Lab 10 RedOx Reactions Laboratory Goals In this laboratory you will develop a basic understanding of what electrochemical cells are develop familiarity with a few different examples of redox reactions develop hypotheses regarding changes to pennies Introduction Oxidation reduction reactions are second fundamental type of chemical reaction Redox reactions as they are often called are simply reactions that involve the transfer of electrons from one chemical species to another This electron transfer will obviously shift the ratio of the protons to electrons on the atoms involved and will thus affect the oxidation number of the atoms involved Atoms that gain electrons are said to have been reduced their oxidation number goes down and atoms that have lost electrons are said to have been oxidized Redox chemistry is the basis of the field called electrochemistry It should be easy to see why the name electrochemistry fits these reactions so well as they always involve the transfer of electrons from one chemical species to another One important consequence of electrochemical reactions occurs if you separate the oxidizing agent from the reducing agent connecting them only with electrodes a wire and salt bridge you create an electrochemical cell Doing so forces the electrons to travel through the wire from the reducing agent to the oxidizing agent and so creates an electrical current the units of current are Amperes or Coulombs second If a particular redox reaction is thermodynamically spontaneous then the electrons will transfer spontaneously and a current will be produced An electrochemical cell that generates a current is called a voltaic or galvanic cell You are probably most familiar with these types of cells as batteries If the reaction is not spontaneous then an electrical current i e electrons are required to make the reaction proceed An electrochemical cell that uses a current is called an electrolytic cell Let s look at the structure of an electrochemical cell a little more closely The redox reaction we will use as an example is the reaction Cu s 2 Ag aq Cu2 aq 2 Ag s The copper here is being oxidized and is therefore the reducing agent meaning that it is causing something else to get reduced Cu s Cu2 aq 2 eand silver ions are being reduced making them the oxidizing agent meaning that they are inducing another material to be oxidized 2 Ag s e Ag aq This reaction will happen spontaneously if we simply put the two materials together However if we were interested in trying to use this reaction to generate electricity we 47 would need to construct an electrochemical cell which would require that we separate the copper from the silver ions We would first make a Cu Cu2 half cell the line indicates the copper and copper ions are in different phases by placing a copper electrode which is just metallic copper in a solution of copper ions We also need to construct an Ag Ag half cell in this case the metallic silver is used only as a means to convey electrons to the aqueous silver ions Since the metallic silver is not important for the reaction to proceed we can use any inert solid that is conductive for the sake of this example we will use silver but silver makes for a very expensive inert electrode Once a wire is in place to connect the copper electrode to the silver electrode we only need a salt bridge to complete the circuit Once the cell is fully connected the copper electrode will be oxidized electrons will spontaneously flow from the copper electrode to the silver electrode and silver ions will be reduced to solid silver which will be observed as solid silver plating out on the silver electrode We call the electrode at which oxidation takes place copper in this case the anode and the electrode at which reduction takes place the cathode Hence electrons in a galvanic cell always flow from the anode to the cathode Since the anode is the source of electrons it is conventionally given a negative sign while the cathode being where the electrons are going to is given a positive sign Since electrons flow to the cathode the cathode s half cell will gain an overall negative charge from the excess of electrons this is not to be confused with the conventional designation of the anode as negative To balance this charge negative ions in the cathode s halfcell will flow via the salt bridge into the anode s half cell and positive ions in the anode s half cell will flow into the cathode s half cell in this way the entire circuit is completed see Figure 1 Figure 1 A Typical Galvanic Cell Electrolytic cells are similar but since the redox reaction is not spontaneous electrons are made to flow against the direction they would normally go Because of this the anode is conventionally given a positive sign and the cathode is given a negative sign compare this to a galvanic cell Electrons still flow from anode to cathode in an electrolytic cell There are many uses for this electrical energy which are of importance in everyday life such as batteries 48 This week in lab you will examine a few different types of redox reactions including making a series of electrochemical cells and performing a couple of small redox reactions Procedure Work in partners for this lab Note that you may do the sections in any order that you wish Part I Making electrochemical cells In this portion you will set up a series of different electrochemical cells and measure their voltage potential For this portion of the lab you will need to create a number of half cells The half cells will consist of each a solid metal and some solution containing the metal cation These half cells will be connected via the wire that you will attach The electrons will flow through the wire from one half cell to the other In order to actually get electrons to flow though the wire we must have a complete circuit To complete the circuit we need what is called a salt bridge This is an object that prevents the two half cells from coming in direct contact and thus reacting without the electrons needing to go through the wire but also allows us to maintain electric neutrality The set up for your electrochemical cells will be very simple You will need to cut a piece of the 70 mm filter paper into a large X shape Each arm of the X will be where you will build one of your electrochemical half cells Wet one arm of your filter paper with a couple drops of the metal cation solution then place a piece of the same solid metal on top of the now wet arm Repeat on other