Video Transcript
In this video, we will learn about the different types of chemical reactions and predict the products likely to form. A chemical reaction is a process where one or more substances, which we call the reactants, are changed into one or more different substances, called the products. This transformation involves the breaking of chemical bonds and the formation of new chemical bonds. Almost everything around us was made either in nature, a factory, or in a laboratory by chemical reactions.
There are many types of reactions with complex names. You may have heard of some of them. For example, fermentation is the type of reaction which occurs in beer and wine production. Combustion is the burning of fuels in oxygen, for example, the burning of gasoline, wood, or charcoal on a fire. Photosynthesis is a complex set of reactions where plants make their own food using carbon dioxide, water, and sunlight. And respiration, another set of complex reactions where plants and animals use food molecules for energy.
These and other types of chemical reactions can be classified into some main categories. In today’s lesson, we will focus on four main types of chemical reactions. We will focus on combination reactions, decomposition reactions, displacement reactions, and combustion reactions. Other reaction types is a subject for another video.
In combination reactions, two or more reactants combine chemically together to form one product. The reactants can all be elements, or they can all be compounds, or they can be a mixture of elements and compounds. Let’s have a look at some different combination reactions and their chemical equations.
A metal can combine with a nonmetal. For example, sodium can react with chlorine gas to form sodium chloride. Sodium chloride is the main component of the table salt that we eat. However, the salt that we eat is not made in a lab by a combination reaction but is rather collected from salt pans. Iron can react with sulfur to produce iron(II) sulfide.
Another example of a combination reaction is when magnesium metal reacts with oxygen gas to form magnesium oxide. Perhaps you’ve seen this in the lab. A pair of tongs hold some magnesium ribbon in a Bunsen burner flame. And the magnesium ignites and burns in oxygen. Much light and heat energy is given off from the burning magnesium. This is an exothermic reaction. This reaction is not just a combination reaction; it is also a combustion reaction.
Combustion reactions typically involve the reaction with oxygen. And these reactions are highly exothermic. Nonmetals can also combine with nonmetals. An example is nitrogen gas reacting with hydrogen gas to form ammonia gas. This specific reaction is called the Haber process and is carried out in industry to produce ammonia, which is the starting material for the synthesis of many products.
Here is another example. Solid carbon can react with oxygen gas to produce carbon dioxide gas. This is also a combustion reaction and is exothermic, releasing energy. You may have performed this reaction in your own backyard by burning charcoal in a barbecue.
Another two specific types of combination reactions are when nonmetal oxides react with water to form acids and when metal oxides react with water to form metal hydroxides, which are basic. When carbon dioxide in the air interacts with liquid water, for example, in rain droplets, carbonic acid can form. This is a component of acid rain. Small amounts of carbonic acid are also found in carbonated beverages like soda. When calcium oxide reacts with water, it produces the hydroxide calcium hydroxide, which is basic.
We’ve looked at a few types of combination reactions and examples. Let’s look at two more to practice predicting what the products will be.
What would be the product formed from the combination of the metal iron and oxygen gas? You guessed it. It will be a combination of the two elements iron and oxygen. The product which would form is Fe2O3, which is the main component of rust. What about when hydrogen gas and oxygen gas react? If you guessed water, that’s great because water is composed of hydrogen and oxygen chemically combined together.
Now, let’s have a look at decomposition reactions. A decomposition reaction is the opposite of a combination reaction. Here, one single reactant breaks apart or decomposes into two or more products. In most cases, these reactions require the input of energy to break the bonds in the reactant. When the energy input is from heat, we call this a thermal decomposition. And we can show this by drawing a small triangle on top of the arrow.
Some metal carbonates when heated decompose into a metal oxide and carbon dioxide gas. For example, when calcium carbonate is heated, it decomposes into white calcium oxide and releases carbon dioxide gas. A similar thing happens when some metal chlorates are heated. For example, when potassium chlorate is heated, it decomposes into a chloride, potassium chloride, and oxygen gas.
Some metal hydroxides and metal nitrates undergo similar decomposition reactions. Let’s look at an example of electrical decomposition, properly known as electrolysis. Electrical energy can be used to drive the decomposition of liquid water into hydrogen gas and oxygen gas. Graphite electrodes are used to deliver energy from a power source to water. And this energy is used to break the bonds in water apart to produce the products.
Let’s try and predict the products of some decomposition reactions. Silver chloride decomposes spontaneously when it absorbs energy from sunlight. What do you think the products will be? Did you guess right? It will decompose into its elements silver and chlorine. What about the decomposition of NH3, ammonia? We saw the reverse of this earlier. In other words, we saw the combination reaction. Ammonia will decompose into its two constituent elements nitrogen and hydrogen.
Perhaps this next one looks familiar to you. We’ve seen that some metal carbonates when heated decompose to give a metal oxide and carbon dioxide. We discussed calcium carbonate just now, but here we have zinc carbonate. This metal carbonate will decompose to give the metal oxide zinc oxide and carbon dioxide gas.
What about this last substance, copper sulfate pentahydrate? When heated, it will break apart into copper sulfate and water vapor. But this is not a decomposition reaction; it’s a dehydration reaction. We know this because no chemical bonds have been broken. This dot over here does not represent chemical bonds.
The third type of reaction we’ll look at are displacement reactions. A displacement reaction occurs when an element or group of elements in one reactant displaces or replaces an element or group of elements in another reactant. There are two types of displacement reactions: single-displacement and double-displacement reactions.
In a single-displacement reaction, a more reactive element displaces or replaces a less reactive element, usually in solution. Here, A is more reactive than B and so displaces or kicks it out of the compound BC. And A takes the place of B. These are also called replacement or substitution reactions. Here, A is substituting for B. Just like in a football match, one player will substitute for another. In a double-displacement reaction, ions from two reactants are exchanged or swapped to form two new products.
Let’s look at some examples of single-displacement reactions. A solid piece of zinc metal is placed in a copper sulfate solution. The zinc, being more reactive than copper, will replace it. Zinc ions will go into solution, and solid copper will form on top of the remaining zinc metal. Many metals will react with a strong acid in a displacement reaction to form hydrogen gas and a salt. For example, when magnesium ribbon is placed in hydrochloric acid, bubbles are seen to form. What’s happening is that hydrogen gas is forming as well as the dissolved salt magnesium chloride. This happens because magnesium is more reactive than hydrogen and so displaces it from hydrochloric acid.
Let’s make some predictions. If we know that chlorine is more reactive than bromine and chlorine gas is bubbled through a sodium bromide solution, what do you think the products will be? Well, the chlorine would displace the bromine and take its place, causing a single-displacement reaction. When potassium reacts with water, a fairly violent single-displacement reaction occurs. Potassium is reactive enough to displace one of the hydrogen atoms from water, producing a basic potassium hydroxide solution and hydrogen gas.
Now, let’s have a look at some double-displacement reactions. When an aqueous solution of barium chloride and an aqueous solution of sodium sulfate are mixed, aqueous sodium chloride is produced as a product as well as barium sulfate, which is an insoluble solid. The barium ions and the sodium ions swapped places with each other. We say that they displaced each other or replaced each other or substituted for each other. The insoluble barium sulfate product is called a precipitate. And this particular double-displacement reaction is also called a precipitation reaction.
Here is another example of a double displacement. Lead in lead nitrate displaces potassium in potassium iodide, and the potassium displaces the lead. The products lead iodide and potassium nitrate are formed. This is another example of a precipitation reaction because one of the products is insoluble in aqueous solution, in this case the lead iodide.
Let’s make a few predictions of what the products could be in these displacement reactions. In the first equation, there are two elements in the first substance and one element in the second substance. This looks like a single-displacement reaction, and it is. Chlorine is more reactive than bromine and so will displace it. In the second equation, there are two elements in the first compound and three in the second compound. The oxygen and hydrogen in the second compound tend to stick together as the hydroxide ion. This is looking like a double-displacement reaction. Copper will displace potassium, and potassium will displace copper. Copper hydroxide and potassium iodide will form as products. Copper hydroxide is an insoluble precipitate.
In the last equation, there are several elements, although nitrogen and the three oxygens tend to stick together as the nitrate ion. And so this also looks like a double-displacement reaction. Ag, which is silver, will displace Na, which is sodium. And sodium will take silver’s place. And the products are silver chloride and sodium nitrate. Again, one of the products is a precipitate or an insoluble solid.
Let’s move on to the last type of reaction, combustion. A simple definition for a combustion reaction is an exothermic reaction between a fuel and oxygen gas. The equation shows that carbon dioxide gas and water vapor are usually the products. The energy that is released can come off in the form of heat, light, or even sound.
An example we are probably all familiar with happens in a Bunsen burner. A fuel supply enters the bottom of the Bunsen burner. The fuel is usually a mixture of propane and butane gas. The air hole allows oxygen gas from the atmosphere to enter and mix with the fuel. The fuel and oxygen gas react with each other and produce much energy in the form of heat, light, and sometimes even sound. We can’t see it, but water vapor and carbon dioxide gas are also released as products.
The balanced equation for the combustion of propane is shown here. When the fuel is an organic compound, such as propane, the balancing of the equation can be quite tricky. In simpler combustion reactions, the balancing is a bit easier. Notice, however, that the products here are not carbon dioxide and water but rather are just oxides of the starting substance. They are still considered combustion reactions because they involve the reaction with oxygen and are highly exothermic.
Combustion reactions involving organic compounds can be considered to be complete or incomplete and have slightly different products. When carbon dioxide and water are the only products, we say this is a complete combustion. In a complete combustion, there is sufficient or enough oxygen gas to convert all the fuel to carbon dioxide gas and water vapor. However, if the products are CO gas, which is carbon monoxide, and water vapor, or if the products are solid carbon and water vapor, we say the combustion is incomplete. This usually occurs when there is insufficient or not enough oxygen gas to convert all the fuel into carbon dioxide and water. When a Bunsen burner flame is blue, we know that a complete combustion is occurring. And this is when the air hole is fully open, letting in sufficient oxygen.
Let’s summarize everything we have learnt. We learnt about four main types of reactions. We learnt that in a combination reaction, two or more substances react together and combine to form one product. In the opposite type of reaction, a decomposition reaction, one reactant breaks apart or decomposes into two or more products. In a displacement reaction, a more reactive element or group of elements in a reactant displaces an element or group of elements in another reactant. And we learnt that there are two types, single- and double-displacement reactions. Lastly, we learnt about combustion reactions, where much energy is released when a substance reacts with oxygen.
