Chemical Reactivity

Q: Write som properties of metals & position of metals in periodic table?

ans:

metals

Metals are the elements (except hydrogen) which are electropositive and form cations by losing electrons. Metals can be categorized.

We can categorize metals by

  1. Very reactive: potassium, sodium, calcium, magnesium and aluminium.
  2. Moderately reactive : zinc, iron, tin and lead.
  3. Least reactive or noble : copper, mercury, silver and gold.

 

Q: Give some important physical and chemical character of metals?

Some interesting facts about metals

The most abundant metal is aluminum

The most precious metal is platinum

The most useable metal is iron

The most reactive metal is cesium

The most valuable metal is uranium

The lightest metal is lithium (d = 0.53 g cm-3)

The heaviest metal is osmium (d = 22.5 g cm-3)

The least conductor of heat is lead.

The best conductor metals are silver and gold

The most ductile and malleable metals are gold and silver

 

 

Important physical characteristics of metals

  1. Almost all metals are solids (except mercury)
  2. They have high melting and boiling points, (except alkali metals)
  3. They possess metallic luster and can be polished.
  4. They are malleable (can be hammered into sheets), ductile (can be drawn into wires) and give off a tone when hit.
  5. They are good conductors of heat and electricity.
  6. They have high densities.
  7. They are hard (except sodium and potassium).

Important chemical properties of metals are:

  1. They easily lose electrons and form positive ions.
  2. They readily react with oxygen to form basic oxides.
  3. They usually form ionic compounds with non-metals.
  4. They have metallic bonding.

Q: what you know about the electropositive character of metals?

ans:

Electropositive Character

Chemically a metal has the ability to lose valence electrons to form a metal cation.

The tendency of a metal to lose electrons is called electropositivity.

where M stands for any metal.

Trends in electropositive

  • Metals have a large atomic size and low ionization energies.
  • As ionization energy decreases down the group, the electropositivity increases.
  • Thus sodium is more electropositive than lithium. Similarly, magnesium is more electropositive than beryllium. On the other hand, the tendency to lose electron decreases as we move from left to right in a period.
  • In the second period Li and Be are metals, B is metalloid, whereas C, N, O, F and Ne are non-metals. Also Be is less metallic than Li.
  • These trends in electropositivity are reflected in the chemical reactivity of metals. In a group the reactivity of metals with oxygen increases. For instance, alkali metals on exposure to air show an increase in affinity for oxygen as we move down the group.

Lithium forms normal oxide. (oxidation # -2)

Sodium forms per oxide. (oxidation # -1)

K, Rb and Cs form super oxide.(oxidation # -1/2)

Trend of metals reactivity in the periodic table

  • In periods as we move from left to right, reactivity of metals with oxygen decreases. For instance, in the third period sodium readily reacts with oxygen to form sodium peroxide, at room temperature.
  • Mg reacts with oxygen only on ignition.
  • Al reacts with oxygen on heating. It is superficially oxidized at room temperature to form Al2O3 layer which serves as a protective covering that prevents further reaction.

Q: How we can understand electropositivity of metals from nature of oxides? 

Ans:

Trends in electropositivity from the nature of the oxides.

  • Metal oxides are generally basic in character. This is because they produce bases in water.
  • Non-metallic oxides are generally acidic since they produce acids in water.
  • The basic character of oxides decreases as we move along a period. This indicates decrease in metallic character. For example, elements of the third period form oxides such as Na2O, MgO,Al2O3, SiO2, P4O10, SO2, Cl2O7. These oxides turn from strongly basic through weakly basic, amphoteric, weakly acidic to strongly acidic. See figure 8.1 to understand this trend.
Group IA IIA IIIA IVA VA VIA VIIA
3rd period Na Mg Al Si P S Cl
Oxide Na2O MgO Al2O3 SiO2 P4O10 SO2 Cl2O7
Nature of oxide Strongly basic Basic Amphotric (both acidic and basic) Weakly acidic Acidic Strongly

Acidic

Very strongly acidic

 Nature of oxides of elements in the period No. 3

  • On the other hand basic character of metal oxides of main group elements increases down the group. For example, in group IIA, the basic character of oxides increases in the following order.

BeO < MgO < CaO < SrO < BaO

Example 8.1: Identifying the more metallic element in a pair of elements.

In each of the following pairs of elements, identify which element is more metallic.

  • Na, K (b)        Be, Na

Solution:

  • K is more metallic
  • Na is more metallic
SELF assessment EXERCISE 8.1
  1. In each of the following pairs identify, which element is more metallic?
  • Be, B (b) Si, Al         (c) K, Li

Ans (a): Be is more metallic              Ans (b): Alis more metallic

Ans(c): K is more metallic

  1. Using the periodic table, rank each set of elements in order decreasing metallic character
  • Na, Li, K (b)        Al, Na, Mg

Ans (a): K> Na>Li                      Ans(b) Na>Mg>Al

  1. Which is more basic Li2O or Na2O?

Ans: Na2O is more basic because basic character increase down the group

Q: How can you compare reactivity of alkali and alkaline earth metals?

Ans:

  • Comparison of reactivity of alkali and alkaline earth metals

To make a comparison of reactivity of alkali and alkaline earth metals. We need to know their position in the periodic table and their relative tendencies to lose electrons.

Alkali metals Alkaline earth metals
·         The group IA elements except hydrogen are known as alkali metals.

·         These include lithium, sodium, potassium, rubidium, cesium and francium.

·         These elements have general electron configuration ns1 in their valence shell.

·         They produce alkaline solutions with water

4Na + 2Na2O ——–à 2Na2O

Na2O  + H2O ——–à 2NaOH

·         The Group IIA elements are known as alkaline earth metals.

·         These include beryllium, magnesium, calcium, strontium, barium and radium.

·          They have general electron configuration ns2 in their valence shell.

 

 

SELF Assessment EXERCISE 8.2

Identify the position of Mg and Ca in the periodic table.

Ans:

Mg Ca
Group# II-A       Period# 3 Group# II-A       Period# 4
Valence Electronic configuration ns2 Valence Electronic configuration ns2
Ionization Energy of alkali and alkaline earth metals
Li

520

Be

899

Na

495

Mg

738

K

419

Ca

590

Rb

403

Sr

549

Cs

376

Ba

503

 

Q:  What you know about ionization energies of alkali and alkaline earth metals?

 Ionization energies of alkali and alkaline earth metals

  • Minimum amount of energy required to remove an electron from outermost shell of an isolated gaseous atom is called ionization energy. Its unit is KJ/mol
  • Because alkali metals have low ionization energies, therefore have a great tendency to lose the single valence electron to form cations.
  • They are so reactive that they are never found in the free state.
  • The alkali metals show increased reactivity down the group. This is because of decrease in ionization energies down the group.

Ex:  Na is more reactive than Li, but less reactive than K. They are all so reactive that they are kept under a liquid such as kerosene oil.

  • Alkaline earth metals also have low ionization energies, so they also have great tendency to lose both the valence electrons to form dispositive cations. They are less reactive than alkali metals.
  • These metals also show increased reactivity down the group.

Ex:Thus Mg is more reactive than Be but less reactive than Ca. Like alkali metals, they are also kept under a liquid such as kerosene oil to prevent contact with moist air.

Q: Write properties and uses of some imortant alkali and alkaline earth metals?

 Ans:

Some important alkali and alkaline earth metals

Sodium (Na):

The most important alkali metal is sodium.

  • Sodium is a soft metal that can be cut with a knife.
  • It melts at 97.8oC and boils at 881.4o

The reason is that in iron, metal atoms are tightly packed due to their small size. They are held through strong metallic bonds due to many electrons in the valence shell. Whereas sodium consists of bigger atoms. Because of large size and only one electron, sodium atoms have weak metallic bonds. Thus sodium is a soft metal.

  • Sodium is a silvery-white soft metal. It is an extremely reactive metal. It reacts with water violently to form sodium hydroxide and liberates hydrogen gas.

This reaction is highly exothermic and proceeds with light explosion. For this reason, it should not be picked with fingers.

  • In a limited supply of oxygen, sodium burns to form sodium oxide (Na2O). But in excess of oxygen it forms pale yellow solid sodium per oxide (Na2O2).
Strontium Use

Strontium-90, a radioactive isotope, is a major product of an atomic bomb explosion. Constant exposure of the body to high energy radiations emitted by the Sr-90 can lead to anaemia, leukemia and other chronic illnesses.

USES

  • Sodium mixed with potassium forms a liquid alloy that is used as a coolant in nuclear reactors. Sodium is also used in sodium lamps to illuminate highways. The petroleum industry utilizes sodium in the production of antiknock compounds for gasoline.

 Magnesium & Calcium

  • Magnesium and calcium are the most common alkaline earth metals.
  • Metallic bonding in these metals involve two electrons. Therefore these metals are relatively soft but are much harder than alkali metals.
  • They are relatively reactive but less reactive than alkali metals. Magnesium reacts with steam but calcium reacts with cold water .

Mg(s)+H2O(g)             MgO+H2(g)

Ca(s)+2H2O(l)             Ca(OH)2+H2(g)

Magnesium reacts with oxygen at high temperature whereas calcium reacts with oxygen at room temperature.

  • Hence Ca is more reactive than Mg.

 USES OF MAGNESIUM

Magnesium has relatively low density and it is used in making light weight alloy with aluminum  that are used for making frames of automobiles , air craft and spaceship, cameras etc. Since magnesium burns brilliantly, it is used in photo flash guns. Mg(OH)2 called milk of magnesia  is commonly used  as antacid.

USES OF CALCIUM   

      

Calcium is an important component of teeth and bones. Calcium ions are found in all living cells. They are involved in the clotting of blood. A proper Ca+2 and K+ ions balance is required for normal heart function.

  1. Describe the inertness of noble metals?

Ans:

inertness of noble METALS

The chemistry of metals is characterized by their ability to lose electrons to form cations. Some metals such as copper, silver, gold and platinum are relatively difficult to oxidize. Therefore, these metals are often called noble metals.

  • Gold and platinum exist mostly as free elements in nature.
  • Copper and silver exist in both free and combined states.
  • All active metals react with HCl but noble metals do not react with HCl.
  • Copper and silver react with strong oxidizing agents such as conc. HNO3 and HClO4.
  • Gold and platinum react only with aqua regia. Aqua regia is a mixture of 3 parts by volume of conc HCl and one part by volume of conc HNO3.
  1. WHAT IS CARAT, WHICH METALIS USED AS A STANDARD FROM WORLD CURRENCY

ANS

Copper, silver and gold have been used for thousand of years to make jewelry and coins. These metals have beautiful high luster and malleability. Pure gold is too soft to be used alone for making jewelry. It is alloyed with or copper to make it harder. A Carat system is used to express purity of gold. Pure gold is 24 Carat. Coinage gold is 22 Carat or = 92% gold. Copper is a common coinage metal though modern pennies are only copper plated. In Pakistan coins are made from Cu-Ni alloy.

Do You Know?

Rolled gold is a thin layer of gold alloy that is bonded onto brass or nickel-silver alloy.

  1. What are the uses of gold, platinum and silver?

Ans:

uses of gold:

  • It used for making ornaments. For this purpose it is alloyed with water. Its purity is counted in CARAT. Pure gold is of 24 carat.
  • It is used in photography.
  • It is used as standard for world currency
  • . Gold alloys are extensively used in dentistry.
  • Gold have useful therapeutic applications. For example salts of gold are used to treat rheumatoid arthritis.

USES OF SILVER

  • Silver have been admired for thousands of years and are still used in jewelry and fine flatware and ornaments.
  • It is used ofr making coins from ancient times.
  • Compounds are used for photographic and X-ray films.
  • Used in medicines
  • Used for making utensils

USES OF PLATINUM

  • Platinum is widely used as catalyst for many types of industrial processes. For example, 100% pure sulphuric acid is prepared by the contact process. In this process platinum is used as catalyst.
  • Platinum is used as electrode as a part of hydrogen electrode and in fuel cells.
  • Automobile exhaust is a major source of air pollution. Therefore, most new cars are equipped with catalytic converters. These converters contain platinum. These converters catalyze the complete combustion of CO and hydrocarbons.
  • A platinum compound called cis-platin is useful as an anti cancer agent.
  1. What are non metals, metalloids?how they are electronegative in nature, also describe their trends in periodic table?

Ans:

 Non-Metals

            Elements that conduct heat and electricity poorly are called non-metals.

Non-metals are generally gases or dull, brittle solids at room temperature except bromine which is liquid.

Metalloids

Several elements near the stepped line have mixed properties of metals and non-metals. These elements are called metalloids.

  • Sme of these play important role in modern electronic materials. For example, silicon and germanium are metalloids. These elements are used in the manufacture of computer chips and solar cells.

 

Q: Write ome important physical properties of non-metals?

Ans:

Important physical properties of non-metals are as follows:

Physical properties of non-metals change gradually but uniquely in a group of non-metals. Nonmetals usually exist in all three physical states of matter. The non-metals at the top of the group are usually gases while others are either liquids or solids.

  1. Solids non-metals are brittle (break easily).
  2. Non-metals are bad conductors of heat and electricity (except graphite).
  3. They are not shiny, they are dull except iodine (it is lustrous like metals).
  4. They are generally soft (except diamond).
  5. They have low melting and boiling points (except silicon, graphite and diamond).
  6. They have low densities.

 

Q: What is position of non-metals in periodic table, how their trend change in periodic table?

Ans:

POSITION IN PERIODIC TABLE

 Non-metals appear in the upper right portion of the periodic table except hydrogen.

 8.2.1 Electronegative character

Silicon in integrated circuit

Computers exist because of an amazing invention called the integrated circuit known as the silicon chip or just chip. Chips are microprocessors which can perform all the function of a computer. These are so small that one will fit on the tip of your finger. A chip is made from a very thin flake of an element called silicon. Specific areas of the chip are cut or etched, in order to make a pattern called circuit. This is a kind of pattern along which electric current flows. Chips contain millions of working parts that are so small that they can be seen only with a microscope.

  • Electronegative character is the reverse of electropositive character.
  • It is the tendency to gain electrons to form anions.
  • As you move from left to right across a period, the tendency to gain electrons increases.

This is due to increase in nuclear charge and decrease in atomic size, which binds the extra electron more tightly to the nucleus.

  • But in a group going from top to bottom, electronegative character decreases, due to increase in atomic size,
  • Halogens are the most electronegative elements in their respective periods.
  • Non-metal oxides are generally acidic since they yield acids in water. Acidic strength of these oxides increases from left to right in a period .
Example : Identifying the more non-metallic element in a pair of elements.

In each of the following pairs of elements, identify which element is more non-metallic.

  • N or O (b) F or Cl

Solution:

  • O is more non-metallic
  • F is more non-metallic
SELF assessment EXERCISE 8.3
  1. In each of the following pairs, identify which element is less non-metallic.
  • B or C             B is less no n-metallic
  • C or Si Si is less non-metallic
  • Cl or Br Br is less non-metallic
  1. Using periodic table rank each set of elements in order of increasing non-metallic character.
  • N, F, O                                             N<O<F, non metallic characters increases in periods
  • Cl, Br, I                                            I<Br<Cl non-metallic character decrease in group
  • Si, S, P Si<P<S non-metallic character increases in periods

Q: What are halogens, compare the reactivity of halogens?

Ans:

            Comparison of Reactivity of the Halogens

            The elements in group VII-A are called halogens. The name halogen is derived from the Greek words “halous” meaning salt and “gen” meaning former.

  • Halogens include fluorine, chlorine, bromine, iodine and astatine (astatine is a radio active element. Little is known about its properties.)
  • All halogens are reactive non-metals.
  • They all exist as diatomic molecular substances.
  • All the halogens react with metals to form salts called halides. In these reactions metals are oxidized. So, halogens act as oxidizing agents.
  • Fluorine is the most reactive element known.
  • Chlorine is less reactive, but combines vigorously with many metals. For instance sodium metal burns in chlorine gas to form sodium chloride.
  • Bromine and iodine react with metals less vigorously. The reactivity of halogens decrease down the group. Thus, the order of decreasing power oxidizing

is F2 > Cl2 > Br2 > I2.

Hydrogen Halides:

The halogens react with hydrogen to form hydrogen halides.

Where X= F, Cl, Br, I

  • Reaction of fluorine with hydrogen is explosive. The hydrogen halides dissolve in water to form hydrohalic acid e.g.,
  • hydrochloric acid HCl,
  • hydrofluoric acid HF etc. Except HF, other hydrohalic are strong acids.
  • The acidic strength increases in the following order:

HF <  HCl <  HBr < HI

Oxidizing power of halogens:

Oxidizing power of F2 is the highest and that of I2 is lowest. Due to the relative strength as oxidizing agent, it is possible for a free halogen to oxidize the ion of halogen next to it in the group. This means F2 can oxidize all the halide ions to free halogen. For example,

Similarly Cl2 can oxidize Br and I ions. But I2 can not oxidize any halide ion.

SELF assessment EXERCISE 8.4

Which of the following displacement reactions will occur?

Ans:

  1. Cl2 cannot displace F
  2. Br2 can displace I
  3. I2 cannot displace Br
  4. Cl2 can displace Br
  5. Cl2 can displace I

Q.1:     Encircle the correct answer:

(i)         Which of the following oxides is the most basic oxide?

  1. MgO
  2. BaO
  3. CaO
  4. SrO

(ii)        Which of the following oxides is amphoteric oxide?

  1. Na2O
  2. Al2O3
  3. MgO
  4. SO2

(iii)      Which of the following elements is most metallic?

  1. Al
  2. Na
  3. Mg
  4. Li

(iv)       Valence shell electronic configuration of alkaline earth metals is:

  1. ns1
  2. ns2np1
  3. ns2
  4. ns2np6

(v)        Which of the following metals is not noble metal.

  1. Au
  2. Cu
  3. Hg
  4. Ag

(vi)       Which of the following elements is the least non-metallic?

  1. Si
  2. S
  3. P
  4. Cl

(vii)      Oxidizing power of _______ is the highest.

  1. I2
  2. Cl2
  3. Br2
  4. F2

(viii)    Chlorine cannot oxidize _______ ion.

  1. Iodide
  2. Flouride
  3. Bromide
  4. Astatine

(ix)       Na is more reactive than Li, but less reactive than

  1. Mg
  2. Al
  3. Rb
  4. All of these

(x)        Which of the following is the strongest acid?

  1. HI
  2. HCl
  3. HF
  4. HBr

 

ANSWER KEY

i. B ii. B iii. A iv. C v. C
vi. A vii. D viii. B ix. C x. A

 

 

 

 

 

 

 

 

 

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