Hi there! I’m Jeremy Krug, and welcome to my review of AP Chemistry Unit 4 – over Chemical Reactions. If you find this video helpful, I’d really appreciate it if you hit that like button, and share this video with the rest of your AP Chem community! And don’t forget that I’ve got the FULL 30-minute review video, guided notes, practice Multiple Choice and Free Response Questions, a full chapter study guide, and MORE – over at Ultimate Review Packet dot com. The link is in the description below. Let’s get started with our 10-minute review of Unit 4 …. There are two main types of changes. Physical changes involve the change in an object’s appearance or its state. Phase changes, like melting and boiling, are physical changes. And anytime you separate a mixture, maybe by chromatography or distillation, that’s a physical change. On the other hand, chemical changes involve the transformation of one or more substances into new substances. Chemical bonds are broken and formed in these chemical changes, or reactions. There are several signals that a chemical change is taking place: light being given off, gas is produced, a large change in temperature, a change of color, a precipitate being formed. These are all good ways to know a chemical reaction is taking place. Chemical equations are used to represent reactions. When writing a chemical equation, always balance it! Balancing an equation shows we conserve mass – and atoms – in every chemical process. Be able to balance equations. When solutions react, some ions that are present may not be reacting, so it’s useful to write a net ionic equation. Let’s say we mix solutions of potassium chloride and silver nitrate. Remember, ionic compounds are completely ionized when dissolved in water. So we actually have a mixture of four ions swimming around: potassium, chloride, silver, and nitrate. These ions will try to swap partners, but only one of them will produce a product. All nitrates and alkali metal ions are soluble, so the precipitate must be silver chloride. We omit the spectator ions, and this is the net ionic equation. Be able to draw a diagram that represents a balanced chemical equation. (2018 FRQ 2) In this question, you’re given a reaction and the particles that represent the products, and you’re asked to draw in the diagram for the reactants. So the first thing you need to do is conserve all your atoms, we’ve got 8 nitrogens and 12 oxygens to work with. It’s important to notice that we have four molecules of nitrogen monoxide that didn’t react from the reactant side. So we need to redraw those on the left side, since they never reacted. That leaves us with 4 nitrogens and 8 oxygens to work with. Respecting the mole ratios, we can draw 2 nitrogen monoxide molecules for every one oxygen molecule. If we repeat that, we get all of our atoms accounted for. So watch those mole ratios, and make sure you conserve all atoms. Chemical reactions usually involve breaking and forming of chemical bonds. We mentioned that in Topic 1, but be aware that some physical changes, like dissolving an ionic compound in water, could also be reasonably considered a chemical change. Think about this on the atomic scale. In an ionic compound like sodium chloride, ionic bonds have to be broken when this crystal lattice is broken apart. And the only way that can happen is if the ion-dipole forces between the ions and water are stronger than the crystal lattice that holds the sodium chloride together. Be able to diagram this. In the case of sodium chloride, the negative poles of water surround the sodium cations, and the positive poles of water surround the chloride anions. We use balanced chemical equations as a recipe to calculate how much product can be made from a certain amount of a reactant. In this problem, we’re trying to produce one gram of nitrogen monoxide. So we write that down, and we go through what I call a ‘three-step process’ to convert to grams of copper in this case. Step One is to convert to moles. Step Two is to use a mole ratio – the numbers for that mole ratio come from the coefficients from the balanced equation – for the substances we’re working with. Then, Step Three is to convert to our final unit, which is grams in this case. There are variations of this; sometimes we’re given two reactants and have to decide which is the limiting reactant, so we do the problem twice and see which reactant produces less product. Or we have a solution and we use molarity as a conversion factor – for example 0.50 molar would be 0.50 moles equivalent to one liter. Sometimes, we have to use the ideal gas law to figure out how many moles we have in lieu of converting to moles in step one. Titrations are laboratory experiments where we use a buret to slowly add a solution to a flask of a different solution. We keep adding the solution until something indicates a reaction has taken place. The most common form of titration in AP Chemistry is the acid-base titration. We usually place the base in the buret, and the acid goes in the flask down below, and we use an acid-base indicator to signal that the reaction is essentially complete. The exact moment where the moles of base that we’ve added from the buret is equal to the moles of acid present in the flask is called the equivalence point. The moment that the acid-base indicator changes color is called the endpoint. If you’re doing your job right, the endpoint will be very, very close to the equivalence point. In AP Chemistry there are three primary types of chemical reactions. Acid-base reactions involve the transfer of a proton. Remember that a proton is essentially a hydrogen ion, or H plus. Oxidation-reduction reactions involve the transfer of electrons. The species that loses electrons is said to be oxidized, and the one that gains electrons is reduced. Be able to determine the charge, or oxidation state, of any atom in a species. Here are some of the basic rules of thumb. In some species, it’s not obvious what the oxidation state of the element is, like the nitrogen in nitrate. So here we’d use algebra to solve, oxygen is nearly always -2 in a compound, and if the whole ion adds up to -1, then nitrogen has to be +5. In precipitation reactions, two soluble ionic compounds combine, and the two of the ions join together to form a solid precipitate, like in this example between solutions of silver nitrate and potassium chloride. You don’t need to memorize all the solubility rules; just remember that compounds containing alkali metal ions, ammonium, and nitrates are always soluble in water, so the other compound must be the precipitate. The Bronsted-Lowry definitions tell us that an acid is a proton donor, and a base is a proton acceptor. Remember, a proton is just an H+ ion. In this example, hydrochloric acid reacts with water to produce hydronium ions and chloride ions. Here, hydrochloric acid is donating the H plus, so it’s an acid, and since water is receiving that H plus, it’s a base. If the reaction were going in reverse, chloride would be receiving protons, so it’s a base, while hydronium donates the H plus, so it’s an acid. We can pair these up into conjugate acid-base pairs, two acid-base pairs for every reaction like this. In every conjugate acid-base pair, the acid has one more H+ than its conjugate base. Realize that water can act as either an acid or base, depending on what it’s reacting with. Since this reaction goes essentially to completion, that means hydrochloric acid is a very strong acid, as it donates pretty much all of its protons to acid. And since water accepts all those protons from the acid, water is actually acting as a strong base in this reaction. On the other hand, chloride hardly accepts any protons at all, so it’s an especially weak base. Remember, the stronger the acid, the weaker its conjugate base. And the weaker the acid, the stronger its conjugate base. We often write redox reactions into half-reactions to help us see where the electrons are going. So in this process, zinc metal is being transformed into zinc two plus ions, and gold three ions are being transformed into gold metal. We balance the charges with electrons, like this. Since zinc is going up in charge, we can say it’s being oxidized. And since gold ions are going down in charge, we say it’s being reduced. We need to get the electrons to cancel, so we multiply the half-reactions by three, and two, and add the two half-reactions to get the overall balanced equation. We can look at this and say that six electrons are being transferred in this process. That’s Unit 4! Thanks for watching, and don’t forget that you can get LOTS more review and practice over at Ultimate Review Packet dot com. Join me next time for my 10 minute review of Unit 5 which will cover Kinetics. Keep up the good work, and keep learning chemistry!