important to consider the position in which these electronegative elements are because the closer they are to the hydrogen atom, the closer the negative charge is to the proton and thus a Weaker Acid. Finally, we must consider the magnitude of the electronegative element. A more electronegative element will deduce a greater pull on the hydrogen atom, thus a Weaker Acid Slide 52 Molecular Structure and Acid Strength Next Slide There are two types of oxo-acids we will consider: The first, INORGANIC, involves the combination of hydrogen, oxygen, and a non-carbon element. EXAMPLE We know already that the molecule on the right is a Strong Acid, but does it make sense? Of course it does! Look at the following diagram. This molecule is has very little polarity because of the equal pull of the oxygen's. Thus we can show that these oxygens are reducing the overall pull on the hydrogen atom because as their numbers increase the force is divided amongst the new oxygen molecules. Thus we can say that as we increase the oxygen count in an inorganic oxo- acid, the Acid Strength increases because the pull on the proton is less. POLARITY? Slide 53 Molecular Structure and Acid Strength Next Slide The second, ORGANIC, involves the combination of hydrogen, oxygen, and Carbon creating carboxylic acids. EXAMPLE Which is the Stronger Acid? It is the left acid. This is because the right acid is pulling more on the proton because there is more electronegative elements pulling on the hydrogen ion to stay than there is in the left atom. Though these molecules pKa values are very similar, the left acid is considered Stronger! The left molecule has less atoms pulling on the hydrogen ion. Slide 54 Molecular Structure and Acid Strength Next Slide QUESTION 11 Rank these molecule in increasing Acid Strength. Slide 55 Molecular Structure and Acid Strength Next Slide QUESTION 11 ANSWER Rank these molecule in increasing Acid Strength. > > The molecule on the left is a strong acid, the molecule in the middle has more electrons pulling on the hydrogen atom, the molecule on the right has the most electron pulling on the hydrogen atom. Slide 56 Molecular Structure and Acid Strength Next Slide QUESTION 12 Rank these molecule in increasing Acid Strength. Slide 57 Molecular Structure and Acid Strength Next Slide QUESTION 12 ANSWER Rank these molecule in increasing Acid Strength. > > The first molecule is the Strongest Acid because it has the most Oxygen Atoms and its atom is more electronegative than the Second molecule which is also has 4 oxygen atoms which is more than third molecule which only has 3 oxygen atoms. Slide 58 Molecular Structure and Acid Strength Next Slide QUESTION 13 Rank these molecule in increasing Acid Strength. HCl, HI, HBr, HF, CH H O NH H Te, H Se. Slide 59 Molecular Structure and Acid Strength Next Slide QUESTION 13 ANSWER Rank these molecule in increasing Acid Strength. HI > HBr > HCl > H Te > HF > H Se > H O > NH > CH This follows the chart on page 17 in our course notes. HF is a special example, which doesnt exhibit the same properties as the other Strong Binary Acids. This is due to a two step process in which HF ionizes in water which is different than the other acids. Slide 60 Polyprotic Acids Next Slide A POLYPROTIC ACID, is an acid that contains more than one hydrogen ion available for ionization. These acids exhibit different properties than other acids which is why they are a popular topic in chemistry. Slide 61 Polyprotic Acids Next Slide As stated before, POLYPROTIC ACIDS are acids who contain more than one proton. Some key points about these acids are: First, K >> K >> Second, We can deduce the pH of a weak polyprotic acid by the first ionization step. Slide 62 Polyprotic Acids Next Slide QUESTION 14 Determine the pH of.9M H S with K = 1.0 x 10 and K = 1.0 x 10 ? Slide 63 Polyprotic Acids Next Slide QUESTION 14 ANSWER Determine the pH of.9M H S with K = 1.0 x 10 and K = 1.0 x 10 ? First off, since this is a weak polyprotic acid the second ionization step isnt necessary in calculating the pH. Slide 64 Polyprotic Acids Next Slide QUESTION 15 Approximate the concentration of PO given x mol/L H PO,where x > 0, with K = 7.1 x 10, K = 6.3 x 10 and K = 4.2 x 10 . Slide 65 Polyprotic Acids Next Slide QUESTION 15 ANSWER Approximate the concentration of PO given x mol/L H PO,where x > 0, with K = 7.1 x 10, K = 6.3 x 10 and K = 4.2 x 10 . First off, we know that H PO is a strong acid, thus: Second, we form an ice table and realize that K is very small, so we can do an approximation: Slide 66 Polyprotic Acids Next Slide QUESTION 15 ANSWER Approximate the concentration of PO given x mol/L H PO,where x > 0, with K = 7.1 x 10, K = 6.3 x 10 and K = 4.2 x 10 . Finally, we will solve for the last ionization step, as done in the second step, dont forget to include the concentration values from the first step: Slide 67 Polyprotic Acids Next Slide QUESTION 16 Approximate the concentration of PO given 1 mol/L H PO using the equation determined in the previous example, with K = 7.1 x 10, K = 6.3 x 10 and K = 4.2 x 10 . Slide 68 Polyprotic Acids Next Slide QUESTION 16 ANSWER Approximate the concentration of PO given 1 mol/L H PO using the equation determined in the previous example, with K = 7.1 x 10, K = 6.3 x 10 and K = 4.2 x 10 . Slide 69 Systematic Treatment of Equilibrium Next Slide When dealing with approximations, we havent considered the complexity of an equilibrium. In this section, we will discuss methods of determining equations that are more accurate when discussing equilibrium calculations. Slide 70 Systematic Treatment of Equilibrium Next Slide A MASS BALANCE equation will contain all species that contain the main element in use. For example, for the mass balance of H PO, we would include all species that contain phosphate. EXAMPLE (H SO) : This value is the ORIGANAL concentration. Slide 71 Systematic Treatment of Equilibrium Next Slide A CHARGE BALANCE equation will contain all species that contain a charge and will also contain a factor of the charge. This relationship deals with amount of electrons in a reaction which must remain constant. EXAMPLE (H SO) : You must remember the equilibrium of water, which is always occurring in the background. Slide 72 Systematic Treatment of Equilibrium Next Slide QUESTION 17 Determine the mass and charge balance equations for NaHSO Slide 73 Systematic Treatment of Equilibrium Next Slide QUESTION 17 ANSWER First off, what chemical equations must we consider for both the mass balance and charge balance equations? NEVER FORGET THIS EQUILIBRIUM Slide 74 Systematic Treatment of Equilibrium Next Slide QUESTION 18 Determine the pH of 1 x 10 M HI using mass and charge balance equations. Slide 75 Systematic Treatment of Equilibrium Next Slide QUESTION 18 ANSWER Determine the pH of 1 x 10 M HI using mass and charge balance equations. By the Quadratic Equation, we find that the concentration of the hydronium Ion is toughly 1.005 x 10 M, thus, when inputted into the pH equation, we find the pH to be 6.998, which satisfies an acid having a pH less than 7. Slide 76 Conjugate Acid-Base Pairs Revisited Next Slide As discussed in the earlier chapters, a conjugate Acid-Base pair is a molecule that differs by a proton (hydrogen ion). In this section, we will discuss more topics of these pairs, including further discussion of Acid Strength. Slide 77 Conjugate Acid-Base Pairs Revisited Next Slide In general, we can tabulate the ionization constant of an Acid-Base pair if we are given one ionization constant of either the Acid or Base. This is quantified in the following equation: From this equation, we see that ACID STRENGTH of an ACID is inversely proportional to the ACID STRENGTH of its CONJUGATE BASE. We will conclude that the conjugate of WEAK is WEAK and the conjugate of STRONG is VERY WEAK!!!! Slide 78 Conjugate Acid-Base Pairs Revisited Next Slide QUESTION 19 The Ka of HCL is 10. What is the ionization constant of this acids conjugate? Is this categorized as a strong, weak, or very weak base? Slide 79 Conjugate Acid-Base Pairs Revisited Next Slide QUESTION 19 ANSWER The Ka of HCL is 10. What is the ionization constant of this acids conjugate? Is this categorized as a strong, weak, or very weak base? This is a VERY WEAK base (i.e. the conjugate of Strong is Very Weak!) Slide 80 Conjugate Acid-Base Pairs Revisited Next Slide QUESTION 20 A Weak Acid reaches a state of equilibrium in solution. What would the strength of its conjugate base be at this state of equilibrium? Slide 81 Conjugate Acid-Base Pairs Revisited Next Slide QUESTION 20 A Weak Acid reaches a state of equilibrium in solution. What would the strength of its conjugate base be at this state of equilibrium? THE CONJUGATE OF WEAK IS WEAK!