Understanding Chemical Reactions and Stoichiometry

In a double replacement reaction, two compounds exchange ions to form two new compounds. These reactions are driven by specific "driving forces" that cause the reaction to proceed to completion.

Driving Forces of Double Replacement

A double replacement reaction will only occur if one of the following three conditions is met:

1. Precipitate Reaction: An insoluble solid, called a precipitate, forms and settles out of the solution.
2. Neutralization: An acid and a base react to form water and a salt.
3. Gas Formation: One of the products produced is a gas that escapes the solution.

Redox Reactions

A redox (reduction-oxidation) reaction involves a change in the oxidation state or charge of the elements involved. In these reactions, electrons are transferred from one substance to another, resulting in a change in the electrical charge of the atoms.

Describing Reactions in Solution: Ionic and Net Ionic Equations

When chemicals are dissolved in water, they may exist as intact molecules or break apart into individual ions. To describe what is actually happening in a beaker, we use more descriptive equations.

The Ionic Equation

To write an ionic equation, we look at each substance in the balanced overall equation. We only "break apart" (dissociate) substances that meet two specific criteria:

1. The substance must be aqueous (labeled as $aq$), meaning it is dissolved in water.
2. The substance must be a strong electrolyte (such as a strong acid, a strong base, or a soluble ionic compound).

Weak electrolytes, such as weak acids, do not break up 100% and should be written in their molecular form.

The Net Ionic Equation and Spectator Ions

In many reactions, some ions do not actually participate in the chemical change. These are called spectator ions. They appear on both the reactant and product sides of the ionic equation in identical forms (same charge, same phase) and can be "canceled out".

The net ionic equation is what remains after you remove the spectator ions. This equation shows only the species that are directly involved in the reaction, such as the ions that form a precipitate.

Stoichiometry and the Limiting Reactant

Once a reaction is balanced, we can use the coefficients to perform stoichiometry, which is the calculation of quantities of reactants and products.

The Limiting Reactant

In many real-world scenarios, you are not given an infinite supply of every reactant. If you have specific amounts of two different reactants, one will run out before the other.

• The limiting reactant is the substance that is completely consumed first. It "limits" or controls how much product can be made.
• The excess reactant is the substance that remains after the reaction has stopped.

To find the limiting reactant, you should calculate how much product each reactant could theoretically produce. The reactant that produces the smallest amount of product is your limiting reactant.

Exam Tip: If a problem provides you with the starting amounts of both reactants, it is a major "alarm bell" that you are dealing with a limiting reactant problem.