Week Five Wrap-Up

Module Review

From watching the lecture videos within this module, we learned that a chemical reaction is a process in which a substance (or substances) is changed into one or more new substances. A chemical equation is a representation of a chemical reaction and uses chemical symbols to to show what happens during a that reaction, showing reactants yielding products,. We learned to balance equations and discussed the phases of substances, such as gas, liquid, solid, and aqueous (dissolved in water).

We took a deeper look at aqueous solutions, learning about homogeneous (all of a substance is dissolved in another), heterogeneous (a substance is not dissolved in another). We also learned about solutes (substance present in a smaller amount) and substances (substance present in a larger amount) and how, when combined together, make the solution. We learned that solutions do not always need to be in a liquid phase. Next, we took a look at different properties of solutes, such as non-electrolytes, weak electrolytes, and strong electrolytes, which are defined by how they dissolve within water. Strong acids, strong bases and ionic compounds are strong electrolytes; they dissociate completely, forming ions, within water. Weak acids and water are weak electrolytes; they partially within water, forming few ions. Larger, organic molecules are usually non-electrolytes (do not break up in solution) which do not dissociate within water, creating no ions. Strong electrolytes tend to conduct very well and non-electrolytes do not conduct electricity.

We then discussed the solubility of ionic compounds by first defining solubility (maximum amount of solute that will dissolve in a given quantity of a solvent at a specific temperature). We then used solubility rules to determine whether compounds are soluble or insoluble. Once we determined which compounds are soluble and which are insoluble, we were able to determine potential precipitates (solids that precipitates out of solution) within reactions.

Up until now, we have written all of our equations as molecular equations, in which the formulas of the compounds are written as though all species existed as molecules or whole units (not necessarily showing us the chemical change that's happening). We then learned how to go to a complete ionic equation, which shows anything dissolved (anything labeled as aqueous) as free ions. Next, we looked for ions that aren't actually involved in the chemical reaction, determined which ions are 'spectators' (do not change) and eliminated them, creating a net ionic equation.

Next, we looked at acid base reactions, or neutralizing reactions, in which the products end up in neutral or near-neutral solution (usually a salt and water). A salt is defined as an ionic compound made up of a cation other than H+ and an anion other than OH- or O2-. We discussed some common properties of both acids (sour taste, causes color changes in plant dyes, etc.) and bases (bitter taste, slippery, etc.). Acids and bases can be defined in several ways. The first way, an Arrhenius acid is a substance that produces hydrodium ion in water and an Arrhenius base is a substance that produces hydroxide in water. The Arrhenius base model must be in water, which does not help us when we are looking at non-aqueous solutions. The Brønsted method is a broader way of defining acids and bases, in which the Brønsted acid is a proton donor and a Brønsted base is a proton acceptor.

We then looked at oxidation-reduction reactions (or redox reactions), or the transfer of electrons, which changes the oxidation number the atoms involved in the reaction. The oxidation number is the number of charges the atom would have in a molecule (or ionic compound) if electrons were transferred completely. When we have an redox reaction, we must have both half reactions: 1) oxidation, in which we lose electrons, and 2) reduction, in which we gain electrons. The oxidation number of elements in an ionic compound is the same as their ionic charge. When determining the oxidation number of the elements in a molecular compound, we have to pretend that the electrons are transferred completely and not shared. We then discussed the rules to help determine oxidation numbers of atoms and compounds within the reaction. We then looked at the reducing agent (the substance that is being oxidized and is causing another substance to be reduced) and oxidizing agent (the substance being reduced and causing another substance to be oxidized). We learned that there are some sub-types of redox reactions, such as combinations (two or more substances combine to form single product), a decomposition reaction (a breakdown of a compound into two or more components), a displacement reaction (an ion or atom in a compound is replaced by an ion or atom of another element), or a disproportionation reaction (an element in one oxidation state which is simultaneously oxidized and reduced).

Supplemental Material

If you would like to learn more on this topic, these resources may help:

Balancing chemical equations

 

Redox Reactions: Crash Course Chemistry # 10

Completion Checklist

Make sure you complete the following learning activities and assessments before moving on to the next topic:

  1. Lecture Videos
  2. Example Videos (where posted)
  3. Practice Problems and Discussion
  4. Quiz