If you have a Mac do whatever you have to do to play it as a slide show I don t know Macs well. Dr. Buckley

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1 For best results please view this as a slide show. You can hit the F5 key or go to the Slide Show tab on the menu bar and click on From Beginning. Page Down and Page Up will move you through the presentation. If you have a Mac do whatever you have to do to play it as a slide show I don t know Macs well. Dr. Buckley gbuckley@cameron.edu

2 Experiment #9 Molar Stoichiometry in a Chemical Reaction Laboratory Overview CHEM 1361 August 2010 Gary S. Buckley, Ph.D. Department of Physical Sciences Cameron University

3 Table of Contents (you may click on any of the topics below to go directly to that topic) The Chemical Reaction Experimental Scheme Calculations Safety Notes Notes on Oxidation-reduction Reactions Use of the Activity Series to Predict Redox Reactions Further Examples of Redox Reactions

4 The Chemical Reaction The reaction you will observe here may be written as: Cu(s) + 2 AgNO (aq) or, in net ionic form: Cu(NO ) (aq) + 2Ag(s) Cu(s) + 2 Ag (aq) Cu (aq) + 2 Ag(s) The primary goal in this experiment is to verify the stoichiometric coefficients in the above equation 1 mol of Cu reacts with 2 mol of Ag +. This reaction is an example of a displacement reaction. Notice the Cu has lost two electrons from the reactant side to the product side and each Ag + has gained one electron. All displacement reactions are also oxidation-reduction (or redox) reactions. In this case the Cu is oxidized loses electrons and the Ag + is reduced since it gains electrons.

5 Experimental Scheme The reaction will be carried out by placing an accurately weighed piece of copper wire into a solution containing a known quantity of silver nitrate. As the reaction proceeds copper will dissolve in the solution causing a blue color and metallic silver will form on the surface of the copper wire. This tends to be one of the students favorite experiments it is interesting to watch Cu(s) + 2 Ag (aq) Cu (aq) + 2 Ag(s) Ag metal coated on Cu wire Blue color due to formation of Cu 2+ ions

6 Calculations + 2+ Cu(s) + 2 Ag (aq) Cu (aq) + 2 Ag(s) The amount of both copper and silver reacted may be determined by comparing the starting quantities and ending quantities of each. From the mass reacted, the number of moles of each reacted may be determined in the usual fashion: # mol # g molar mass From this information, the ratio of mol of Ag reacted to mol of Cu may be determined, as well as the mass ratio of grams of Ag reacted to grams of Cu reacted.

7 Safety Notes A few key safety notes: 1. Step 1, the making of the silver nitrate solution, must be carried out in a fume hood. The lab book insinuates this, but does not come right out and say it. 2. You will be working with 6 M HNO 3 which can burn if it comes in contact with your skin. Wear gloves for the bulk of this experiment. 3. Another good reason to wear gloves is that the silver ion solution will cause discoloration of your skin if it comes in contact. This is not a safety issue, but more cosmetic in nature. The discoloration will eventually go away.

8 Notes on Oxidation-Reduction Reactions The occurrence of an oxidation-reduction (redox) reaction depends on the relative ease with which the species involved can give up (or take on electrons). From a terminology standpoint: Oxidation the loss of electrons (this species is also called the reducing agent) Reduction the gain of electrons (this species is also called the oxidizing agent) In the reaction considered here: + 2+ Cu(s) + 2 Ag (aq) Cu (aq) + 2 Ag(s) Cu is oxidized (and is the reducing agent) because it loses two electrons to Ag. Ag + is reduced (and is the oxidizing agent) since it gains electrons from the Cu. This reaction occurs because Cu is more easily oxidized than Ag.

9 The reverse reaction: Notes on Oxidation-Reduction Reactions (continued) 2+ + Cu (aq) + 2 Ag(s) Cu(s) + 2 Ag (aq) does not occur since the Cu is more readily oxidized than Ag it will not accept electrons from silver. A fairly easy way to consider whether or not an oxidation-reduction reaction occurs is to look at an activity series. You will find one on page 124 of your lab book or on page 141 of your textbook. The following slide discusses the use of the activity series to determine in which direction a redox reaction will occur.

10 Use of the Activity Series to Predict Redox Reactions A small portion of an activity series is included below. The reactions are written as oxidations - the species on the left gives up an electron. Those higher on the list are more easily oxidized. Notice that copper is above silver in the activity series. This means that copper is more easily oxidized and, when the two are put together, copper will donate electrons to silver as we have already seen. Since copper is above silver, when writing the chemical equation for the reaction between the two the silver reaction will be reversed Cu(s) + 2 Ag (aq) Cu (aq) + 2 Ag(s)

11 Further Examples of Redox Reactions Consider the reaction of iron and lead. Since iron is higher in the activity series, its equation will remain as written and the nickel chemical equation will be reversed. Fe + Ni 2+ Fe 2+ + Ni Balancing the redox equations requires ensuring the charges are the same on both sides of the equation. (This is always true think about your net ionic equations.) Consider the reaction between chromium and lead. The chromium equation stays as written while the lead equation is reversed Cr Cr +3e and Pb + 2e Pb Electrons that are given up (3 per chromium) must be accepted by lead (2 per lead). Since electrons cannot be left floating around, one looks for the common multiple of 3 and 2, which is six. Thus, the chromium equation is multiplied by 2 and the lead equation by 3 to get: Cr + 3 Pb 2Cr + 3 Pb