In this presentation you will: explore the principles of electrochemistry

Electrochemistry • In this presentation you will: • explore the principles of electrochemistry Next >

Introduction Electrochemistry is the branch of chemistry that studies the chemical action of electricity, for example, in electrolysis and the production of electricity. Next >

Electroneutrality Nature seems to oppose unbalanced processes. When a positive or negative electric charge is generated, a charge neutralizing effect often takes place. Zn(s) → Zn2+(aq) + 2e- Next >

Metal ions go into the solution, leaving their electrons behind. Electroneutrality If a piece of metal, such as zinc (an electrode), is immersed in pure water, oxidation occurs, and a small number of metal atoms go into solution as ions. Their electrons are left behind in the metal. Zn(s) → Zn2+(aq) + 2e- Next >

Electroneutrality As the process continues, a negative electric charge builds up in the metal. This negative charge, together with the build up of positive charges in the water, makes it more difficult for additional ions to leave the metal. Next >

Ions returning to the metal. Electroneutrality Similarly, positive ions from the water return to the negatively charged metal. Overall, the process results in a neutralizing effect with an insignificant concentration of ions in solution. Next >

Question 1 In the oxidation of an electrode,... A) …a positive electric charge builds up in the metal. B) …metal atoms go into solution as ions, constantly building up a considerable negative charge in the electrode. C) …a number of electrons go into solution as ions, leaving the atoms behind in the metal. D) …a charge neutralizing effect takes place in the metal. Next >

Single Electrode Potential The charge (potential) developed in an electrode-solution system is impossible to measure. This is because one lead of a meter would need to be connected to a second electrode. Doing this would create a second potential difference that would contaminate the reading. Next >

Single Electrode Potential Single electrode potentials are, therefore, not directly observable. It is possible, however, to measure the potential difference between the two electrodes. Next >

ZnSO4 solution CuSO4 solution Zinc metal anode (oxidation) Copper metal cathode (reduction) ZnSO4 solution Zinc metal anode (oxidation) Illuminated bulb CuSO4 solution Copper metal cathode (reduction) Electrochemical Cell An electrochemical cell takes advantage of the oxidation-reduction reactions (redox). The potential differences between the two electrodes (often zinc and copper) generate electricity. Next >

ZnSO4 solution CuSO4 solution Zinc metal anode (oxidation) Copper metal cathode (reduction) ZnSO4 solution Zinc metal anode (oxidation) Illuminated bulb CuSO4 solution Copper metal cathode (reduction) Electrochemical Cell The electrodes are immersed in two salt solutions (electrolytes) of the metal. An electrochemical cell is effectively two half-cells joined together to the sum of their potentials. Next >

ZnSO4 solution CuSO4 solution Zinc metal anode (oxidation) Copper metal cathode (reduction) Cu2+ SO42- SO42- Zn2+ ZnSO4 solution Zinc metal anode (oxidation) Illuminated bulb SO42- Zn2+ Cu2+ SO42- CuSO4 solution Copper metal cathode (reduction) Electrochemical Cell In a zinc-copper electrochemical cell, the zinc metal anode oxidizes. That is, it loses positive zinc ions (Zn2+) to a solution of zinc sulfate. It gains a negative charge due to the electrons (2e-) left in the metal. Next >

ZnSO4 solution CuSO4 solution Zinc metal anode (oxidation) Copper metal cathode (reduction) SO42- Cu2+ Zn2+ SO42- ZnSO4 solution Zinc metal anode (oxidation) Illuminated bulb Zn2+ Cu2+ SO42- SO42- CuSO4 solution Copper metal cathode (reduction) Electrochemical Cell At the same time, the copper metal cathode reduces, obtaining ions from the solution and subsequently gaining a positive charge. Next >

Zn(s) + Cu2+(aq) + 2e-→ Cu(s) + Zn2+(aq) + 2e- ZnSO4 solution CuSO4 solution Zinc metal anode (oxidation) Copper metal cathode (reduction) ZnSO4 solution Zinc metal anode (oxidation) Illuminated bulb SO42- SO42- Zn2+ Cu2+ CuSO4 solution Copper metal cathode (reduction) Electrochemical Cell Cu2+(aq) + 2e-→ Cu(s) In a zinc-copper electrochemical cell, the two half cell reactions are: Zn(s) → Zn2+(aq) + 2e- The overall reaction is: Electrons are balanced. Next >

Question 2 What reaction is necessary in an electrochemical cell in order to generate an electromotive force around a circuit? A) Only a neutralizing reaction B) Only an oxidation reaction C) Only a reduction reaction D) Both oxidation and reduction reactions Next >

ZnSO4 solution CuSO4 solution Zinc metal anode (oxidation) Copper metal cathode (reduction) ZnSO4 solution Zinc metal anode (oxidation) Illuminated bulb CuSO4 solution Copper metal cathode (reduction) Electrochemical Cell Connecting the two metals by means of a conductor closes the circuit. With the two metals connected, the excess electrons in the zinc electrode (anode) are attracted by the excess positive atoms and flow through towards the copper electrode (cathode). Porous partition Next >

ZnSO4 solution CuSO4 solution Zinc metal anode (oxidation) Copper metal cathode (reduction) ZnSO4 solution Zinc metal anode (oxidation) Illuminated bulb CuSO4 solution Copper metal cathode (reduction) Electrochemical Cell This electron flow is generated by an electromotive force(emf),which is measured in volts. Porous partition Next >

Voltmeter e- e- Cu Zn SO42- Na+ ZnSO4(aq) CuSO4(aq) Cu2+ Zn2+ Zn2+ SO42- Electrochemical Cell In order to sustain the cell reaction, the charge carried through the external circuit must be compensated by the transport of ions between the two solutions. The two solutions can be separated by a porous partition that prevents mixing but allows ions to diffuse through. Next >

Voltmeter e- e- Cu Zn SO42- Na+ ZnSO4(aq) CuSO4(aq) Salt bridge [Na2SO4(aq)] Cu2+ Zn2+ Zn2+ SO42- Electrochemical Cell It is more effective, though, to use two separated containers for the two half cells, linked with a salt bridge. A salt bridge is an intermediate compartment filled with a solution that allows drift and diffusion of ions. Next >

Question 3 What is needed in an electrochemical cell to sustain the cell reaction? A) A voltmeter to measure the emf B) The transport of ions between the two solutions C) A mix of the two solutions D) A separate container to keep the two solutions in Next >

Glass tube to contain H2(g) H2(g) Pt(s) electrode H+ (aq) Bubbles of H2(g) Reference Cell The absolute value of a half cell potential can not be determined, but its value can still be measured in relation to the potentials of other half cells. Next >

Glass tube to contain H2(g) H2(g) Pt(s) electrode H+ (aq) Bubbles of H2(g) Reference Cell The standard half cell to which any other half cell can be connected for comparison is the hydrogen electrode. The potential of the standard hydrogen electrode (SHE) is defined as zero. When this reference cell is connected to another half cell, the measured emf is the electrode potential of that cell. Next >

Question 4 What electrode is normally used as the standard half cell, to which the potential of any other half cell is compared? A) Zinc electrode B) Copper electrode C) Hydrogen electrode D) The absolute value of a half cell potential can not be determined Next >

Eo In V Reduction Half-reaction Reference Cell Standard electrode potentials can be listed in order to produce a table of electrode potentials, called electrochemical series. This table shows the ability of different ions and molecules to act as oxidizing agents in reference to a defined potential. In this way, reducing agents can be predicted. Next >

Weaker reducing agent Stronger oxidizing agent Eo In V Reduction Half-reaction Weaker oxidizing agent Stronger reducing agent Reference Cell Good oxidizing agents appear positive in the electrochemical series, whereas good reducing agents appear negative. Next >

Question 5 Which of the following is the strongest oxidizing agent? A) Copper B) Hydrogen C) Nickel D) Cadmium Next >

Electrochemical Cell Notation Using the same example of the zinc-copper cell, the shorthand representation of the half cell reactions that make up the electrochemical cell is: Cell potential Left hand half cell (LHS) Right hand half cell (RHS) Zn(s) I Zn2+ (aq) II Cu2+(aq) I Cu(s) Eө cell = +0.10 V Boundary between two faces Salt bridge Boundary between two faces Electrons always flow from the half cell on the left to the half cell on the right. In an electrochemical cell, the voltage obtained is the difference in reduction potentials (Eө). Eөcell = Eө reduced (RHS) - Eө oxidized (LHS) Next >

EӨ In V Reduction Half-reaction Cu2+(aq) + 2e- → Cu(s) Eө = +0.34 V Zn2+(aq) + 2e- → Zn(s) Eө = -0.76V Electrochemical Cell Potential Calculation By looking at the reduction potentials of the cell reaction, it is easy to determine which is Eө reduced and which is Eө oxidized. The more negative half cell potential is Eөreduced, the more positive potential is Eө oxidized. Therefore, in a zinc-copper electrochemical cell: Eөcell = 0.34 – (-0.76) = 1.10 V Next >

Summary In this presentation you have seen: • the electroneutrality tendency • the principles of an electrochemical cell • a reference cell End >

In this presentation you will: explore the principles of electrochemistry