CHEMICAL REACTIONS

Unit Introduction

The purpose of this unit is to allow you to investigate chemical reactions and learn how to write and balance chemical equations.

Chemical Reactions

A chemical reaction occurs when two or more substances interact in a way that produces new substances with characteristics that are different from those of the individual components. When a chemical reaction happens, the law of conservation of mass is obeyed. That means that matter is not created or destroyed in a reaction, but is conserved.

This video provides an excellent foundation of knowledge on chemical reactions:

How do you know when a chemical reaction has taken place? There are a few types of evidence you can look for to know whether one has occurred.

EVIDENCE OF CHEMICAL REACTION

Color change; for example, iron rusts and turns from a gray to a brown color

Temperature change; reactions will either take in or give off heat energy, which affects the temperature of the substances

Formation of a precipitate; this is when a solid forms in a liquid solution

Formation of a gas; example is the fizz produced from dropping Alka-Seltzer in water

Energy can either be absorbed or released in a chemical reaction. An endothermic reaction occurs when energy is absorbed as a result of the reaction. The prefix endo- means into, and heat is taken in or absorbed in an endothermic reaction. The temperature is perceived to be cooler at the end of the reaction than it was in the beginning. An exothermic reaction occurs when energy is released because of the reaction. The prefix exo- means outside, and heat is given off during an exothermic reaction. The temperature at the end is higher than it was before the reaction took place.

Endothermic Reaction	Exothermic Reaction

Chemical Equations

In previous units, you have learned how to identify and write formulas for compounds. The next step is the writing of a chemical equation. The function of a chemical equation is to show changes that occur during a chemical reaction. There are two ways to write a chemical equation. The first is a word equation that simply uses the words for the elements and compounds involved. An example of a chemical equation with words would be “hydrogen plus oxygen yields water.” The second type of chemical equation uses the symbols of those elements and compounds involved rather than the words. An example of a chemical equation with symbols would be 2H₂ + O₂ à 2H₂O.

When a reaction occurs, there are two types of substances: reactants and products. The substances to the left of the arrow are called the reactants. Reactants are the elements and compounds that must interact for the reaction to take place. The elements and compounds to the right of the arrow are called the products. Products are the new substances produced in a chemical reaction. The reaction happens in one direction, which is indicated by the arrow. The arrow separates the reactants and products and is read as “yields.” So, the previous example would read “hydrogen plus oxygen yields water.”

Here are some basic guidelines for writing chemical equations:

Writing Chemical Equations (00:44)

A number that precedes (or comes before) the symbol or formula for an element or compound is called a coefficient. The coefficient is distributed throughout the entire molecule. In the example above, you have a coefficient of 2 preceding H₂. This means that you have 2 H₂ molecules. In the example you also see a coefficient of 2 preceding H₂O. This means that you have 2 molecules of H₂O. The coefficients can be manipulated to balance the equation.

A subscript is a number that follows an element symbol and is written in a somewhat lower position. The subscript indicates the number of atoms for just the element it follows. So, in H₂O, the subscript of 2 indicates that there are 2 H atoms in the molecule, but only 1 O atom. The subscripts cannot be manipulated to balance the equation.

A coefficient and a subscript are multiplied to get the total number of atoms of that element. For example, 2SO₄ has 2 sulfur atoms (coefficient 2 multiplied by a single S atom) and 8 oxygen atoms (coefficient 2 multiplied by subscript 4).

Here is a pictorial representation:

One SO₄ molecule:

2SO₄, or two SO₄ molecules:

Balancing Chemical Equations

As mentioned previously, when a chemical reaction occurs, the law of conservation of mass must be obeyed. The way this is indicated in chemistry is to balance the chemical equation at the molecular level. The number of atoms of each element in the reactants must equal the number of atoms of each element in the products to show the balance. Follow these steps to properly balance chemical equations.

1. Write out the formulas for all substances involved in the reaction.

2. Count the number of atoms of each element on both sides of the reaction.

3. Do the numbers balance? Yes = you are done! No = move on to step 4.

Ex: The number of carbon atoms on each side are equal, but the number of iron and oxygen atoms on each side are different.

4. Add coefficients to make the quantity of atoms of each element equal on both sides of the reaction. Note that the red coefficients below apply to the whole molecule that comes after, and the coefficients and subscripts get multiplied.

5. Double check your numbers to make sure the equation is balanced.

Watch the following video clip for additional explanation on how to balance chemical equations.

Now, try a few practice problems to test your ability to balance equations. Keep going until you get 5 right in a row.

Here is even more practice. Play the game to see if you have mastered the skill of balancing equations.

QUIZLET VOCABULARY