# Understanding Chemical Reactions

Hey guys, welcome to this Mometrix video over chemical reactions.
In this video, we will take a look at the definition of a chemical reaction, characteristics of chemical reactions, types of chemical reactions, and other things that are pretty useful to know.

## Chemical Reaction Definition

A chemical reaction has taken place when a chemical substance alters its initial chemical identity and takes on a new chemical identity. So, basically what is happening is you have, at least, two molecules that come together perfectly (hitting each other in the exact right way) and they (these two molecules) change into something else.

That’s typically how all types of reactions work. For example, you have an eager and excited teacher who says, “science is very important, you need to learn, so do your homework, do your homework, do your homework!” Then let’s add to the equation that one guy in the class who says, “I didn’t do my homework.” When these two come together and react, the product will be a very unhappy teacher, and a slightly terrified young boy.

In this example, we can see some observable differences after the reaction has occurred. Well, this same thing is true with chemical reactions. Every chemical reaction yields a different result, which inevitably means that chemical reactions will not all have identical observable properties.

## Chemical Reaction Characteristics

However, if you mix two chemicals together and want to know if they reacted with one another, you can look for these observable characteristics:

### Irregular Change in Color

If you mix white and red together and get pink or light red, then that’s probably not a sign of a chemical reaction. That’s just what happens when you mix the colors red and white together. However, if you mix a red and white substance together and get blue, then you may have a chemical reaction.

### Change in Consistency

When two substances are mixed together and there is a change in consistency, then this may reflect a change in the molecular structure. So, if you take two runny liquids and mix them together and the product is a thick jelly substance, then you might have a chemical reaction.

### Change in Luster

Luster refers to the reflective quality of a substance. So, if you mix two dull murky substances together, and they produce a really glossy or polished substance, then you might have a chemical reaction.

### Appearance of Bubbles

The formation of froth or bubbles may suggest that a gas has been composed in the chemical reaction.

### Changes in Odor

Now, if it’s just a slight change in odor, it’s probably not a chemical reaction. So, if I mix a substance that smells like honey (now this is just a silly example), with a substance that smells likes vanilla, and the ending result smells like some type of syrup then it’s more than likely not a chemical reaction. If you take those same two substances together, and their product smells like a smelly sock, then you may have a chemical reaction.

### Change in Temperature

You can use a thermometer to take the temperature of each substance before, and if there is a change after the two have been combined, then there may be a chemical reaction.

### Precipitate Formation

This just means that an insoluble solid is formed from a liquid solution. If this happens, then you probably are witnessing a chemical reaction.

So, those are some observable characteristics that may signify that a chemical reaction has taken place.

## 5 Types of Chemical Reactions

Now, let’s take a look at 5 different types of chemical reactions.

### Combination Reaction/Synthesis Reaction

A combination reaction is pretty straightforward. It can be defined as when substances are mixed together and form a new compound. Here is what the combination reaction equation looks like:

$$A + B → AB$$

As you can see, the two substances literally combine to form a new substance. An example of this is Carbon solid and oxygen gas reacting together to form carbon dioxide gas:

$$( \text{C(s)} + \text{O2 (g)} → \text{CO2 (g)} )$$

### Single-Replacement Reaction

A single-replacement reaction (also known as a displacement reaction) can be defined as a chemical reaction in which one element displaces or replaces (whichever word you would like to use) a comparable element in the compound causing a chemical change in the compound. Here is what that looks like in equation form:

$$AB + C → AC + B$$

As you can see in this equation, $$C$$ has stepped in and replaced or displaced $$B$$. An example of this is the combination of Copper (Cu) and Silver Nitrate (AgNO3).

$$\text{Cu} + \text{AgNO3} → \text{Ag} + \text{(CuNO3)} 2$$

### Double-Replacement Reaction

A double replacement reaction can be defined as, a chemical reaction in which the ions of two compounds swap with one another within a liquid solution and configure two new compounds. Here is what that looks like in equation form:

$$AB + CD → AC + BD$$

An example of this is the combination of Sodium Sulfide (Na2S) and Hydrogen chloride (HCl) to produce sodium chloride and hydrogen sulfide:

$$\text{Na2S} + \text{HCl} → \text{NaCl} + \text{H2S}$$

Sodium (Na+) and Hydrogen (H+) are cations, and sulfur (S2-) and Chlorine (Cl-) are anions. The cations and anions switch places during the reaction to form new compounds.

### Decomposition Reaction

Decomposition and composition are opposites. In a decomposition reaction, a single complex compound is broken down (or decomposed) into more basic substances. Here is what that looks like as an equation:

$$AB → A + B$$

Now, there are actually three different subtypes of decomposition reactions: thermal decomposition, photo decomposition, and electrolytic decomposition. The terms that proceed the word decomposition refer to the type of energy used to make the decomposition possible. So, in thermal decomposition heat is added to the complex compound to decompose it, in photo decomposition photons (or light) are added to the complex compound to decompose it, and in electrolytic decomposition an electric current passes through a liquid solution to decompose it. For example, the electrolysis of water: water decomposes into hydrogen and oxygen when an electric current is added.

### Combustion Reaction

A combustion reaction can be defined as, a chemical reaction where a substance mixes with oxygen and produces substantial amounts of energy that take place in the form of heat and light. An example of this would be the combining of Methane and oxygen to produce carbon dioxide and water.

$$(\text{CH4}+\text{2O2} → \text{CO2}+\text{2H2O})$$

Now, the last thing we will talk about, as promised, is “other things that are pretty useful to know.” There is a lot I could go into here, but I just want to talk about two things. The first is basic language, and the second is reaction rates.

## Basic language

You should already know this, but here it is: the individual substances, before they are combined together to react, are called reactants or reagents. These reactants, once combined yield a product or products.

## Reaction Rates

Now, there is a lot that goes into this, but I will give you the most basic idea of what this means.

Chemical reactions are all over the place when it comes to the speed or rate it takes for the reaction to occur. Some reactions happen instantaneously, and others take years and years. Well, reaction rates are dependent upon certain characteristics of the reactants.

There are two variables the reaction rate is dependent on: the change in concentration of a substance, and the time it takes for the change to be observed. Well, this rate of change can be altered via a catalyst that speeds up chemical reactions, or an inhibitor that slows down chemical reactions.

Understanding how chemical reactions work is important, but understanding the rate at which they take place is equally important.

I hope that this video over chemical reactions was helpful!

See you guys next time!