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Zinc oxide is an inorganic compound which has the chemical formula ZnO. This compound is also known as calamine or zinc white. It is just like a white powder which is not soluble in water. It is used as an additive in various products and materials which includes food supplements, rubbers, ceramics, plastics, pigments, batteries, first-aid tapes, lubricants, paints, cosmetics etc. Zinc oxide is mainly a wide-band gap semiconductor of II-VI semiconductor group. This compound is generally found in mineral zincite. It is mostly produced synthetically.
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Key Takeaways: Zinc Oxide, Metallurgy, Chemical Reactions, Corrosion, Reducing Agent, Calcination
Uses of Zinc Oxide
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- Zinc oxide is used in a wide range of cosmetics and personal care products including nail products, makeup, body lotions, bath soaps etc.
- It is also used in sunscreen products, skin protectants like diaper rash ointments.
- It is used as a colorant and bulking agent.
- Zinc oxide works like a sunscreen by reflecting and scattering UV radiations, sunscreens reduce or prevent sunburn and premature ageing of the skin.
- It is also used in cigarette filters, breakfast cereals, spintronics, light emitting diodes (LED’s), piezoelectricity etc.
Read More: Electrochemistry
Physical Properties of Zinc Oxide
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- Zinc oxide crystallizes in two different forms, hexagonal wurtzite and cubic zincblende. The wurtzite structure is most stable at ambient conditions. The zincblende form can be stabilized by growing ZnO on substrates with cubic lattice structure.
- In both the cases, the zinc and oxide centers are tetrahedral, the most characteristic geometry for Zn(II).
- Hexagonal and zincblende polymorphs have no inversion symmetry.
- ZnO is a soft material with an approximate hardness of 4.5 on Mohs scale.
- The high capacity and heat conductivity, low thermal expansion and high melting temperature of ZnO is beneficial for ceramics.
- Boiling Point of Zinc Oxide -1,974 °C
- Melting Point of Zinc Oxide -1,974 °C
Zinc Oxide in Powdered form
| Concepts Related to This Topic | ||
|---|---|---|
| Types of Solutions | Solubility | State of Matter |
| Types of Solid | Covalent bonds | Metallic Bonds |
| Properties of Bases | Refining of Metals | Biochemistry |
Chemical Properties of Zinc Oxide
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- Crystalline zinc oxide is thermochromic, which is responsible for changing the color from white to yellow when heated in air and reverting to white on cooling. Zinc oxide is an amphoteric oxide and it is not soluble in water but can be easily dissolved in most acids like hydrochloric acid.
ZnO + 2HCl → ZnCl2 + H2O
- Solid zinc oxide dissolves in alkalis to give soluble zincates:
ZnO + 2NaOH + H2O → Na2[Zn(OH)4]
- Zinc oxide further decomposes into zinc vapor and oxygen at a temperature which is around 1975*C with a standard oxygen pressure. In a carbothermic reaction, when carbon is heated it converts the oxide into zinc vapor at a much lower temperature which is around 950*C.
ZnO + C → Zn(vapor) + CO
Applications of Zinc Oxide
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- It might come as a shock but over 50% of the Zinc Oxide used is in the rubber industry.
- ZnO along with stearic acid is used in the vulcanization of rubber to produce such things as tires, shoe soles, and even hockey pucks.
- A very important use is that Zinc Oxide is widely used as the buffer layer in CIGS (Copper Indium Gallium Selenide) solar cells.Another main use in concrete manufacture. The addition of Zinc Oxide aids the processing of concrete and also improves water resistance.
- Zinc oxide also has antibacterial and deodorizing properties. For this reason it is employed in medical applications such as in baby powder and creams to treat conditions such as diaper rash, other skin irritations and even dandruff.
- Due to its reflective properties it is also used in sunblocks and can often be seen on the nose and lips of lifeguards at the beach.
- Zinc oxide is commonly found in the paint and pigment industry as it helps in providing anti-corrosive coating for different metals.
- Zinc oxide rich minerals increase crop productivity and life. The rich nutrients of zinc are used for animal feeds as they form an integral part of the enzyme system of the body.
- More than half of the zinc oxide used is in the rubber industry. Along with stearic acid, it is used in the vulcanization of rubber to produce tires, shoe soles, and even hockey pucks.
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Things to Remember
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- The most commonly found ore of zinc is zinc blende which is further a mineral of zinc sulphide. Other ores which can be found are calamine, zincite etc.
- Concentration of zinc sulphide takes place by the froth flotation method which is based on the principle of wetting ability of ore as well as impurities.
- Extraction of zinc metal from its ore takes place in three steps: concentration of ore, isolation of metal from the concentrate and purification of metal.
- Zinc should be consumed in an enough amount and it plays a major role in maintaining the human body.
- Zinc is majorly used in galvanizing iron and steel which further prevents iron from rusting. The galvanized iron is used in manufacturing of vehicles.
Sample Questions
Ques: Out of C and CO, which is considered as the better reducing agent for ZnO? (3 Marks)
Ans: The two reducing reactions are:
ZnO(s) + C(s) → Zn(s) + CO(g)
ZnO(s) + CO(g) → Zn(s) + CO2(g)
In the first reaction, there is an increase in the magnitude of ΔS* while in the second reaction, it remains the same. In simple words, ΔG* will have more negative value in the first reaction when C(s) is used as a reducing agent but in the second reaction, CO(g) is used as the reducing agent. Hence, C(s) is the better reducing agent.
Ques: Why is zinc not extracted from zinc oxide through reduction using CO? (2 Marks)
Ans: The reaction which is involved in reduction of ZnO by CO is:
ZnO(s) + CO(g) → Zn(s) + CO2(g)
The process is not achievable thermodynamically because there is hardly any change in the entropy as a result of the reaction. This is quite noticeable from the physical states of the reactants and products involved in the reaction.
Ques: How is zinc extracted from its ore? (5 Marks)
Ans: Zinc is obtained from its ore from the following given steps:
- Concentration: The ore is crushed into a fine powder and further converted into a slurry mixture after adding water. Collectors and froth stabilizers are the main ingredients in this process. The concentration of ore mainly depends upon the wetting ability of ore and impurities.
- Roasting: In this process, concentrated ore is roasted in the presence of air which results into the formation of the compound named as zinc oxide.
- Reduction: ZnO reduction takes place in the process of powdered cake.
- Purification: Electrolytic refining is used in the process mainly in which impure zinc is made anode and pure zinc is made the cathode. The electrolytic solution formed is ZnSo4 in which a small quantity of dilute sulphuric acid is added. When we pass electricity, pure zinc is deposited on the cathode.
Ques: What are the different alloys which are formed by zinc metal? (3 Marks)
Ans: Some alloys which are formed from zinc metal are as follows:
- Zamak: This alloy is a mixture of zinc as a base metal with aluminium, magnesium and copper.
- Tombak: It is a brass alloy which consists of a huge amount of copper and a low amount of zinc.
- Zinag Alloy: This alloy is formed by the combination of zinc, aluminium and silver.
- Nickel Silver: This alloy consists of nickel, copper and zinc but doesn’t consist of silver.
- Lead-free Solder: This mainly consists of tin, copper, silver, bismuth, zinc, indium, antimony and some traces of other metals.
Ques: What are some applications of zinc oxide? (3 Marks)
Ans: Zinc oxide is used as a buffer layer in CIGS(Copper Indium Gallium Selenide) solar cells. It is also used in concrete manufacture. The addition of zinc oxide aids the processing of concrete and also improves water resistance. It also has some antibacterial and deodorizing properties. Due to this reason, it is employed in medical applications such as baby powder and creams to treat conditions such as diaper rash, other skin irritations and even dandruff.
Ques: How does distillation play an important role in the extraction of zinc? (3 Marks)
Ans: The process of distillation plays an important role in the extraction of zinc in the following ways:
- Zinc has a low boiling point which is usually purified by this method.
- Impure metal is heated above the boiling point so that it can form vapors which leave behind the impurities. Impurities don’t get vapourised with metals.
- The vapors formed by the pure metal are collected and cool down, after that it is condensed to form pure metal.
Ques: How Calcination plays an important role in the extraction of zinc metal? (3 Marks)
Ans: The process of calcination takes place only when zinc is present in carbonate form. In this process, zinc carbonate is strongly heated when air is absent which further leads to the formation of zinc oxide. The reaction involved in calcination is given as follows:
ZnCO3 → ZnO + CO2
The zinc oxide obtained in the above reaction is further reduced to get Zn metal. ZnO is made to react with carbon so that it can get reduced. It leads to the formation of the zinc metal:
ZnO + C → Zn + CO2
Ques: Write some advantages and disadvantages of the Froth floatation method. (5 Marks)
Ans: The advantages of froth flotation method is as follows:
- This method is applicable for almost all types of minerals.
- Surface reagents control and alter the surface properties.
- Sulphide ores get the best results from the method of Froth floatation.
The disadvantages of froth floatation method is as follows:
- One of the major drawbacks is that this process is highly complex and expensive.
- Slime affects the result in this process.
Ques: How crude zinc metal is isolated from the concentrated ore? (3 Marks)
Ans: The isolation of crude zinc metal takes place in the methods given below:
- Roasting: In this process, the compound called zinc sulphide is reduced into zinc oxide. The concentrated ore is ground into smaller pieces which are further suspended in the stream of air so that it reacts with oxygen which is present in the air. Following reaction takes place:
2ZnS + 3O2 → 2ZnO + 2SO2
- Reduction: The zinc oxide formed from the above reaction is reduced to form zinc. The zinc oxide is made to react with powdered coke now which results into below given reaction:
2ZnO + C → 2Zn + CO2
Ques: Explain the terms: Zone refining and column Chromatography. (5 Marks)
Ans: Zone refining: Zone refining is the method used for production of semiconductors and other metals of very high purity, for example Ge, Si, B, Ca and In. This method is based on the principle in which impurities are more soluble in the molten state than in the solid state of the metal. The impure metal which exists in the form of a bar is heated at one end with a moving circular heater. This process is repeated various times till the impurities are completely driven to one end of the rod which is then cut off and discarded.
Column Chromatography: Chromatography is based on the principle that different components of a mixture are absorbed to different extents on an absorbent. In column chromatography, an absorbent such as alumina (Al2O3) or silica gel is packed in a column. The component which is more strongly absorbed on the column takes much longer time to travel through the column than a component which is weakly absorbed.
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