CHAPTER-08

CHEMISTRY SSC-I                               CH:08

chemical reactivity                      LEC# 01

metals

  • Metals occupy left and lower three-quarters of the periodic table.
  • They are typically shiny solids, have moderate to high melting points and are good conductor of heat and electricity.
  • They tend to lose electrons in chemical reactions.                

Electropositive Character

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

where M stands for any metal.

  • Metals have 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.
  • Tthe tendency to lose electron decreases as we move from left to right in a period.
  •  For instance 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 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.

Sodium forms per oxide.

K, Rb and Cs form super oxide.

  • 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.

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
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  • The 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

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

  • Na, K                       (b)           Be, Na

Hint:

Apply the trends of increasing electropositive character down a group and decreasing electropositive character across a period. Check the periodic table to see which elements is more metallic.

Solution:

  • K is more metallic
  • Na is more metallic
Text Box: SELF ASSESSMENT EXERCISE 8.1
  1. In each of the following pairs identify, which element is more metallic?
  2. Be, B               (b) Si, Al (c) K, Li
  3. Using the periodic table, rank each set of elements in order decreasing metallic character
  4. Na, Li, K                                                          (b)           Al, Na, Mg
  5. Which is more basic Li2O or Na2O?

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.

 position of alkali metals and alkaline earth metals in the period table

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  • 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.
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  • 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.

HEMISTRY SSC-I                                 CH:08

chemical reactivity                      LEC# 02

Ionization energies of alkali and alkaline earth metals

  • 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. This means 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. Think Why? These metals also show increased reactivity down the group. 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.

Some important alkali and alkaline earth metals

Sodium (Na): The most important alkali metal is sodium.

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

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 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

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.

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

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.

Importance of silver,  gold and platinum

Silver and gold have been admired for thousands of years and are still used in jewelry and fine flatware and ornaments. These metals have beautiful luster and high malleability, therefore can be transformed easily into different shapes. However both are too soft to be used for these purposes. Both are alloyed with copper to make them harder. In former times gold and silver were used in coins But now-a-days copper-nickel alloys are universally used for making coins. Silver is an excellent conductor of both electricity and heat. It is used for making utensils. You might have had the painful experience of using a silver spoon to take soup. The light sensitive silver halides are used in photographic processes. Gold alloys are extensively used in dentistry. Some compounds of gold have useful therapeutic applications. For example salts of gold are used to treat rheumatoid arthritis.

Text Box: Do You Know?
Rolled gold is a thin layer of gold alloy that is bonded onto brass or nickel-silver alloy.

                Platinum is widely used ascatalyst for many types of industrial processes. For example, 100% puresulphuric acid is prepared by the contact process. In this process platinum isused as catalyst. Platinum is used as electrode as a part of hydrogen electrodeand 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. Theseconverters catalyze the complete combustion of CO and hydrocarbons. A platinumcompound called cis-platin is useful as an anti cancer agent.

EXERCISE

Give short answers

  1. In a group the reactivity of metals with oxygen icreases? Give example.

Ans: Reactivity of metals increase from top to bottom with oxygen and they form following oxides with oxygen.

 Lithium forms normal oxide.

Sodium forms per oxide.

K, Rb and Cs form super oxide.

  • Which element is more metallic Mg or Al? Explain.
  • Ans: The tendency to lose electron decreases as we move from left to right in a periodi.e metallic character decrease from left to right in perids so Mg is more metallic than Al.
  • What is the importance of Mg?

[See Lecture 2]

  • Arrange the following in order of increasing acidic strength HF, HI, HBr, HCl?
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  1. Ans: The acidic strength increases in the following order:

HF <  HCl <  HBr < HI

  • Can F2 oxidize all the halides ions to free halogen?

Ans: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,

Q.3:        Arrange the following oxides in order of decreasing basic character.

                BeO, CaO, MgO, SrO

Ans: SrO>CaO>MgO>BeO

Q.4:        Rank the each set of elements in order of increasing metallic character.

(a)           Al, Na, Mg              (b)           Na, Li, K

Ans          Al<Mg<Na                            Li<Na<K                

Q.5:        Which of the following displacement reaction will not occur? Give reason.

  • Will not occur because F has more oxidation power than Cl so Cl cannot displace F
  • It will take place
  • It will also not take place because I cannot replace Br due to less oxidation power than Br.

Q.6:        Give some important applications of platinum.

Ans: 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.

Q.7:        Identify the position of potassium and calcium in the periodic table.

Ans: Potassium belong to Group I-A period (4th )and is an alkali metal.

Calcium Belong to Group II-A period (4th ) and is an alkaline earth metal

Q.8:        What is aqua regia?

 Aqua regia is a mixture of 3 parts by volume of concHCl and one part by volume of conc HNO3.it is also knwn as royal water

Thinktank

1:            Why it is advisable, not to pick sodium metal with fingers.

Ans: Pure sodium metal is extremely hazardous. It is very sensitive even to minor moisture content. Our air contains enough moisture (water) in it to react with the sodium metal. Sodium metal reacts violently with water and can explode. Even our hands contain some minor moisture that sodium can react with thus burning our hands

2:            Arrange the following elements in order of increasing non-metallic character.    Si, Al, P.

Ans:        Al<Si<P

 (Non metallic character increase from left to right)

3: Design an experiment to show that iron is more reactive than copper.

4:            Design an experiment to differentiate between Cl and I.

5:          An iron bar is to be silver plated which metal would be the anode and which the cathode?