Electrochemistry

Electrochemistry • Voltaic cell • Standard reduction potential • Electrochemical Series قسم الكيمياء – مجمع عمر بن الخطاب التربوي

Voltaic cell قسم الكيمياء– مجمع عمر بن الخطاب التربوي

Name: Hamza Eid – chemistry department Title of Lesson: Voltaic cell Date of Lesson: 18 – 26/3/ - 2007 Length of Lesson: 4- blocks Description of Class: Level 11 Advance Chemistry قسم الكيمياء– مجمع عمر بن الخطاب التربوي

Overview This lesson allows students to learn how to set up voltaic cells and explore the electrochemistry that is involved. Students will practice writing half reactions, learn to designate the cathode and anode, and make a reduction table. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Outcomes • Students will be able to: • Setup, conduct, and explain voltaic cell reactions • Write equations for half-reactions and balanced full redox reactions • Identify the anode and cathode • Identify the type of reactions that occur at the anode and cathode أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Teacher Does • Demo - How to set up a half-cell • The half cell consists of the oxidized and reduced form of an element or species. • In our case a solid metal and a solution of its ion. (i.e Zn and Zn2+) • The half cell will be constructed in a 100 mL beaker with 50 mL of solution and a strip of the metal. • Two half cells are used to make a voltaic cell. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

The voltaic cell needs a salt bridge connecting the two half cells. Our salt bridge will consist of strips of paper soaked in a brine (saturated salt) solution. Two half cells can be connected via a wire. This experiment will simply use the wire that connects the cells to the voltammeter. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

The voltammeter measurers the potential as electrons move through the cell. It will measure negative or positive flow depending on which way the wires are connected. Be sure to take positive measurements only. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Student Does • Students will be divided into groups of 3-4 • Ask students, in groups (each group consist of 3-members) to do the following activity (activity 1) then each group demonstrates: • How to set up an electrochemical cell using two metal /ion half-cells. • How they can identify the anode and the cathode in a cell أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Activity 2- Voltaic Cells Lab Resources, materials and supplies needed • For each group (3-4 students): • 1 Voltammeter (able to read voltage below 1V) • 20 mL potassium chloride Solution • 10 - strips of filter paper • 6 - 100 mL beakers • 50 mL of each of the following 1M solutions: ZnSO4, FeSO4, NiSO4, CuSO4, Ag2SO4, Al2(SO4)3 • 1 - 2" long wire of each of the following wires: Zn, Fe, Ni, Cu, Ag, Al أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Supplementary materials, handouts Lab Sheets Discussion Questions أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Students will be divided into groups of 3-4 and given a worksheet that outlines the procedure. For each voltaic cell students will report the half reaction equations, the balanced full equation, the anode and the cathode, the sites of oxidation and reduction, the positive voltage. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Activity-3You can get electricity from a lemon. • Make two slits in the skin of a lemon and insert a piece of aluminum and a piece of copper in the other slit. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Make sure the two metals are not touching each other inside the lemon. If you touch your tongue to the two strips of metal you will feel a tingle of electricity. The current flows because of the chemical reaction that takes place between the metals and acid in the lemon juice. The lemon juice acts in the same way as Volta's salt water or the chemical paste in a battery أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Assessments أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

For each of the above create a sketch of the cell similar to the following أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Vocabulary • Battery - a cell that produces an electrical charge from a chemical reaction • Electrodes - conductors of an electrical charge • Current - a continuous flow of electrical charges • Voltage - potential difference in a chemical cell which produces current • Anode - negative electrode in a chemical cell • Cathode - positive electrode in a chemical cell أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Standard electrode potentials أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Outcomes • Define standard reduction potential and cell potential • Construct, and label the parts of a standard hydrogen electrode • predict the voltage (E° ) of an electrochemical cell using the table of standard reduction half-cells أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Teacher Does • Standard Reduction Potentials: The tendency of a half reaction to occur as reduction under standard conditions • standard conditions: • Temperature = 25oC • concentrations of all ions = 1 M • pressure of all at gases =1 atm) أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

To measure relative electrode potentials, we must establish an arbitrary standard. That standard is the Standard Hydrogen Electrode (SHE). The SHE is assigned an arbitrary voltage of 0.000000… V Hydrogen Half-Cell reversible reaction أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Hydrogen Electrode The standard hydrogen electrode consists of a piece of platinum foil coated with fine particles of platinum. It is immersed into a solution of hydrogen ions and hydrogen gas is bubbled over it. . أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Cell Potential • the potential difference, in volts, between the electrodes of an electrochemical cell • positive value indicates a spontaneous reaction أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

The Copper-SHE Cell • The cell components are: • A Cu strip immersed in 1.0 M copper (II) sulfate. • The other electrode is a Standard Hydrogen Electrode. • A wire and a salt bridge to complete the circuit. • The initial cell voltage is 0.337 volts. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

The Copper-SHE Cell • In this cell the SHE is the anode • The Cu2+ ions oxidize H2 to H+. • The Cu is the cathode. • The Cu2+ ions are reduced to Cu metal. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

The Zinc-SHE Cell • For this cell the components are: • A Zn strip immersed in 1.0 M zinc (II) sulfate. • A wire and a salt bridge to complete the circuit. • The other electrode is the Standard Hydrogen Electrode. • The initial cell voltage is 0.763 volts. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

The Zinc-SHE Cell • The cathode is the Standard Hydrogen Electrode. • In other words Zn reduces H+ to H2. • The anode is Zn metal. • Zn metal is oxidized to Zn2+ ions. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

In an electrochemical cell. we can think that in an electrode the following situation occurring: The better the reducing agent the more the equilibrium lies over the right hand side. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Student Does Ask students, in groups (each group consist of 3-members) to do the following activity (activity 2) Each group demonstrate their observation. Use the following work sheet أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

From the activity we conclude that: If we had used a voltmeter in the circuit we would find that the bigger voltage would be developed for Zn/Ag. the second biggest by Zn /Cu and the smallest by Cu/Ag. So we can see that the voltage in a cell is governed the tendency of the metal ions to go into solution represented by the following equilibrium. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

So by measuring the voltage (and the direction of flow of the electrons) in a series of cells we will be able to arrange oxidising and reducing agents in an electrochemical series. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

The activity series of metals أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

electrochemical series: a series of half reactions arranged in the ordering of their reducing ability. The metals are arranged in the order of their reducing power. Teacher does أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

With the most reactive metals at the top. A metal placed in solution can displace a metal ion of a metal below it in the reactivity series so Zn(s) + CuSO4(aq) →ZnSO4(aq) + Cu(s) Will proceed أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Teacher does - Activity-4 • Ask students, in pairs, to design an investigation to determine the order of reactivity of the metals provided by constructing a potential difference chart أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Activity -5 • From Video 1 • Ask how they could improve the design of their investigation. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Activity-6 • Provide students with data of standard electrode potentials for a range of half-cells. • Do a worked example to show how to determine the direction of reaction for each half-cell, and how to write a balanced equation for the reaction and determine the standard cell potential by combining the two relevant standard electrode potentials. • Ask students to work in pairs and challenge each other to do the same with a different set of half-cells and check their partner’s answer أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

conclusions أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Uses of Standard Electrode Potentials • Electrodes that force the SHE to act as an anode are assigned positive standard reduction potentials. • Electrodes that force the SHE to act as the cathode are assigned negative standard reduction potentials. • Standard electrode (reduction) potentials tell us the tendencies of half-reactions to occur as written أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

For example, the half-reaction for the standard potassium electrode is: The large negative value tells us that this reaction will occur only under extreme conditions أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Compare the potassium half-reaction to fluorine’s half-reaction: The large positive value denotes that this reaction occurs readily as written. Positive E0 values denote that the reaction tends to occur to the right. The larger the value, the greater the tendency to occur to the right. It is the opposite for negative values of Eo. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Uses of Standard Electrode Potentials أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Use standard electrode potentials to predict whether an electrochemical reaction at standard state conditions will occur spontaneously. Example 21-3: Will silver ions, Ag+, oxidize metallic zinc to Zn2+ ions, or will Zn2+ ions oxidize metallic Ag to Ag+ ions? أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Steps for obtaining the equation for the spontaneous reaction. Choose the appropriate half-reactions from a table of standard reduction potentials. Write the equation for the half-reaction with the more positive E0 value first, along with its E0 value. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Add the reduction and oxidation half-reactions and their potentials. This produces the equation for the reaction for which E0cell is positive, which indicates that the forward reaction is spontaneous أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Write the equation for the other half-reaction as an oxidation with its oxidation potential, i.e. reverse the tabulated reduction half-reaction and change the sign of the tabulated E0. Balance the electron transfer. أ.حمزة الفار – مجمع عمر بن الخطاب التربوي

Electrochemistry