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Chemistry 30 Notes Introduction to Electrochemistry During redox reactions, electrons pass from one substance to another. The flow of electrons - current - can be harnessed to do work. Electrochemistry is the branch of chemistry that deals with the conversion between chemical and electrical energy. 1. Electrochemical Cells The basic unit of all batteries is the electrochemical cell (also called a galvanic cell). Electrochemical cells convert the energy of a spontaneous redox reaction into electricity. An electrochemical cell consists of two half-cells, each having an electrode (solid electrical conductor) in contact with a solution of ions. The electrodes are where the redox half-reactions occur. 1. Anode – is the site of oxidation and the source of electrons, making it the negative post of the electrochemical cell 2. Cathode – site of reduction and the positive post as it consumes electrons The electrodes are connected with wire. The force that moves the electrons from the anode to the cathode via the wire is known as the electrical potential and it is measured in volts, using a voltmeter. It is generally defined as the cell potential or voltage for a source in a circuit. Chemistry 30 Notes Once the half-cells are connected, there is an immediate flow of electrons (current) but then it abruptly stops as the charges build up in the half-cells. With nowhere to go, the build-up of charge prevents further electron transfer and the redox reaction stops. To solve this problem, the two half-cells are connected with a salt bridge, a U-shaped tube filled with non-reactive electrolyte that is designed to keep ions flowing and the cells electrically neutral. The ends of the tube are plugged with cotton balls to prevent the solution from falling out but are porous enough to permit some fluid and ions to flow between cells. 2. Electrochemical Cell Example Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s) This reaction involves two half-reactions: Zn → Zn +2 + 2 e- Cu2+ + 2e- → Cu oxidation reduction One half-cell contains a strip of zinc metal (which acts as an electrode) in a Zn(NO3)2 solution. The other half-cell contains copper metal in Cu(NO3)2. KNO3 is used as the salt bridge. The zinc half-cell undergoes oxidation. Here, the solid zinc electrode disintegrates, forming zinc ions and releasing electrons. The copper half-cell undergoes reduction. Here, copper ions from the electrolytic solution become deposited on the copper electrode, forming more solid copper. At the anode, Zn2+ ions are being produced and go into solution. This causes a build-up of positive ions in this solution. The excess positive charge attracts the negative NO3ions from the salt bridge, thereby keeping the solution electrically neutral. Chemistry 30 Notes At the cathode the opposite occurs. As positive Cu2+ ions are removed from solution, to form solid Cu, the solution becomes overly negative. This attracts the positive K+ ions from the salt bridge, keeping this side of the cell neutral. External Circuit Electrons flow from A to C Anode to Cathode Internal Circuit Anions to the Anode Cations to the Cathode As the reaction proceeds, the voltage will eventually decrease and finally become zero when the concentrations of ions in solution reach equilibrium. The cell will then be considered ‘dead’. 3. Predicting Cell Reactions and Equations To identify the anode and cathode and to calculate the voltage of the cell, we will need to use a Table of Reduction Potentials. Standard Potentials (E0) are measured against a hydrogen ion reduction reaction, which is arbitrarily assigned a potential of zero volts. The cell’s total voltage (E0cell) is thus calculated by subtracting the E0 for the oxidation from the E0 for the reduction. OR The half-reaction with the greater potential to be reduced is higher on the table. As such, the higher half-reaction will happen at the cathode and the lower half-reaction will occur at the anode (stronger reducing agent therefore more likely to be oxidized). Chemistry 30 Notes Zn2+ + 2e- Zn Cu2+ + 2e- Cu For example: On the table: E = -0.76V E = +0.34V The less positive, or more negative reduction potential, occurs at the anode. so…Zn + Cu2+ Zn2+ + Cu E0cell = E0cathode - E0anode = +0.34V – (-0.76V) = + 1.10 V The + sign of the cell potential tells us the redox reaction is spontaneous, meaning the cell does work. 4. Representing Cell Reactions