Electrochemistry - Fundamentals PDF

LESSON 07: ELECTROCHEMISTRY UNIT 01: Fundamentals of Electrochemistry Introduction: Chemical reactions involve the transfer of electrons from one atom or molecule to another – a process called The Reduction-Oxidation (Redox) Reaction. In this chapter, we will discuss first the fundamentals of ele...

LESSON 07: ELECTROCHEMISTRY UNIT 01: Fundamentals of Electrochemistry Introduction: Chemical reactions involve the transfer of electrons from one atom or molecule to another – a process called The Reduction-Oxidation (Redox) Reaction. In this chapter, we will discuss first the fundamentals of electrochemistry: the redox reactions and the notations needed. Learning Objectives: After successful completion of this lesson, the students are expected to: 1. Balance redox reactions in acidic or basic media. 2. Identify the two general types of cell: voltaic cells and electrolytic cells. 3. Construct cell notations from chemical equations or cell diagrams. Course Materials: Electrochemistry is the branch of chemistry that deal with the inter-conversion of electrical energy and chemical energy. Electrochemical processes are redox reaction in which the energy released by spontaneous reaction is converted to electricity, or in which electrical energy is used to cause a nonspontaneous reaction to occur. In order to understand this further, we will have a recap on redox reactions. Redox reactions are chemical reactions where electrons are transferred from one chemical species to the other. As they do so, their oxidation state changes. A loss of electron by oxidation reaction results in the increase of oxidation state of an atom, making it more positively charged. The lost electron is then transferred to the other chemical species by reduction reaction, causing a decrease in the oxidation state of an atom, making it less positively charged or negative at times. In order to balance redox reactions, we should be able to recognize which species undergo reduction or oxidation process. For example, for the reaction The oxidation state of zinc went from (0) to (+2), hence it underwent oxidation reaction. For copper, the oxidation state is reduced to (0) from (+2) by reduction reactions. To balance the reactions, we first split the equation into two half-reactions by combing those with the similar atoms in it. That is The first step in balancing redox reactions is to balance first the atoms. Since they are already balanced, we then balance them by their charges. Notice that for (Ox), the reactant side do not have any charge at all, while the product side has a net charge of (+2). To balance the charges, we add enough electrons to the side with the highest charge to bring down the charge similar to that of the lowest. For (Ox) two electrons will be added to the product side, and two electrons for the reactant side for (Red). That is, Finally, we combine the two equations, cancelling out similar terms The next reaction requires more steps, as other atoms such as oxygen and hydronium ions are needed to balance them. Consider the reaction We will notice that the oxidation number of iron increases from (2+) to (3+), thus iron underwent an oxidation process. Consequently, this means that chromate ion underwent reduction process to chromium (III) ion. We can write these two half-reactions as Then, balance each half-reaction for the number and type of atoms and charges. Add 5=< to balance O atoms first, then add 5˘ to balance H atoms. To balance charges, add electrons. The resulting balanced half-reactions are shown as below. Please note that the coefficient should be multiplied to the charge to get the total charge for a chemical species. For (Ox), the reactant side only has a (+2) charge and (+3) charge for the product side. Addition of one electron in the product side will be sufficient to balance the charges. For (Red), to obtain the charge for the reactant side, we multiply the coefficient by the charge, that is 14(+1) + 1(−2) = +12 charge for the reactant side. For the product side, that is 2(+3) + 7(0) = +6 charge. Addition of six electrons in the reactant side will make the equations balanced in terms of charges Finally, add the two half-reactions together. The electrons must cancel out in the final equation, so you may multiply a factor to any of the two equations. In this case, we multiply (Ox) by six, so the electrons will cancel out eventually. The balanced equation is This reaction is in acidic medium since there are 5˘ ions present. To make the reaction in basic medium, just add same amount of