What is a Reaction?
On the previous page, several examples of chemical reactions were given: water boiling, gasoline burning, and microwaves heating food. So just what is a reaction? Chemical reactions are simply processes that change one type of matter into another. For example, when you burn gasoline, it is converted from one form (gasoline) into another (water, carbon dioxide, and other exhaust products). Changes like boiling water (H2O) are usually considered processes, because the water has not undergone a fundamental change, since you can easily recondense the steam.
Reactions and processes are generally written in an "equation" as follows:
H2O (l) --> H2O (g)
In this case, water is boiling. The substances on the left side are called the reactants, the arrow represents the actual change, and the substances on the right are the new products of the reaction. In chemistry, the term species is often used in place of "substance." The letters in the parenthesis signify the state of the species: an s signifies solid, l means liquid, g stands for gas, and aq signifies an aqueous, or dissolved, species.
Example Problem 1
Identify the products, reactants, and the state of each in the following process.
CO2 (s) --> CO2 (g)
Since the CO2 (s) (commonly known as dry ice) is on the left side, it is a reactant. The "s" in the parenthesis tells us that the CO2 is a solid. The CO2 (g) is the product of the reaction, and the "g" means it is a gas.
Using the above sample problem, you can see a reaction equation can tell us a lot about that reaction. We know the initial substances in a reaction, the products of the reaction process, and what state of matter they are both in. In the problem above, we can tell that solid carbon dioxide is becoming a gas. You have likely seen this reaction in person, when dry ice becomes gaseous, surrounded by a white cloud of condensed water vapor.
When two or more species are products or reactants, they are separated by a plus (+) sign. If more than one "unit" of a species is needed, you could write it twice on one side. However, this is inefficient. Therefore, as in mathematics, the number in front of that species (its coefficient) tells how many are present. The example problem below illustrates a reaction with several reactants, several products, and a species with a coefficient.
Example Problem 2
Identify the products, reactants, and the state of each in the following reaction. Also tell which species has a coefficient, and what this indicates.
2 H+ (aq) + Zn (s) --> Zn2+ (aq) + H2 (g)
Don't be confused by the small "+" signs above some species or the subscript "2" after the H. Also, don't worry if you don't know what these species are. These facts will all be explained later. The three reactants of this reaction are two units of aqueous (dissolved) H+ and one unit of solid Zinc. The two products of the reaction are one unit of aqueous (dissolved) Zn2+ and one unit of gaseous H2. The "2" coefficient on the H+ indicates that there are two units of this species present.
Now, you may think that something strange has happened here: three reactants were converted into two products! It's important to note that the number of products and reactants don't have to be equal. Here's a quick explanation of the above problem, but don't worry if it doesn't make much sense right now. In Example Two, the two "H"s (hydrogen ions) were converted into a hydrogen gas molecule. The subscript "2" indicates that each hydrogen gas molecule has two H atoms. Therefore, the same number of H's (hydrogen atoms) is present on both sides; thus, the reaction is "balanced." The structure of molecules will be explained later in this chapter; balancing reactions is found in the "Chemical Reactions" unit. For now, we will move on to the international units of chemistry and how to use them.