Chemistry might seem like magic occasionally, but you don’t need a crystal ball to predict products of reactions!
Chemistry might seem like magic occasionally, but you don’t need a crystal ball to predict products of reactions! Episode 6 recaps types of chemical reactions and describes how you can identify them. The five types of reactions are: Synthesis (1:29), decomposition (2:34), single displacement (3:33), double displacement (5:00) and combustion (7:29) reaction.
Question: Acid-Base reactions are very common in chemistry. Name one of the products of the reaction between HCl and NaOH? (8:44)
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap types of chemical reactions.
Let’s zoom out:
Turning cheap metal into gold, finding the philosopher’s stone, producing an elixir to live forever… wouldn’t that be awesome? For hundreds of years alchemists tried their luck and couldn’t figure it out! Alchemy was the forerunner of chemistry and a mixture between science and magic. Today, chemistry still seems like magic to some, especially when your teacher gives you two reactants and you are to predict the products. But no worries, you won’t need a crystal ball to do that! All you need is today’s episode in which we will identify and classify some common types of reactions and therefore recap types of chemical reactions.
Let’s zoom in:
There are five types of chemical reactions we will be talking about today: synthesis or combination, decomposition, single displacement, double displacement and combustion reactions. For all of these you will learn how just by looking at the reactants and you can classify the reaction and therefore will be able to predict the products. So let’s dive in and start with the synthesis or sometimes called combination reaction.
It’s name actually already gives it away: in this type of reaction, two or more elements or simple compounds combine to form a single product. Let’s look at two examples: first - Burning a strip of magnesium metal in the bunsen burner combines magnesium with the oxygen in the air to form magnesium oxide. As a sign of this reaction, you will see a bright white light - but don’t look into it, it can harm your retina! Let me pick another example, that is actually very important for the environment: the formation of sulfuric acid, which contributes to “acid rain”. In a first synthesis reaction sulfur dioxide, which is released into the atmosphere during the burning of fossil fuels, combines with water to form sulfurous acid. In a second reaction, the sulfurous acid then combines with oxygen to form sulfuric acid. For synthesis reactions, it is common that your teacher gives you two elements and asks you to predict the product by combining them - but be careful when deriving the chemical formula! Don’t just add them up! You might have to balance the equation.
Next is the reverse process of synthesis: the decomposition reaction. Here we have a compound decomposing into either elements or smaller compounds. For Example: When heating Silver(I) oxide, it decomposes into its elements, silver and oxygen. To test for oxygen, you can hold a wooden stick that is glowing over the vessel - the glow intensifies or the wooden stick starts lighting up when oxygen is produced. You might be familiar with another decomposition reaction if you wear soft contact lenses. Sometimes, contact cleaning solutions contain hydrogen peroxide, H2O2. In a cleaning vessel catalyzed by platinum, H2O2 decomposes into water and gaseous oxygen. You can see the O2 bubbles! The H2O2 is also the reason why this solution should never be used directly on your eyes. It hurts. Trust me. In class, teachers typically give you a single compound, which indicates that it is a decomposition reaction.
The third type of chemical reaction is the single displacement or sometimes single replacement reaction. Again, the name gives it away: One of the elements in one compound will be replaced by another element. In single displacement reactions, the reactants are usually aqueous solutions that are either ionic or acids as well as a metal or halogen by itself. Let’s talk about it using an example: When we put a strip of Zinc into hydrochloric acid, HCl, we will observe the formation of a gas, it bubbles. The zinc loses 2 electrons and becomes a 2+ ion. It replaces the hydrogen and forms Zinc chloride. The hydrogen gains the electrons and is now in its elemental form as hydrogen gas. The zinc therefore has replaced the hydrogen in the compound with chlorine. So whenever we see an aqueous ionic compound or acid and a metal or halogen as reactants, we can replace the metal cation or hydrogen ion with the element to form one product. The second product is the elemental metal or hydrogen that were replaced. There is, however, a caveat: precious metals. These are less reactive and, in many cases, do not replace the cation. To determine if a reaction is taking place, chemists use an activity series that lists how reactive the elements are.
The fourth type of chemical reaction is the double displacement or double replacement reaction. In this type of reaction your reactants are usually two aqueous solutions of ionic compounds. Most ionic compounds dissociate in water , meaning we actually have the ions “floating around”. That’s why ionic compounds in aqueous solutions are able to conduct electrical current. Let me make an analogy to how this reaction proceeds by talking a bit more about 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 settle down.
This is similar for our ions in the aqueous solution. Some combinations of ions experience such strong attractive forces that the water molecules cannot break them apart. They form an insoluble ionic compound and settle out - they precipitate on the bottom of your flask. 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. And now the “magic” happens: lead(II) cations and iodide anions form an insoluble ionic compound. It is actually very pretty, because this precipitate is bright yellow! The products of our reaction are therefore Lead(II) iodide and Potassium Nitrate. Potassium and nitrate are, however, eternal bachelors… but this would go beyond the scope of our episode today. As you can see, uhm, hear, we switched partners: Lead(II) is no longer with nitrate, but with iodide and potassium is no longer with iodide, but with nitrate. That’s why the type of reaction is called double displacement reaction. It doesn’t have to produce a precipitate, double displacement reactions can also result in the formation of water, or a gaseous compound. To identify them, look for two aqueous solutions of ionic compounds as reactants.
The last type we are talking about in this episode are combustion reactions. Combustion reactions are reactions with elemental oxygen that usually release large amounts of energy in the form of heat and light. To identify combustion reactions, look for hydrocarbons, molecular compounds that consist of carbon and hydrogen, as reactants. When they combust, or react with oxygen, the products are water and carbon dioxide. You experience combustion reactions all around and frankly also inside of you: in your engine when driving, when lighting the bunsen burner during a lab activity or, for our AP Biology friends, during cellular respiration, when glucose in your cells reacts with oxygen to release carbon dioxide, water and energy.
To recap……
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.
Coming up next on the Apsolute RecAP Chemistry Edition: Introduction to Gas Laws
Today’s Question of the day is about predicting products.
Question: Acid-Base reactions are very common in chemistry. Name one of the products of the reaction between HCl and NaOH?