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Unit 3: Chemical Reactions Overview: Balancing chemical reactions Equilibria and the concept of reactions not going to completion Terminology of solutions Dissolution of ionic solids in solvents such as water to give electrically conducting solutions Dissolution of molecular solids in water to give non-conducting solutions Concept of weak and strong acids according to the extent of ionization Concept of net ionic reactions as applied to precipitation and gas-forming reactions Oxidation-reduction reactions and the concept of oxidation numbers Learning Outcomes: Understand information conveyed in a balanced chemical reaction Be able to balance chemical reactions – remember, must know reactants and products to do this Understand that not all reactions go to completion but result in a “dynamic equilibrium” Understand nature of chemical reactions in solution and the terms solution, solute and solvent Understand that when an ionic solid dissolves in water the solid is fully dissociated into separate ions and that such solutions are electrical conductors. Understand that aqueous solutions of most molecular compounds do not conduct electricity are called non-electrolytes. Understand that some molecular compounds when dissolved in water break apart into ions. In some instances they break apart completely (e.g. HCl to produce H+ + Cl-). These are called strong electrolytes. In other cases, e.g. acetic acid (CH3COOH) we develop an equilibrium between the neutral CH3COOH and the ions H+ and CH3COO-. This is an example of a weak electrolyte. Know the electrolyte solubility guidelines. Be able to predict the outcome of precipitation reactions using the solubility guidelines Be able to write precipitation reactions as net ionic reactions and understand the concept of spectator ions. Know the common strong and weak acids and bases. Understand the Arrhenius definition of acids and bases Understand the Brönsted-Lowry definition of acids and bases Understand the idea that a strong acid dissociates 100% into H3O+ in water whereas a weak acid only partially dissociates. Know how to write a net ionic reaction for the reaction of a strong acid (e.g. HCl) and a strong base (e.g. NaOH) Understanding how non-metal and metal oxides act as acids and bases respectively Be able to write net ionic reactions for gas forming reactions Be able to identify oxidation-reduction reactions and to recognize oxidizing and reducing agents Understanding the connection between oxidation-reduction reactions and electron transfer Know the rules for determining the oxidation number for an element in a compound and be able to use them to determine oxidation numbers Understand the role of oxidation numbers in identifying an oxidation-reduction reaction. Be able to balance oxidation-reduction reactions Readings: Sections 3.1 – 3.10 Assignment: From Chapter 3: # 2, 6, 10, 20, 24, 26, 30, 38, 42, 46, 50, 70 Introduction: Now that we know something about atoms and molecules, it's time to see what kinds of reactions they can undergo and how we can describe these reactions. This unit will cover chemical equations, the nature of solutes in aqueous solutions and a few types of common reactions. A balanced chemical equation neatly summarizes a reaction. Consider the equation, 2 H2 (g) + O2 (g) → 2 H2O (l). From this, don't know everything about the reaction, but we do know the reactants, the products, the states of the reactants and products and the stoichiometry of the reaction. The stoichiometry is an essential component of the equation if we are to perform calculations such as determining the mass of water formed from a mixture of hydrogen and oxygen. Remember that atoms are not created or destroyed in reactions, but they are traded. This means that the number of each type of atom on the reactant side of the equation must equal the number of each type on the product side. In our example, four hydrogen atoms on the lefthand side means there must be four hydrogen atoms on the right-hand side. If you know the reactants and products, you will be able to balance many chemical reactions by inspection, but be aware that some reactions require a more rigorous approach to balancing their equations. As long as the reactions proceed to completion (so that at least one of the reactants is completely consumed), we can easily calculate the amount of products formed. Some reactions do not proceed to completion. We say that the reactants and products are in chemical equilibrium. In this case, while some products are formed, reactants will also be present, even after an infinite amount of time. Calculating the amounts of products formed is more involved and will be covered in another unit in this course. Since many reactions occur in solution, and often the solvent is water, it is worth knowing something about the nature of dissolved solutes in aqueous solutions. Ionic compounds such as NaCl and some covalent compounds such as HCl dissolve in water to completely produce ions. These are known as strong electrolytes. Other compounds such as acetic acid (CH3COOH) dissolve in water, but only a small fraction of the acetic acid exists as the ions, CH3COO- and H3O+. Compounds that do not completely ionize in water are weak electrolytes. Other compounds such as ethanol and glucose dissolve in water without producing ions. These are non-electrolytes. In the previous unit, you tried to develop a mental image of what atoms and molecules looked like, now you should try the same thing, but with the molecules and ions as they exist dissolved in water. For the first time in this course, some solubility guidelines are provided to assist you in determining when a precipitate forms as a result of a reaction. Many factors determine whether a compound will be soluble, and these factors will be discussed in subsequent units, but for now you can simply apply the guidelines. Note the exceptions to the guidelines, and be aware that other exceptions may exist that are not listed in your book. Even though we are only in unit three, it is time to discuss acids and bases. Most people have an idea of the properties of an acid or base, but their chemical nature remained a mystery until relatively recently. Today, even the word, acid, has more than one meaning. The Arrhenius and the Bronsted-Lowry definitions describe acids (and bases) in terms of chemical species. The Bronsted-Lowry definition of an acid is fairly comprehensive and is worth knowing at this point. By this definition, an acid is a species that donates a proton (H+), and a base is a species that accepts a proton. Acids and bases, when mixed, neutralize each other. Consider HCl (aq) + NaOH (aq) → H2O (l) + NaCl (aq). Note that this can also be described by the net ionic equation, H+(aq) + OH- (aq) → H2O (l). Before leaving this unit, be sure to study the tables listing common strong and weak acids and bases. It is important that you are able to identify which compounds are strong and weak acids and which are strong and weak bases. The final section in this unit deals with redox reactions. Be able to define reduction and oxidation. From here, you should be able to identify which compounds should be good oxidizing and reducing agents. You will notice that a set of rules helps you determine the oxidation number of atoms within molecules or ions. Once you have mastered these concepts, you will be able to balance redox equations. Just as acid-base reactions are very common, so are redox reactions. Consider that through redox reactions, plants oxidize water to form oxygen, and then animals reduce oxygen to form water. This should emphasize the importance of redox reactions. Concluding Remarks: This unit introduces some general types of reactions such as acid-base and redox reactions. Such reactions are neatly described by their balanced chemical equations. And since many of these reactions occur in water as the solvent, we have considered the nature of the species in solution. Remember that some compounds simply dissolve in water to produce free molecules while other compounds (electrolytes) dissolve to produce ions. It may appear that you should memorize many of the reactions presented in this unit, but instead make an attempt to learn a few of them. Later, as you learn more theory, you will be able to more accurately predict which reactions will occur and the products of those reactions.