Chemical Reactions

When we mix two chemical solutions together there is always the possibility that there will be a reaction. It is also equally possible that there will not be a reaction. Knowing what will happen when two chemicals or chemical solutions are combined goes to the very heart of what it means to be a chemist. While you are not here to become a chemist, it does make sense that you be shown some of the most common ways to determine whether a reaction will or will not take place ; so let's start with the question, "what is a chemical reaction?"

A Chemical Reaction is the process that takes place when a substance (or substances) are brought into contact with each other and produce a new substance(s).

The substance (or substances) initially involved in the chemical reaction are called reactants or reagents. Chemical reactions are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants.

Chemical reactions are represented by Chemical Equations. Chemical equations are balanced to show the same number of atoms of each element on each side.

In chemistry there is a Law called the Law of Conservation of Mass that states that matter cannot be created nor destroyed. What this means is that even though different substances are created during the process of a chemical reaction, none of the reactant atoms are lost. They are simply rearranged into the new substances.

So how can you tell when a reaction has taken place?

There are five (easy) ways to detect a reaction:

1. Color Change
2. Precipitate Formation (solid formation falling out of solution)
3. Gas Formation (bubbles and odor)
4. Temperature Change
5. pH Change

Beyond these methods, spectroscopy and other more technical methods can be employed to detect the reaction.

Chemists observe chemical reactions and have come up with a way to represent or model what is happening.

For example, take a look at the reaction for making NaCl. In this reaction, solid Sodium (Na) combines with Chlorine (Cl₂) gas to make solid Sodium Chloride (NaCl):

2Na (s) + Cl₂ (g) → 2NaCl

Chemical Equations are different from Numerical Equations. In a numerical equation, 2x + 4y = 65, the product is just a number and does not in any way indicate the components that it was derived from. In a chemical equation, the products must represent the component atoms they were derived from and the amounts on both sides of the reaction must be equal.

Reactant A + Reactant B → Product

The reactants are used up in forming the product. The arrow → shows the direction of the reaction.

Remember that in a chemical reaction, matter is neither created nor destroyed and atoms cannot change their identity (e.g. a carbon atom can�t become an Iron atom); this means that you have to have the same number of each type of atom on each side of the chemical equation.

Balancing a Chemical Equation

1. Write the Skeleton Equation
   • Li(s) + H₂O(l) → H₂ (g) + LiOH (aq)
2. Count the atoms of the elements in the reactants
   • 1 atom Li, 2 atoms H, 1 atom O
3. Count the atoms of the elements in the products
   • 1 atom Li, 3 atoms H, 1 atom O
4. Change to Coefficients to make the number of atoms of each element equal on both sides of arrow
   • 2Li(s) + 2 H₂O → H₂(g) + 2LiOH(aq)
5. Write the Coefficients in their lowest possible ratio
6. Check your work

Once you have balanced the reaction it can then be used to complete calculations necessary for experimental work. In order to make these calculations we will need to introduce a couple of new concepts: Avagadro's Number and The Mole.

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