Chapter 4 - Reactions in Aqueous Solutions

For reactions in a solution, by the end of this chapter, you will be able to identify compounds as acids or bases, and as strong, weak, or non-electrolytes. You will also be able to recognize reactions by type and predict the products of simple acid-base, precipitation, and redox reactions. You'll be able to calculate molarity and use it to convert between moles of a substance in solution and the volume of the solution. You will describe how to carry out a dilution to achieve a desired solution concentration, and finally, you will describe how to perform and interpret the results of a titration. Let's talk first about solutions. Solutions are defined as homogeneous mixtures of two or more pure substances. The solvent is present in the greatest abundance, and all other substances are the solutes. When water is the solvent, the solution is called an aqueous solution. Here, we have an example of an aqueous solution of methanol. Methanol is the solute in this solution, while water is the solvent. So this is a solution of methanol, and as you can see, the molecules of methanol are surrounded by molecules of water. They are not close together because there is a lot of water that can surround those molecules of methanol. This is an example of an aqueous solution because the solvent is water. Substances can dissolve in water in different ways. First, ionic compounds dissolve by dissociation, where water surrounds the separate ions. Dissociation means that the ions separate. Remember that we are talking about ionic compounds, which are formed by ions. When you dissolve the salt in water, the ions separate and are surrounded by water. This is also called dissociation. Molecular compounds, on the other hand, interact with water but do not dissociate. They simply dissolve in water. Some molecular substances react with water when they dissolve. So these are the three ways substances can dissolve in water: ionic compounds by dissolving and dissociation, molecular compounds by dissolving, and other molecular substances by reacting with water in a way that they dissolve. Here we have an example. This is a salt, the crystal structure of sodium chloride. We also have molecules of water, oxygen, and hydrogen. The green ion is the chloride ion, which has a negative charge because chlorine is a nonmetal. Water has a polar bond between oxygen and hydrogen, where oxygen is more electronegative than hydrogen. This makes hydrogen partially positive because oxygen pulls away the electrons from hydrogen. The interaction between chloride and water is through hydrogen because all the hydrogens are partially positive due to the presence of the polar bond between oxygen and hydrogen. Sodium, on the other hand, is positive, so it interacts with the oxygen part of water, which is the more electronegative atom in the covalent bond between oxygen and hydrogen. That's why sodium is surrounded by water through the oxygen part, while chloride is surrounded by water through the hydrogen part. Now, let's introduce electrolytes and non-electrolytes. An electrolyte is a substance that dissociates into ions when dissolved in water, while a non-electrolyte may dissolve in water but does not dissociate into ions when it does so. So, an electrolyte is an ionic compound that can dissociate in water, while a non-electrolyte cannot dissociate in water and remains molecular. Compounds, so here, the ionic compounds are all strong electrolytes. None of them are weak electrolytes or non-electrolytes. On the other hand, molecular compounds, strong acids, weak acids, and weak bases are all weak electrolytes. The rest of the compounds are non-electrolytes. Therefore, most molecular compounds are non-electrolytes. Strong acids are strong electrolytes, while weak acids and weak bases are weak electrolytes. To identify whether a compound is an electrolyte or not, we can look at water. Pure water, H2O, does not conduct electricity as it does not have any ions. If we add a molecular compound like glucose or sucrose (sugar) to water, it can dissolve but does not dissociate into ions. Therefore, it also does not conduct electricity. However, if we add sodium chloride to water, it dissociates into ions, allowing the flow of electric current and conducting electricity. Sodium chloride is a strong electrolyte. Now, let's talk about solubility of ionic compounds. Not all ionic compounds dissolve in water. A list of solubility rules can help determine which combinations of ions will dissolve. Here is a table illustrating these rules. Soluble ionic compounds include those with nitrate, acetate, chloride, bromide, iodide, and sulfate ions. However, if chloride, bromide, iodide, or sulfate ions form compounds with silver, mercury, or lead ions, they become insoluble. Sulfate compounds with calcium, strontium, or barium ions are also insoluble. Exceptions to this are compounds with ammonium or alkali metal ions, which are soluble. Insoluble ionic compounds include those with sulfite, carbonate, phosphate, and hydroxide ions. However, if these compounds have ammonium or calcium, strontium, or barium ions, they become soluble. These are the basic rules of solubility for identifying whether an ionic compound will dissolve in water. Remember that if an ionic compound dissolves in water, it will dissociate and be a strong electrolyte. Now, let's talk about precipitation reactions. When two solutions containing soluble salts are mixed, sometimes an insoluble salt is produced. This solid, called a precipitate, falls out of solution. An example is shown here with potassium iodide and lead nitrate solutions. As the solutions are mixed, a yellow precipitate forms. Once all the solution is added, the precipitate settles at the bottom of the beaker. In solution, you would have potassium nitrate ions (which are soluble and dissociate) and lead iodite ions (which are insoluble). These are the rules of solubility and precipitation reactions, which help us identify whether a compound is soluble or insoluble in water and whether a precipitation reaction will occur. That we mentioned in the table before, and this is a precipitation reaction. So, if you have these two reactants, you can switch. Okay, we're going to see this in the next slides. We are switching this C with this anion and this C with this anion. Then, we evaluate using the table and the rules to see if those possible products are soluble or insoluble. So, this type of reaction is known as metathesis or exchange reactions. Metathesis comes from the Greek word that means to transpose. It appears as though the ions in the reactant compounds exchange or transpose ions, as seen in the equation below. Here, we have the silver nitrate and the potassium chloride, and we are going to produce silver chloride and potassium nitrate. This is the C, so we can combine this C with this anion and this C with this anion. It is important that you can identify those reactants as aqueous. Remember, when they are aqueous, that means that they are soluble. So, these compounds are dissociated, and I know that they are dissociated because they both are ionic compounds. Okay, both of them are ionic compounds, and it says here in the equation that they are aqueous. That means that silver nitrate and the potassium chloride, both of them are in solution, both of them are dissociated. But when we mix this with this solution, then a solid will be produced. That is known as silver chloride plus potassium nitrate. This one is a solid. Okay, and we can look for the exceptions in table one. That for the chloride, that's when you combine chloride with silver, this will be insoluble in water. So, that's why you have an S as a solid. And you will have potassium nitrate. That means that when all these react, you will have this solid and this in solution, dissociated. You will have ions of potassium and ions of nitrate. You will not see ions of silver and chloride at this point here. Yes, they are in two separate beakers. We have one beaker with the silver nitrate and a white beaker with the potassium chloride. So, here we will see the silver ions, and here we will see silver chloride ions. But here, when they react, the only ions that you will see are going to be the potassium and the nitrate, because this combined to produce the silver chloride that is a solid that will be insoluble in water. Completing and balancing metathesis equations follow these steps: 1. Use the chemical formulas of the reactants to determine which ions are present. 2. Write formulas for the products: cation from one reactant and anion from the other. Use charges to write the proper subscript. 3. Check your solubility rules. If either product is insoluble, a precipitate will form. 4. Finally, balance the equation. We have three different ways to write metathesis reactions: 1. The molecular equation: we combine the two initial reactants and write the expected products. The molecular equation lists the reactants and products without indicating the ionic nature of the compounds. 2. The complete ionic equation: we represent this molecular equation but in ions. We separate those that can be dissociated. 3. The net ionic equation: the equation that really explains the reaction that is being produced in that beaker or that reaction. For example, the molecular equation lists the reactants and products without indicating the ionic nature of the compounds. You will see just the molecules or the compounds. In this case, they are ionic compounds. Remember, this one is aqueous and this one is aqueous. That means that we have this one as ions dissociated and this one can't be dissociated because it's a solid. It's insoluble in water. And this one will be dissociated. So, it's important to write the next equation, which is the complete ionic equation. In the complete ionic equation, all strong electrolytes, strong acids, strong bases, and soluble ionic salts are dissociated into their ions. We just separate them. So, we have the silver nitrate. We separate the silver from the nitrate and write the substrate of the state of that ion. That is aqueous. The potassium chloride, we also dissociate this and create the ions potassium and chloride. This will produce the silver chloride, which did not dissociate because it's insoluble in water. So, we keep it this way. Meanwhile, the potassium nitrate is soluble in water, so we dissociate this one again, and we have the potassium and the nitrate ions separate. The complete ionic equation contains all the ions possible in the reactant and product, and we also keep the ones that are insoluble as solid. We don't separate or dissociate that one because it's insoluble in water. Water, and finally, we will write the net ionic equation. To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right, or something that is repeating on both sides. If we have an ion that is the same in the reactants and the product, we will need to remove those. The ions crossed out are called spectator ions, and in this case, the potassium and the nitrate are examples of those. The remaining ions are the reactants that form the product and a soluble salt in a precipitation reaction, as we can see in this example. So that means that we have silver here, we don't have the ion of silver in the reactants, in the products, we have the nitrate here, and we have nitrate in reactants and products. So we need to cancel this one. Potassium, we have potassium ion here, potassium ion here, so we need to cancel those. The chloride ion here in the product is part of the solid, so we don't have an ion of chloride in the product, so we can't cancel the chloride, neither the silver. So we just draw a line on top of those spectator ions. These are spectator ions, and we just write the net ionic equation, which will be the silver plus the chloride in solution will produce the silver chloride solid, the precipitate. And this is how we write those kinds of precipitation reactions. So write a net ionic equation, write a balanced molecular equation, dissociate all strong electrolytes, cross out anything that remains unchanged from the left side to the right side of the equation, and write the net ionic equation with the species that remains. That's how we write the net ionic equation, starting from the molecular, then to the complete ionic equation, and finally the net ionic equation. Let's talk now about acids. There are two definitions of acids. One of them was developed by a chemist named Arrhenius. He defined acids as substances that increase the concentration of protons or H+ or hydrogen ions when dissolved in water. So when he found a compound that increases the concentration of protons or hydrogen ions in water, he defined that as an acid. On the other hand, Bronsted and Lowry defined acid as a proton donor. So Bronsted and Lowry defined an acid as a proton donor, while Arrhenius defined an acid as a substance that increases the concentration of protons in the water. Here, we have some examples of acids: hydrochloric acid, nitric acid, sulfuric acid, and acetic acid. Most of the time, it's very common to see compounds with protons at the beginning. Those are acids. Acetic acid is one of the exceptions because it is a carboxylic acid, and the proton is usually written before those two oxygens. Most of the time, the acid has those hydrogens at the beginning of the chemical formula. Bronsted and Lowry defined bases as substances that increase the concentration of hydroxide ions or OH- when dissolved in water. Those are bases. Bronsted and Lowry defined bases as proton acceptors. So, for Bronsted and Lowry, acids are proton donors, and bases are proton acceptors, while for Arrhenius, acids are the ones that increase the concentration of protons, and bases are the ones that increase the concentration of hydroxide in solution. Also, we can identify a liquid as an acid or as a base using litmus paper. The one that turns the litmus paper blue is a base. The one that turns the litmus paper red is an acid. That's another way to identify a solution as an acid or a base. Now, we have strong and weak acids and bases. Strong acids completely dissociate in water, while weak acids only partially dissociate. So, strong acids will be dissociated 100%. For example, hydrochloric acid (HCl) means that when you add HCl to water, it will completely dissociate into H+ and chloride ions. You will never see HCl in water. You will see hydrogen and chloride as ions. It's completely dissociated, so that means it's a strong electrolyte. On the other hand, weak acids dissociate in really small quantities. Most of the time, maybe around 1 to 10% of the concentration will be dissociated. That's why they are weak acids. Strong bases dissociate to metal cations and hydroxide ions in water, while weak bases only partially react to produce hydroxide anions. Strong bases will completely dissociate. For example, sodium hydroxide is one of them. When you add that to water, you will separate those ions, inducing dissociation. Because these are ions, the sodium will be surrounded by water, specifically with the oxygen that will interact with sodium. Sodium is a cation and oxygen is the negative part of the water molecule, while hydroxide will then interact with the proteins of water with the hydrogens. So, we also have an acid and base reaction in this case. We have a compound that is receiving one hydrogen from water. Here, water is acting as an acid and ammonium is acting as a base. As a product, when you remove this hydrogen from water, you will have OH-. Before you have H2O, you remove one hydrogen. That hydrogen is positive, so that means that you will keep OH-. You will have a negative charge, so we have an OH- (hydroxide) ion. Ammonium, on the other hand, is neutral. It doesn't have a charge, but it receives one hydrogen, which is a positive ion. So, it will produce NH4+ (ammonium ion). This is an acid-base reaction. In an acid-base reaction, the acid (water in this case) will donate a proton to the base (ammonium). Reactions between an acid and a base are called neutralization reactions. When a strong acid like HCl (hydrochloric acid) reacts with a strong base like sodium hydroxide, the net equation is going to be something like this: HCl + NaOH -> NaCl + H2O. Here, we have the molecular equation. Hydrochloric acid and sodium hydroxide produce sodium chloride and water. Remember, these are strong acid and strong base, so they will be completely dissociated. The cation (Na+) will react with the anion (Cl-) and the H+ will react with OH-. That will form water (H2O), while sodium and chloride will form sodium chloride (NaCl). All three will be dissociated in water, while water will be a liquid, one of the products. When we write the net ionic equation, we need to cancel all the ions that repeat in the products and reactants. So, we cancel Cl- and Na+ and the resulting net ionic equation will be H+ + OH- -> H2O. This is known as a neutralization reaction because we are going to produce water and water has a pH of 7, which is known as a neutral pH. That's why it's called a neutralization reaction. We're going to produce water and salt. Most of the time, when you're doing neutralization, the two possible products will be a soluble salt and water. For a neutralization reaction between a strong acid and a strong base, there is also another type of reaction known as a gas-forming reaction. In this reaction, some metathesis reactions do not give the expected products. When a carbonate or bicarbonate reacts with an acid, the products are a salt, carbon dioxide, and water. So, for example, calcium carbonate and HCl will produce calcium chloride, CO2, and water. Here, CO2 is a gas. Similarly, sodium bicarbonate reacts with HBr to produce sodium bromide, CO2, and water. Once again, a production of gas. So, if you see carbonate ions or bicarbonate, that will give you a hint that one of the products will be carbon dioxide, a gas compound. Now, this reaction gives the predicted product, but you had better carry it out in a hood (lab fume hood) because the gas produced has a foul odor. H2S (hydrogen sulfide) has a really strong and unpleasant odor. For example, sodium sulfide reacts with sulfuric acid to produce sodium sulfate and H2S. The other type of reaction is known as an oxidation-reduction reaction. Here, we have a metal, calcium, that reacts with oxygen to produce calcium oxide. Oxidation is when you lose electrons, while reduction is when you gain electrons. Here, if we can see, we have an ionic compound. The calcium is from Family 2, so that means that this ion is +2, while this ion is -2. Oxygen at this point is 0. It has a charge oxidation number of zero because it's an element. In its natural state as a solid, calcium is zero, but when it turns into calcium oxide, it becomes plus two. Calcium loses two electrons, so it undergoes oxidation. Oxygen is also in its natural state, with an oxidation number of zero. However, when it turns into oxygen, its oxidation number becomes minus two. This means that oxygen gains two electrons and undergoes reduction. So, calcium is oxidized while oxygen is reduced. The electrons released by calcium are received by oxygen to produce calcium oxide. The loss of electrons is oxidation, while the gain of electrons is reduction. Both of these processes need to happen simultaneously; you cannot have oxidation without reduction. If something undergoes reduction, there has to be something else releasing those electrons. Thus, calcium gets oxidized while oxygen gets reduced. These reactions are often called redox reactions, short for reduction and oxidation. To determine if an oxidation-reduction reaction has occurred, we assign oxidation numbers to each element in a neutral compound or charged entity. Here are the rules for assigning oxidation numbers: 1. Elements in their elemental form have an oxidation number of zero, as seen with solid calcium and oxygen gas, O2. 2. The oxidation number of a monatomic ion is the same as its charge. For example, lithium in group 1 has a charge of plus one, so its oxidation number is also plus one. Fluoride, on the other hand, has a charge of minus one, so its oxidation number is minus one. 3. Nonmetals typically have negative oxidation numbers, although some can be positive in certain compounds or ions. Oxygen, for instance, usually has an oxidation number of minus two, except in the peroxide ion, where it has an oxidation number of minus one. Hydrogen can have different oxidation numbers as well. It is minus one when bonded to a metal but commonly plus one when bonded to a nonmetal. 4. Fluorine always has an oxidation number of minus one since it belongs to group 7, the halogens. The other halogens also have an oxidation number of minus one when they are negative. However, they can have positive oxidation numbers, especially in oxyanions. Most of the time, they are minus one, except when they are part of oxyanions with positive oxidation numbers. 5. The sum of the oxidation numbers in a neutral compound is zero. This is evident in the example of calcium oxide, where calcium is plus two and oxygen is minus two. When you add them together, the sum is zero since calcium oxide is a neutral compound. 6. In a polyatomic ion, the sum of the oxidation numbers equals the charge of the ion. So, for polyatomic ions, the sum of the oxidation numbers of the atoms involved is equal to the charge of the ion. Another type of reaction is a displacement reaction, where an ion oxidizes an element. In this example, silver ions oxidize copper metal. Silver turns from an ion to a solid, indicating reduction, while copper goes from zero to plus two, indicating oxidation. Silver ions use copper as the reducing agent, while copper gets oxidized. Overall, redox reactions involve both oxidation and reduction processes, with elements gaining or losing electrons to form new compounds. The silver ions start to oxidize the copper, turning copper solid into copper in solution. Here, the blue solution is due to the presence of copper ions. In the wire, you will see a solid silver that comes from the solution, from a positive ion to a solid. So, the reverse reaction does not occur. Why not? Silver may induce copper to oxidize, but copper cannot make silver oxidize. Each metal will react differently through oxidation reactions. In the table, the higher you are, the more reactive you are to oxidation reactions. Copper and silver are both present, but copper is more reactive, so it prefers to be oxidized while silver is reduced. This is why the reaction goes in one direction instead of the reverse. In the reverse reaction, silver is not reactive enough to undergo oxidation like copper. Lithium, on the other hand, has the highest reactivity for oxidation, so any metal that reacts with lithium will be reduced to a metal. Elements higher on the activity series are more likely to exist as ions, rather than solids, because they are more reactive for oxidation reactions. Another kind of reaction is metal-acid displacement reaction. The element above hydrogen will react with an acid to produce hydrogen gas. The metal is oxidized to a cation, while the acid is reduced. In this case, hydrochloric acid reacts with magnesium to produce bubbles of hydrogen gas and magnesium chloride in solution. Hydrogen goes from +1 to 0, indicating reduction, while magnesium goes from 0 to +2, indicating oxidation. The oxidation state of chloride remains -1. Concentration is also important in solutions. Molarity is one way to measure the concentration of a solution. Molarity (M) is equal to moles of solid divided by the volume of the total solution. With this equation, you can calculate moles, volume, or molarity if you have the other two variables. Calculate the most moles of solute that you need to add to create a solution of specific molarity with a specific volume. Alternatively, you can determine the volume that you will need to create a solution of any molarity with a specific number of moles. To create a solution of known molarity, we need to weigh out a known mass and determine the number of moles of the solid. We then add that amount to a volumetric flask of a specific volume, which we determined using the molarity. For example, if we want a 250 M solution, we use a volumetric flask of 250 milliliters. We weigh out, for example, 39.9 grams of copper sulfate and add it to the volumetric flask. We then add enough water to dissolve the solid, and once it is dissolved, we fill the flask up to the mark to reach the desired concentration. We can also create a diluted solution from a concentrated solution by using a pipette to deliver a specific volume of the concentrated solution to a new volumetric flask, and then completing it with water. In this case, we have a stock solution that is 1 molar, and we take a sample of 25 M and add it to a new volumetric flask of 250 M. We fill the remaining volume with water until it reaches the calibration mark on the flask. This gives us a solution of 0.1 M, which is a dilution of the initial 1.0 M solution. The molarity of the new solution can be determined using the equation M1V1 = M2V2, where M1 is the concentration of the concentrated solution, V1 is the volume of the concentrated solution, M2 is the concentration of the diluted solution, and V2 is the volume of the diluted solution. We can also use molarity for stoichiometric calculations. For example, if we have substance B and substance A, and we know the molarity of substance B, we can determine the number of grams of substance A using the volume. We can convert from molarity to moles of substance B using the volume, and then use the coefficients in the balanced equation to convert from moles of substance B to moles of substance A. Finally, we can convert from moles of substance A to grams of substance A using the molar mass. Similarly, if we have the mass of substance A, we can convert it to moles of substance A using the molar mass, and then convert from moles of substance A to moles of substance B using the coefficients in the balanced equation. If we need to calculate the molarity, we can use the volume by dividing moles by volume. If we need to calculate the volume, we can multiply moles by molarity. Moles divided by molarity will be equal to the volume. Okay, so in that way, we can calculate the volume of the substance B. So, we can do this as a big picture. Okay, of all the possibilities that we can have from substance B to substance A by using the molarity of the concentration of the solution okay, to look for the grams of the substance another substance in the reaction. Titration is one of the techniques that maybe we can use that map that we saw in the slide before. And titration is an analytical technique in which one can calculate the concentration of a solid in a solution. So here we have, for example, a sample of acid. The concentration of this acid, we don't know the concentration. There's a known concentration, and we have a the sample is 20 Ms. Okay, 20 Ms of the acid. We're going to add a few drops of an acid-base indicator, most of the time it's known as phenolphthalein. This one, when phenolphthalein is in acid, is clear. It doesn't have color. Okay, so the phenolphthalein in acid is, it has, it's clear. Now, once we're going to have B here with a standard solution of sodium hydroxide, we know the concentration of the sodium hydroxide, the base that we're going to use to do the titration of this solution. And we're going to start to add this solution until we reach this pinkish color. This one is a little bit strong, it has to be more clear than that. But at this point, we have used, we have reacted all the hydrogens from the acid with the base. So basically, the volume that we use here, we can calculate that volume, multiply it by the concentration of this sodium hydroxide, and we're going to calculate the moles of that base. And if the reaction is one-to-one, that means that if we have, for example, we use 30 moles of base, those react with the 30 moles of hydrogen of the acid. And by using the volume, we can calculate the concentration of the acid. So we use basically the titration, where we have a sample of an unknown concentration of the acid. The only data that we know about the acid is the volume that we add, that was 20 Ms. Okay, we have the indicator. And then we're going to start to use the titration agent that, in this case, is going to be a base because this is going to be an acid-base reaction, to titrate all of the hydrogens that we have in solution. And because we know the concentration of that base, because it's a standard sodium hydroxide, and we know the volume of that base, we can multiply the volume with the concentration, and that will be the number of moles of the base. And if the reaction is one-to-one, with the coefficients, that means that that amount of moles of base is equal to the amount of hydrogen. So, those moles of hydrogen, divided by the initial volume, that is 20, because the sample was 20 Ms, then we can calculate the concentration of the acid. So basically, here we have that map where we have the volume of the standard solution that was needed to reach the equivalence point. That equivalence point is where all the sample was consumed by the titration agent. That, in this case, was the sodium hydroxide. So we have that volume, we use the molarity, so volume times molarity will be equal to the moles of the standard solution. By using the coefficients, we can turn those moles of the standard solution to moles of the unknown solution. And once we have the moles, we divide by the volume of the sample that we add to do the titration, and this will give us the concentration of the unknown solution. The reaction is complete at the equivalence point. At that point, it turns from clear to pink because we don't have any more protons in the solution. That's why it turns pink. And this will be the end of chapter four, reactions in aqueous solution.