Sunday, October 17, 2010
CONCEPT OF ELECTROCHEMISTRY
Oxidation is the process of removal of electrons from a species, and reduction is the process of addition of electrons to a species. A redox reaction is a reaction in which there is a transfer of electrons from one species to another. The electron donor is called the reducing agent or reductant and the electron acceptor is called oxidizing agent or oxidant.
An electrochemical cell consists of two electrodes ( metallic conductor ) in contact with an electrolyte ( an ionic , solution , liquid or solid ) . An electrolyte and its electrolyte from an electrode compartments of the electrochemical cell may be joined by a salt bridge ( to complete the electrolyte circuit ) . The electrode at which oxidation occurs is called the anode , the electrode at which reduction occurs is called the cathode .
A galvanic cell is an electrochemical cell that produces electricity as a result of the spontaneous reaction occurring inside it. An electrolytic cell is an electrochemical cell in which a non – spontaneous reaction is driven an external source of current .
Galvanic cell diagram
Electrochemical cell diagram
The salt bridge or porous disk is necessary to maintain the charge neutrality of each half-cell by allowing the flow of ions with minimal mixing of the half-cell solutions. As electrons are transferred from the oxidation half-cell to the reduction half-cell, a negative charge builds in the reduction half-cell and a positive charge in the oxidation half-cell. That charge buildup would serve to oppose the current from anode to cathode- effectively stopping the electron flow--if the cell lacked a path for ions to flow between the two solutions.
Example of the reaction by galvanic cell:
In this example, when zinc is placed into a salt solution (in the half-cell), the zinc tends to lose electrons. This loss of electrons is called oxidation and takes place at the anode. Conversely, for the half-cell containing copper in the salt solution, the copper tends to gain electrons. This is referred to as reduction and happens at the cathode. When the two cells are connected via the salt bridge or electric conductor to form a closed circuit, the arrangement becomes a galvanic cell or battery. The voltage generated for this battery is 1.1Volts.
Example of reaction by electrochemical cell :
An ammeter, placed in the circuit, will register the flow of current. From this indication, the electric circuit can be determined to be complete. It is clear that copper sulfate solution obviously has the property that it can conduct electricity. Such a solution is termed as electrolyte. The wires are called electrodes, the one with positive polarity being the anode and the one with negative polarity the cathode. The system of electrodes and electrolyte is referred to as the electrolytic cell, while the chemical reactions which occur at the electrodes are called the anodic or cathodic reactions or processes.
Simulation is one of the way in teaching skill . It has many benefits by using this way compared to ordinary explanation . Nowadays , educational institution start using simulation in their learning and teaching process. The benefits by using simulation are :
a) It can make the teaching and learning process become more interesting. So , it will make the environment in the class lively . The students does not felt bored.
b) It can increase the level understanding among the students . It is because the animation can stimulate their mind to think more.
c) It is the easy way to teach because teacher use technology . So , they can imagine the process easily because the simulation can show how it is happen start from the beginning until the end of the process.
d) Mostly , the simulation are colourful . As we all know , colour can stimulate our mind. It will help the student to understand what the teacher teach about .
This video will show you how the reaction occur in galvanic cell.
Answer all the question below which are in part A and part B .
PART A : Answer all the equation below based on the video
As electric current is generated in the cell , the zinc electrode slowly _____________ , and more copper is ____________ on the copper electrode, and the ________________ solution become fades .
PART B : Based on answer in part A
i) Explained how is this explained .
ii) Write down the equation that are involved.
Dissolves , deposited , blue colour of copper (II)
i. Zinc metal loses its electrons . It is oxidized to zinc ions ( Which is soluble in water ). Thus , zinc metal is the anode . Anode is the negative electrode since it is a source of electrons . The oxidation reaction at the zinc anode may be represented by a half cell – notation , Zn (s) / Zn2+(aq) . The slash (/) is used to separate the different phase of the species involved.
The electron from the zinc metal flow through the wire , enter the copper metal and are taken up by copper (II) ions , which become reduced to copper ( which is insoluble which water ) . Thus , copper method is the cathode . Cathode is the positive because it attracts electrons . The reduction reaction at the copper cathode may be represented by a half – cell notation ., Cu2+(aq) , Cu (s) .
The flow of electrons in the wire constitute electricity . The voltage and the direction of electron flow are shown by a voltmeter .
As electricity is generated , the zinc half – cell will have an excess of positive charges as more zinc ions are formed . On the other hand , the copper half – cell will have an excess of negative charges as more of copper ( II ) ions are deposited as copper atoms . The negative ions in the salt bridge then move into the zinc half-cell , and the positive ions move into the copper half – cell .
ii . At anode : Zn(s) Zn2+ (aq) + 2e
Overall reaction : Zn(s) + Cu2+ (aq) Zn2+ (aq) + Cu(s)
Fill in the blank with a suitable answer:
1. An electrochemical cell converts chemical energy from a spontaneous redox reaction into _____________.
2. In galvanic cell , the two metals are connected by a ___________ , while the two containers are connected by a ______________.
3. ________________ reaction occurs when a species loses one or more electrons .
4. The oxidation number of oxidation reaction will ____________ while oxidation number of reduction reaction will _______________.
5. In electrochemistry , the reduction and oxidation reaction are called ______________.
Answer either true or false :
1. Both cell use electricity to perform redox reaction
2. Both cell produce electric current as a result of redox reactions
3. Both cell perform reduction at the cathode and oxidation at the anode
4. An electrolytic cell performs reduction at the anode while the galvanic cell has its reduction at the cathode