Episode 22 starts with a recap of the five types of chemical reactions described in episode 6. But then we are getting in deep!
Episode 22 starts with a recap of the five types of chemical reactions described in episode 6. But then we are getting in deep! The episode introduces acid-base reactions and defines them as reactions in which protons are being transferred (1:53). Not protons, but electrons are being transferred in redox reactions (3:15). To identify which atom has been oxidized and reduced, we recap oxidation numbers (4:12) and look at the combustion of methane (8:19). Last, but not least, we recap precipitation reactions, compare it to dating and learn about eternal bachelors (9:11).
Question: (11:38) Which laboratory technique is often used to determine the concentration of an unknown acid?
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap Types of Chemical Reactions.
Lets Zoom Out:
Unit 4 - Types of Chemical Reactions
Topic 4.7 - Types of Chemical Reactions
Big idea - Transformations
In episode 6 we introduced five types of chemical reactions: synthesis, decomposition, single and double replacement as well as combustion reactions. So why another episode? In AP Chemistry, as you’ve realized by now, we need to dive a bit deeper. Therefore, this episode will recap acid-base reactions, precipitation reactions, which are subclasses of double replacement reactions as well as reduction-oxidation reactions. Combustion reactions are one subclass of redox reactions.
Let’s zoom in:
Today’s episode again starts with a recap of a recap - this time of Episode 6. As you can see, those pre-AP episodes are really handy when preparing you for your AP course! In synthesis reactions two elements or simple compounds react to form a single compound. The reverse process is happening in decomposition reactions. In single displacement the cation is replaced, in double displacement reactions two aqueous ionic compounds switch partners. Reactions with oxygen resulting in the release of large amounts of energy are combustion reactions.
That’s the big picture, but let’s add to it and zoom in even more.
We’ll start with acid-base reactions. Acids and bases have different definitions, depending on who you ask: Svante Arrhenius, Johannes Nicolaus Brønsted and Thomas Martin Lowry or Gilbert N. Lewis. Using the Brønsted-Lowry definition, acids are defined as substances that dissociate to give H+ ions in aqueous solutions. Since the H+ ion is a hydrogen atom that has lost its only electron and only has one proton in its nucleus, the term “proton” is often used for the H+ ion. This proton can now be transferred or donated to a second substance. Let’s look at an example: strong acids like HCl dissociate completely in water. That means that you have H+ ions and Cl- ions in the solution. If we now add NaOH, which is a strong base and dissociates into Na+ ions and OH- ions, the H+ bonds with the OH- to form H2O! OH- reacts as a base: it accepts a proton. So the substance that donates the proton is the acid, the substance that accepts the proton is the base. An acid-base reaction therefore involves the transfer of protons between substances! This is just an amuse-bouche, a little taste. Unit 8 will be all about acids and bases!
The second type we are looking at are oxidation-reduction reactions or for short, redox reactions - I guess oxred reactions didn’t sound as good. They follow the same principle: it is a transfer! But in this case, it is not about a proton, but one or more electrons that are being transferred. Redox reactions are happening all around us, for example during photosynthesis, when iron is rusting and during combustions! The substance that is losing its electrons is oxidized. How can you remember that? LOOOOSing is OOOOxidation. Both with O. Or: OIL RIG - Oxidation is losing, Reduction is gaining, because the substance that accepts the electrons is reduced. What? That doesn’t make sense! It might not at a first glance, but it does when we use oxidation numbers! Oxidation numbers can be assigned to each atom in the reactants and products. By looking at the change in oxidation number, we can determine if a substance has been oxidized or reduced. Oxidation numbers are a form of “bookkeeping”, they help us to determine the electron distribution of the compound.
Let’s briefly summarize the ground rules used to determine the oxidation numbers of covalently bonded substances: 1. atoms in pure elements have an oxidation number of 0. 2. In a binary compound, a compound consisting of two different elements, the more electronegative element gets the oxidation number it would have as an anion. 3. Fluorine, the most electronegative element, will always have the oxidation number -1. 4. Oxygen usually has an oxidation number of -2, unless it is bonded with Fluorine - then the Fluorine rule trumps the oxygen rule, or unless it is a peroxide, O22-, as in hydrogen peroxide, H2O2, then it has an oxidation number of -1. 5. Hydrogen has an oxidation number of +1 when bonded with nonmetals - because it is usually less electronegative. When bonded with metals, for example in LiH, it has an oxidation number of -1, since it is more electronegative. 6. The algebraic sum of the oxidation numbers for neutral compounds is 0, for polyatomic ions it equals the charge of the ion. Oxidation numbers can even be fractions!
This sounds like a lot. So let’s look at a few examples: C2H8. We know Hydrogen has an oxidation number of +1 with nonmetals. Since there are eight hydrogens, it adds up to +8. To get to overall 0, each of the two carbons has to be -4. We assign the oxidation numbers for each atom individually. Another example: SO42-: We know oxygen has an oxidation number of -2. There are four oxygens which adds up to -8. Since the polyatomic ion has an overall charge of -2, sulfur has to have +6. -8 + +6 = -2.
What about ionic compounds? We can also assign oxidation numbers for ionic compounds - since they show a distribution of electrons, we would just use the ionic charges. Speaking of ionic charges: oxidation numbers are written with sign first, then numeral. Ionic charges are written with numbers first, then the sign!
Alright, so what does that have to do with the redox reaction? If the oxidation number of an atom gets more negative from reactant to product, then the substance has accepted electrons - and is therefore being reduced. Vice versa, if the oxidation number of an atom is getting more positive, the atom has lost electrons and is being oxidized. Let’s use an example: the combustion of methane gas.
Methane, CH4, reacts with oxygen, O2, to form water, H2O, and carbon dioxide, CO2. In methane, each hydrogen has the oxidation number +1, carbon is -4. Oxygen is a pure substance and therefore 0. In water, each hydrogen is still +1, but Oxygen is now -2. In carbon dioxide, oxygen is also -2, and carbon is now +4. So, carbon changed from -4 to +4 and must have lost electrons. Carbon has been oxidized. Those lost electrons are being transferred: Oxygen changes from 0 to -2 and therefore has accepted electrons. It has been reduced. Combustion reactions are one subclass of redox reactions. You will learn A LOT more about redox reactions in Unit 9!
Last, but not least for today: Precipitation reactions. Precipitation reactions are double replacement reactions: We have two aqueous solutions, each with ions. Mixing those two yields a precipitate - a substance that is insoluble or sparingly soluble and usually settles out at the bottom of the flask. I would like to use the same analogy we’ve already used in episode 6: dating. What, dating? Yes! The ions floating around in the aqueous solutions are like you floating around in the dating pool. You meet some people - other ions - but nothing really sticks. Until, one day, you meet THE ONE, your heart's true desire. You become a couple, you date, and eventually you decide to spend the rest of your lives together and you precipitate, uhm, settle down, I mean.
Let’s look at an example: We are combining an aqueous solution of potassium iodide, KI, and lead(II) nitrate. Our combined solution contains potassium cations, lead(II) cations, iodide anions and nitrate anions. The lead(II) cations and iodide anions form a bright yellow insoluble ionic compound due to strong attractive forces between the ions that the water molecules cannot break. What about the potassium cation and the nitrate anion? These two are “eternal bachelors”: all sodium, potassium, ammonium and nitrate salts are soluble in water - no matter who their partner is. How can we memoize this? NAA are always soluble - and I do not mean the National Apartment Association, but Nitrate, Alkali metal, Ammonium. Often we add another A for Acetate.
To recap…
In an acid-base reaction, one or more protons are transferred from an acid to a base. During redox reactions one or more electrons are being transferred. To determine which atom is being oxidized aka loses electrons and which atom is being reduced, we use oxidation numbers. Several rules govern the assignment of oxidation numbers. Precipitation reactions are double replacement reactions that form an insoluble salt. Nitrate, Alkali and Ammonium salts are always soluble.
Coming up next on the APsolute RecAP Chemistry Edition: Reaction rate and rate law
Today’s Question of the day is about acid-base reactions.
Question: Which laboratory technique is often used to determine the concentration of an unknown acid?