HW #5: 5.28, 5.34, 5.36, 5.38, 5.46, 5.52, 5.54, 5.58, 5.68, 5.72, 5.74, 5.76, 5.78, 5.80

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1 Chemistry 121 Lectures 11 & 12: Chemical Equations; Balancing Chemical Equations; Classes of Chemical Reactions: Precipitation, Acid-Base, and Redox Reactions Chapter 5 in McMurry, Ballantine, et. al. 7 th edition HW #5: 5.28, 5.34, 5.36, 5.38, 5.46, 5.52, 5.54, 5.58, 5.68, 5.72, 5.74, 5.76, 5.78, 5.80 Objectives 1. Define a balanced chemical equation and describe why this must follow from the conservation of mass 2. Develop a method for balancing straightforward chemical reactions 3. Describe precipitation reactions 4. Relate solubility of ionic compounds to opposite charge interactions in the ionic solid 5. Introduce the concept of the net ionic equation 6. Write simple net ionic equations based on solubility guidelines 7. Use Brønsted-Lowry theory to define acids and bases 8. Define neutralization 9. Define oxidation and reduction 10. Define oxidizing agent and reducing agent 11. Understand the relationship between oxidation and oxidizing agent, reduction and reducing agent 12. Know oxidation and reduction occur together 13. Define oxidation number 14. Define half-reaction 15. Utilize oxidation numbers and half-reactions in order to balance overall simple redox reactions 1

2 5.1: Chemical Equations Under standard conditions, sodium hydrogen carbonate and sodium carbonate are solids, dihydrogen oxide is liquid, and carbon dioxide is a gas, important information which is often included in the chemical equation as follows 2NaHCO3 (s) Na2CO3 (s) + CO2 (g) + H2O (l) The capital delta over the is transformed into arrow is another way of indicating the reaction only occurs when heated (aq) following a compound means the compound is in aqueous solution Na2CO3 (aq) + CaCl2 (aq) CaCO3 (s) + 2NaCl (aq) Notice in the above examples there are the same number of a particular type of atom on the reactant and product side of the equation (recall the tinker toys in a box analogy) 5.2: Balancing Chemical Equations 1. Write the chemical equation without stoichiometric coefficients (reactants on the left proceeding to products on the right, of course). As an example, let s consider the combustion of butane, C 4H 10, which requires O 2 and generates CO 2 and H 2O C4H10 + O2 CO2 + H2O 2

3 2. Choose one of the reactants to have a stoichiometric coefficient of 1, and then place a coefficient in front of a product that allows you to balance one of the atoms in that reactant on both sides of the equation. Repeat as needed. In the butane example, give C 4H 10 a one (recall the multiplicative identity); we then need to account for the 4 C in butane on the product side, as well as the 10 H in butane C4H10 + O2 CO2 + H2O At this point only the elemental species oxygen remains to be balanced. On the product side of the reaction there are 13O (8 in 4CO 2, 5 in 5 H 2O). Thus, on the reactant side we need 13 O or 13/2 O 2. While the generally accepted form is to have integer stoichiometric coefficients, it is perfectly acceptable to leave it as a simple fraction, since in actuality we are mixing large (mole scale) ratios of species. C4H10 + (13/2)O2 4CO2 + 5H2O Other important considerations Non-nuclear reactions proceed with a conservation of mass. This makes sense if you think of reactions as reorganizations of valence electrons amongst a collection of atoms in molecular systems, or swapping charges in ionic systems o Alternatively stated, if you start with a given collection of atomic tinker toys you must end up with the same number of tinker toys, whether they are connected differently or not In chemical reactions you are combining numbers of things. Since different collections of atoms have different masses associated with them, we need to correct for mass differences The problem with chemistry is that it occurs on such a tremendously small scale. As such it is important to scale-up reactions in order to put them on the human scale. Notice that if 1 sodium carbonate reacts with 1 calcium chloride in the 2 nd reaction above, then 100 Na2CO3 would react with 100 CaCl2, or 6.02 x Na2CO3 would react with 6.02 x CaCl2 enter the mole 3

4 5.4, 5.8: Precipitation Reactions, Solubility Guidelines, and Net Ionic Equations An ionic compound will dissolve into solution if the ion-dipole interactions between water and the ions are greater than the ion-ion interactions in the ionic compound Recall that ions in solution are free to move independently of one another they will support an electric current and as such are referred to as electrolytes So what happens when we mix 2 solutions where ions which interact more strongly come into contact? Consider the reaction between a solution of Pb(NO3)2 and KI Demonstration of dissolving CaCl2 in water, Na2CO3 in water, then mixing them together: 4

5 So how do we know which species will precipitate? Just look for those with the greatest charge interactions [shaded green] on the table below In the table above notice that [as a 1 st order approximation] the lower the electrostatic interaction in the ionic solid, the greater the water solubility. Exceptions make for useful diagnostic tests when you are looking for a particular ion The Barn Dance Analogy and Net Ionic Equations 5

6 5.5: Acids, Bases and Neutralization Reactions An acid is a proton donor o Examples Nitric acid, HNO3, strong acid - generates an equivalent amount of H3O + in water Interestingly, NO3 - is on the table above come to think of it, 5 of the 6 anions listed correspond to the conjugate bases of common acids hmmm Acetic acid or any compound with the RCO2H motif, where R can be anything. Weak acids - generates a small amount of H3O + in water A base is a proton acceptor A base must have a pair of non-bonded electrons in order to accept a proton o Examples Sodium Hydroxide (or other alkali metal hydroxides) Standard strong base - generates an equivalent amount of OH - in water Ammonia (or carbon based derivatives like CH3NH2) Standard weak base generates a small amount of OH - in water 6

7 A Brief Word on Acid/Base Strength and Conjugate Acid/Base Pairs Rationalizing acid/base strength if by definition a neutral compound acting as an acid loses H +, it will be left with a negative charge, since the H + will form a coordinate covalent bond with the base, leaving its electron and thus a negative charge behind. How well the compound can stabilize this negative charge determines how well it can lose H + in the first place; that is, how strong an acid it is. We refer to the compound stabilizing the negative charge as the conjugate base of the [initial] acid since it must have a pair of electrons with which it could conceivably pick up the H + if the reaction were reversed. Similarly, by picking up H +, the base is transformed into the conjugate acid o HCl + H2O H3O + + Cl - o CH3CO2H + H2O CH3CO2 - + H3O + o NH3 + H2O NH4 + + HO - o NaOH (s) + H2O Na + (aq) + HO - (aq) + H2O Notice that we define acids in a practical sense by the ability to donate protons to water, generating the hydronium ion H3O + ; we define bases by their ability to generate hydroxide in water The 2 principal determinants of base stability are charge delocalization (primary) and placing negative charge on an electronegative element (secondary) o Notice that HCl is a much stronger acid than HF, in spite of the fact that fluorine is more electronegative than chlorine. Why? Chloride is much larger than fluoride, thus dispersing charge to a greater extent Question: Which is a stronger base, NaH or NaOH? 7

8 Acid-Base Neutralization Reactions HCl + NaOH H2SO4 + 2NaOH HCl + NH3 H2SO4 + 2NH3 H3PO4 + 3NaOH H2CO3 + 2NaOH 5.6, 5.7: Redox Reactions Three very good ways to keep oxidation and reduction clear The oxidation of iron (or any other metal) The oxidation of ethanol (reverse reaction is reduction) The 4 electron reduction of O2 (reverse reaction is oxidation) And 2 disturbing ways to keep oxidation clear: LEO (the loin) says GER OLE likes BEER 8

9 Oxidation and reduction always occur together; that is, if something is being oxidized, something else is being reduced Oxidizing agents are reduced in a redox reaction, reducing agents are oxidized There can be no net change in the number of electrons in a redox reaction This is to say that viewing a reaction in redox terms is a method for looking at the transfer of electrons, just like looking at a reaction in acid-base terms is a method for looking at the transfer of protons Question: Is Cl2 an oxidizing agent or a reducing agent? Question: Na 0 is a strong reducing agent why? 9

On Oxidation Numbers The oxidation number of an element is 0 The oxidation state of a monatomic ion is equal to its charge Certain elements have the same oxidation number in (almost) all compounds in

10 On Oxidation Numbers The oxidation number of an element is 0 The oxidation state of a monatomic ion is equal to its charge Certain elements have the same oxidation number in (almost) all compounds in particular, the group 1 metals and hydrogen have an oxidation number of +1, the group 2 metals have an oxidation number of +2. O almost always has a -2 charge o When assigning oxidation numbers, hydrogen actually supersedes O, which is why the oxidation number of O in H2O2 is 1 Notice that this system for tracking the flow of electrons is not identical to determining the charge on an atom H always covalently bonds, yet has an oxidation number of +1 The sum of the oxidation numbers in a neutral species must equal 0; in a polyatomic ion it must equal the charge on that ion 10

11 Half-Reactions Half reactions separate a chemical reaction into the species being oxidized and the species being reduced. Consider the reaction that occurs when Zn is placed in a CuSO4 solution: In order to balance the overall redox reaction, multiply one of the half-reactions so the electrons cancel Question: Write the overall redox reaction for the reaction of aluminum with oxygen to form aluminum oxide 11

Lecture 0402 Acid -- Base, Formation of a Gas, Redox Concept

Chemistry, The Central Science, 11th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten Lecture 0402 Acid -- Base, Formation of a Gas, Redox Concept John D. Bookstaver St. Charles Community