Ch#7 : Electrochemistry Flashcards
Learn about oxidation , reduction , oxidization states , electrolytic processes and electroplating
Define oxidation and reduction in terms of gain and loss of oxygen and hydrogen.
Oxidation is defined as the gain in oxygen or loss in hydrogen atoms by an element.
4Fe + 3O². —> 2Fe²O³
Reduction is defined as the loss of oxygen or gain of hydrogen atoms.
Fe²O³ + 3CO —> 2Fe + 3CO²
Define oxidation and reduction in terms of loss and gain of electrons.
Oxidation is the process which involves the loss of electrons by a elements.
Na —> Na+1 + e
Reduction is the process which involves the gain of electrons by a element.
Cl + e —> Cl-1
Define oxidation states and state the 4 rules for assigning oxidation states.
Oxidation state is the number of charges an atom will have in a molecule or compound.
Rules:
1. The oxidation state of any uncombined or free elements is always zero.e.g S in S⁸.
- In simple ions , the oxidation state is the same as the charge on the ion. Oxidation state of Mg+2 is +2.
- In a complex ion , the total sum of oxidation states is equal to the charge on the ion. Example , in CO³-2 , the sum of the oxidation states of C and 3O is -2.
- The oxidation number of each of the atoms in a molecule or compound is counted seperately and their algebraic sum is zero. Example , in HCl , the sum of the oxidation states of H + Cl atoms is zero.
What are oxidizing and reducing agents? Give 2 examples for each.
An oxidizing agent is the reactant containing the element that is being reduced(gain electrons) in a chemical reaction.
Examples : F² , Cl²
A reducing agent is the reactant containing the element that is being oxidized in a chemical reaction.
Examples : CO , H²
What are electrochemical cells and their two types? , Also describe the nature of electrochemical processes.
Devices which convert chemical energy into electrical energy or vice versa are called electrochemical cells.
Electrolytic Cell : A electrochemical cell which uses electrical energy to drive a chemicak reaction.
Galvanic Cell : An electrochemical cell which converts chemical energy into electrical energy.
Nature if electrochemical processes :
Electrochemical processes are redox reactions in which chemicak energy released by a spontaneous reaction is converted to electricity or in which electrical energy is used to drive a non-spontaneous reaction. Whether an electrochemical cell releases or requires energy , it always involves the transfer of electrons or redox reactions.
Define electrolytes and give a example.
Electrolyte : A substance which can conduct electricity when dissolved in water or in its molten state is called an electrolyte.
Example : NaCl , KCl , HCL , NaOH
Non electrolytes : glucose , sucrose , benzene.
Explain the reactions in a electrolytic cell and state what they consist of.
Electrolytic Cell: A electrochemical cell where electrical energy is used to drive a non spontaneous chemical reaction.
Consists of :
- A vessel containing an electrolyte (MX)
- Two inert electrodes
- A battery
The electrons move from the anode to the cathode in the outer circuit. In the solution , the cations move towards thw catode and the anions move towards the anode. At the cathode , the cations reduce by gaining electrons. At the anode , the anions oxidize by losing electrons.
Oxidation occurs at anode and reduction at the cathode.
At Anode : X-1 —> X + 1e
At Cathode : M+1 + 1e —> M
State 7 uses of electrolytic cells.
- Down’s cell is used for the commercial preparation of sodium metal. It produces chlorine gas as a by product.
- Nelson’s cellis used for the commercial preparation of sodium hydroxide. It produces chlorine and hydrogen gas as a by product.
- Electrolytic cells are used for the commercial preparation of calcium and magnesium metals.
- It is used to produce aluminum metal commercially.
- It is used to purify copper.
- It is used to electroplate metals such as tin, silver , nicket on steel.
- Electrolytic cells are used to prepare anodized aluminum. Anodized aluminum can absorb dyes. It can produce metal red , blue , green and other metalic colors.
What are Voltaic Cells? , What do they consist of? Explain the reactions in a daniel cell.
The cell which converts chemical energy from a spontaneous reaction into electrical energy is called a Galvanic or Voltaic Cell. It is called a Voltaic Cell because Alessandro Volta first discovered such cell. The English chemist Frederick Daniel contructed the world’s first voltaic cell using copper and zinc electrodes. So this cell is named as Daniel cell.
It consists of :
- A zinc bar dipped in a 1M ZnSO⁴ solution.
- A copper bar dipped in a 1M CuSO⁴ solution.
- A salt bridge which is an inverted U tube containing an inert electrolyte such as KCl. The electrolyte doesn’t react with the electrodes or the ions in the solution. It makes the electrical contacts between the solution through which the ions can move.
- A voltmeter to measure current.
Each compartment of a daniel cell is called a half cell. A Daniel cell consists of two half cells joined in series. When the circuit is complete , Zn half cell acts as anode and electrons flow from the zinc rod to through the wire to the Cu rod. The Cu half cell acts as cathode.
Reactions : In a Daniel Cell , electrons flow from the Zn rod , through the external wire to the Cu rod. This is because Zn has a greater tendency to oxidize than Cu.Zn atoms from the rod into the solution as Zn+2 ions leaving electrons on the rod. These electrons flow through the external circuit. The oxidation half reaction occurs at the anode compartment. Cu+2 ions in the CuSO⁴ solution capture electrons from the rod and are reduced. The reduction half reaction occurs at the cathode compartment.
At Anode : Zn —> Zn+2 + 2e
At Cathode : Cu+2 + 2e —> Cu
Briefly describe electroplating. Explain Zinc and Chromium Plating.
Electroplating : is the electrolytic process used to deposit one metal on another metal. An object can be electroplated hy making it cathode in an electrolytic tank containing ions of the plating metal. The plating metal is made anode. On passing electricity through the electrolytic tank , a thin layer of anode metal is deposited is deposited on the surface of the object.
Zinc Plating : Zinc plating is done by making zinc metal the anode. A solution of potassium zinc cyanide K²[Zn(CN)⁴] containing little sodium cyanide is used as electrolyte. During the electrolysis , zinc at the anode enters the solution as Zn+2 ions which are deposited on the cathode.
The electrolyte ionizes as following:
K²[Zn(CN)⁴] 2K+1 + [Zn(CN)⁴]-2
[Zn(CN)⁴] Zn+2 + 4CN-1
At Anode: Zn —> Zn+2 + 2e
At Cathode : Zn+2 + 2e —> Zn
Sodium Cyanide prevents hydrolysis.
Chromium Plating : Since chromium doesnt adhere strongly with steel , it is first plated with copper or nickel and then chromium. For electroplating chromium , chromium metal is made anode and chromium sulphate Cr²(SO⁴)³ as electrolyte. Few drops of diluted H²SO⁴ are added to prevent hydrolysis.
Cr²(SO⁴)³ 2Cr+3 + 2SO⁴-2
At anode : Cr —> Cr+3 + 3e
At cathode : Cr+3 + 3e —> Cr
Explain the rusting of iron and aluminum.
Txtbook pg143
State all methods for preventiing corrosion.
Pg143-144
