sign in sign up
<<
Home , Salt bridge

Unit 7 :

17 Jan 2019 Agenda

● Three Classes of reactions ○ Spontaneous redox ○ Electrochemical cells ○ and... Spud-u-like labette 1. Insert 4 metal strips (zinc, copper, magnesium and lead) into a potato. 2. Use a voltmeter to measure the voltage between all possible combinations of two of the metals. 3. Generate a complete results table for all the possible combinations. 4. Order the metals from the most likely to be oxidized to least likely to be oxidized - with justifications from evidence. Results table: Potato + (red) → Cu Pb Zn Mg - (black)↓ Cu x -0.31 V -0.88 V -0.97V -0.33 V -0.80 V -0.88V -0.32 V -0.87 V -1.61 V Pb 0.34 V x -0.48 V -0.63V 0.37 V -0.63 V -0.57 V 0.34 V -0.46 V -0.50 0.32 V -0.55 - 0.34 V Zn 0.85V 0.43 V x -0.18V 0.92 V 0.56 V -0.02 V 0.80V 0.48 V -0.03 V 0.96 V 0.55 V -0.75? 0.84 V 0.87 V

Mg 0.88 V 0.64 V 0.14V x 0.98 1.39 V 0.02 V 0.99 V 1.29V 0.76 V 0.90V 0.52 V? 1.59 v 1.69 V 1.68 V 1.70 V + (red) → Cu Pb Zn Mg - (black)↓ Cu x -0.31 V -0.88 V -0.97V -0.33 V -0.80 V -0.88V -0.32 V -0.87 V -1.61 V Pb 0.34 V x -0.48 V -0.63V 0.37 V -0.63 V -0.57 V 0.34 V -0.46 V -0.50 0.32 V -0.55 - 0.34 V Zn 0.85V 0.43 V x -0.18V 0.92 V 0.56 V -0.02 V 0.80V 0.48 V -0.03 V 0.96 V 0.55 V -0.75? 0.84 V 0.87 V

Mg 0.88 V 0.64 V 0.14V x 0.98 1.39 V 0.02 V 0.99 V 1.29V 0.76 V 0.90V 0.52 V? 1.59 v 1.69 V 1.68 V 1.70 V Most likely to be oxidized (lose electrons)

Mg

Zn

Pb

Cu

Least likely to be oxidized ____V

1.30V 0.86 V Cell potential - the “pull” or driving force, on the electrons is called the cell

potential (Ɛcell), or the (emf) of the cell. The unit of electrical potential is the volt (V) 1 joule of work per coulomb of charge transferred. Measuring cell potential

● Traditional voltmeter ○ Draws current through a known resistance ○ Frictional heating wastes some of the potentially useful energy of the cell ○ Measures potential < maximum cell potential ● Digital voltmeter (replaced potentiometers) ○ Draw only minimal current 17.1 Galvanic Cells

● Uses a spontaneous (thermodynamically favorable) redox reaction to produce a current that can be used to do work ● Oxidation occurs at the ○ Zn →Zn2+ + 2e- ● Reduction occurs at the ○ Cu2+ + 2e-→Cu

1.1 2e- - a device in which chemical energy is changed to electrical energy. Electrons flow through the wire from anode to cathode, and flow through salt bridge from one compartment to the other to keep net charge zero Galvanic Cell At anode - loss of electrons occurs OXIDATION Zn → Zn2+ + 2 e-

At cathode - gain of electrons occurs REDUCTION Cu2+ + 2 e -→ Cu 1.1 2e- What is going to happen to the zinc over time?

What is going to happen at the copper electrode over time? 1.1 2e- What is going to happen to the zinc electrode over time? Reduce in size (mass)

What is going to happen at the copper electrode over time? Increase in size (mass) 1.1 2e- Let us set up a galvanic cell -

String soaked in

As salt bridge.

How close are we to reported standard 1.10V for this cell?

Scale? Figure 17.3 An electrochemical process involves electron transfer at the interface between the electrode and a solution. a) The species in the solution acting as the reducing agent supplies electrons to the anode. Oxidation Reduction occurs here occurs here . a) The species in the solution acting as the reducing agent supplies electrons to the anode. b) The species in the solution acting as the oxidizing agent receives electrons from the cathode.

Porous disk allows ions to flow Oxidation Reduction without extensive mixing of occurs here occurs here solutions Exam question A student is given a standard galvanic cell, represented above, that has a Cu electrode and a Sn electrode. As current flows through the cell, the student determines that the Cu electrode increases in mass and the Sn electrode decreases in mass.

(a) Identify the electrode at which oxidation is occurring. Explain your reasoning based on the student’s observations. A student is given a standard galvanic cell, represented above, that has a Cu electrode and a Sn electrode. As current flows through the cell, the student determines that the Cu electrode increases in mass and the Sn electrode decreases in mass.

(a) Identify the electrode at which oxidation is occurring. Explain your reasoning based on the student’s observations. Since the Sn electrode is losing mass, Sn atoms must be forming Sn2+(aq). This process is oxidation. (b) As the mass of the Sn electrode decreases, where does the mass go?

(c) In the expanded view of the center portion of the salt bridge shown in the diagram below, draw and label a particle view of what occurs in the salt bridge as the cell begins to operate. Omit solvent molecules and use arrows to show the movement of particles. (b) As the mass of the Sn electrode decreases, where does the mass go? The atoms on the Sn electrode are going into the solution as Sn2+ ions.

(c) In the expanded view of the center portion of the salt bridge shown in the diagram below, draw and label a particle view of what occurs in the salt bridge as the cell begins to operate. Omit solvent molecules and use arrows to show the movement of particles. c) In the expanded view of the center portion of the salt bridge shown in the diagram below, draw and label a particle view of what occurs in the salt bridge as the cell begins to operate. Omit solvent molecules and use arrows to show the movement of particles.

KNO 3 salt bridge K+ NO - Cu 3 Sn

The response should show at least one K+ moving toward the Cu compartment on the left − and at least one NO3 ion moving in the opposite direction.

The missing component of the cell is the salt bridge. A salt bridge is important to complete the circuit and allows for the migration of ions to maintain charge balance in each half-cell of the galvanic cell. Homework: Review naming systems. (Quiz on naming systems next week - Friday).

Figure 2.23 A flowchart for naming binary compounds

Figure 2.24 Overall strategy for naming chemical compounds

Table 2.7 Names of Acids that do not contain oxygen

Table 2.8 Names of some oxygen containing acids