TLDR;
This video provides a comprehensive revision of S-block elements for Class XI Semester One, covering Group 1 (Alkali Metals) and Group 2 (Alkaline Earth Metals), along with key aspects of hydrogen, hydrogen peroxide, and hydrides. It emphasizes electronic configurations, trends in physical and chemical properties, reactivity, and important compounds.
- Electronic configuration and oxidation states of Group 1 and Group 2 elements.
- Trends in atomic and ionic radii, ionization enthalpy, electronegativity, melting and boiling points, and density.
- Chemical properties including reactions with oxygen, water, hydrogen, and halogens.
- Flame test colors for Group 1 elements.
- Types of hydrides (ionic, covalent, metallic) and their properties.
- Structure, preparation, and reactions of hydrogen peroxide.
Intro [0:12]
The instructor, Rakhi Dutta, welcomes students to a revision session on S-block elements, focusing on Group One Alkali Metals and Group Two Alkaline Earth Metals, including hydrogen, hydrogen peroxide and hydrides. The session aims to provide a short live revision, covering key topics within the syllabus.
Syllabus Overview [1:22]
The importance of following the syllabus is emphasized, noting modifications such as the removal of the hydrogen chapter. The current focus includes Group One Alkali Metals and Group Two Alkaline Earth Metals, with specific attention to hydrogen peroxide and hydrides. Key areas within the S-block include electronic configuration, occurrence, trends in properties like ionization enthalpy, and chemical reactivity with oxygen, water, hydrogen, and halogens.
Group One: Alkali Metals [3:57]
Group One elements, including Lithium, Sodium, Potassium, Rubidium, and Cesium, are discussed. Their general electronic configuration is NS1, with one electron in the outermost shell, leading to a +1 oxidation state. The position of these elements in the periodic table is reviewed, noting their placement in consecutive periods. The electronic configuration is further detailed, explaining how to represent it using the nearest noble gas element in square brackets, followed by the configuration of the outermost cell.
Trends in Properties: Atomic and Ionic Radii [8:50]
As one moves down Group One, new electron shells are added, increasing the distance between the nucleus and the outermost cell, which increases the atomic radius. The ionic radius also increases down the group as electrons are lost to form cations. This general trend is important for understanding the character of the compounds formed.
Trends in Properties: Key Characteristics [14:15]
Several properties increase down Group One, including nuclear radius, ionic radius, ionic character, metal character, and flexibility. Ionization energy decreases due to the increasing distance between the nucleus and the outermost electron. Electronegativity decreases, while electropositive character increases. Melting and boiling points decrease down the group.
Density Trends and Anomalies [18:14]
Density generally increases down the group, but potassium is an exception. The atomic volume of potassium is higher than sodium, leading to a decrease in density. The density order is Lithium < Potassium < Sodium < Rubidium < Cesium. Lithium is the lightest metal and floats in kerosene, so it is wrapped in wax. Cesium remains liquid in the summer due to its low melting point of 302 Kelvin (28.5 degrees Celsius).
Oxidizing and Reducing Properties [34:03]
The tendency to oxidize increases down the group, making Cesium the most powerful reducing agent in the gaseous state. However, in aqueous solution, Lithium is the strongest oxidizing agent due to its high reduction potential. Sodium is the weakest reducing agent in aqueous solution, with the trend being Lithium > Potassium > Rubidium > Cesium > Sodium.
Hydration Enthalpy [21:58]
Hydration enthalpy is very important. Smaller cations have higher hydration levels because they can accommodate more water molecules. Hydrated radii decrease down the group, with Lithium having the highest hydrated radii. The order of hydration energy is Lithium > Sodium > Potassium > Rubidium > Cesium.
Flame Test [29:19]
Different alkali metals impart different colors to a flame. Lithium gives a dark red color, sodium gives a golden yellow color, potassium gives a light purple color, rubidium gives a reddish-purple color, and cesium gives a purple color.
Reactions with Oxygen [47:31]
Lithium reacts with oxygen to form monoxide (Li2O), sodium forms peroxide (Na2O2), and the remaining members (Potassium, Rubidium, Cesium) form superoxide (MO2). The oxidation state of oxygen in monoxide is -2, in peroxide is -1, and in superoxide is -0.5.
Reactions with Hydrogen [52:50]
Alkali metals react with hydrogen to form metal hydrides (MH), where hydrogen has a -1 oxidation state. Lithium hydride is covalent, while the ionic character increases down the group. Thermal stability and ionic character increase down the group.
Reactions with Halogens [1:01:21]
Alkali metals react with halogens to form halides (MX). Covalent character increases as the size of the anion increases. The smaller the cation, the more polarizing it becomes, and the more covalent character there will be.
Reactions with Water and Liquid Ammonia [1:02:57]
Reactivity with water increases down the group, with the order being Lithium < Sodium < Potassium < Rubidium < Cesium. Alkali metals dissolve in liquid ammonia to form blue solutions that are strong reducing agents and paramagnetic. Concentrated solutions are copper-bronze colored and diamagnetic. Lithium forms lithium imide (Li2NH), while other alkali metals form amides (MNH2).
Hydroxides, Carbonates, and Nitrates [1:08:15]
Oxides react with water to form hydroxides. The alkalinity of hydroxides increases down the group. Thermal stability and solubility of carbonates and nitrates increase down the group. Lithium carbonate is insoluble in water and decomposes to lithium oxide and carbon dioxide upon heating.
Hydrides: Classification and Properties [1:18:52]
Hydrides are classified into ionic (saline), covalent (molecular), and metallic (interstitial). Ionic hydrides are formed by S-block elements, covalent hydrides by P-block elements, and metallic hydrides by D and F-block elements. Covalent hydrides are further divided into electron-deficient, electron-precise, and electron-rich.
Hydrogen Peroxide: Preparation and Structure [1:38:15]
Hydrogen peroxide (H2O2) can be prepared in the laboratory using barium peroxide and dilute sulfuric acid, or industrially using 2-ethyl anthraquinone. The structure of H2O2 is non-planar and open book-like, with an angle of 111.5 degrees in the gaseous state and 90.2 degrees in the solid state.
Hydrogen Peroxide: Oxidizing and Reducing Properties [1:48:35]
H2O2 acts as both an oxidizing and reducing agent. It undergoes disproportionation, where it is simultaneously oxidized and reduced. It can convert ferrous sulfate to ferric sulfate and is used as a bleaching agent due to its oxidizing properties.
Group Two: Alkaline Earth Metals [2:07:30]
Group Two elements include Beryllium, Magnesium, Calcium, Strontium, and Barium. Their general electronic configuration is NS2, with two electrons in the outermost shell, leading to a +2 oxidation state.
Group Two: Trends in Properties [2:08:41]
Atomic and ionic radii, ionic character, metal character, and flexibility increase down the group. Ionization energy and electronegativity decrease down the group. Density increases from Beryllium to Calcium, then decreases to Strontium, and increases again to Barium.
Group Two: Flame Test and Chemical Reactivity [2:11:40]
Beryllium and magnesium do not impart color to the flame due to their high ionization enthalpy. Calcium gives a brick red color, strontium gives a crimson red color, and barium gives a green apple color. They react with oxygen and nitrogen to form oxides and nitrides. Beryllium oxide is amphoteric, while the alkalinity of other oxides increases down the group.
Group Two: Hydroxides and Halides [2:16:02]
They react with water to form hydroxides. Beryllium hydroxide is amphoteric, while the alkalinity of other hydroxides increases down the group. The tendency of halides to form hydrates increases down the group. Anhydrous calcium chloride is used as a dehydrating agent.
Group Two: Beryllium Chloride Structure [2:21:48]
Beryllium chloride (BeCl2) exists as a monomer in the vapor state at high temperatures (1200 K or more) and as a dimer in the vapor state at lower temperatures. In the solid state, it exists as a polymer. The dimer structure involves coordinate bonds, with chlorine atoms donating electron pairs to beryllium atoms.
Group Two: Carbonates and Sulfates [2:27:07]
They form carbonates (MCO3) and sulfates (MSO4). Thermal stability of carbonates increases down the group, while solubility decreases. Beryllium and magnesium sulfates are soluble, while calcium, strontium, and barium sulfates are poorly soluble or insoluble.
Volumetric Strength of H2O2 [2:30:27]
The volumetric strength of H2O2 is defined as the volume of oxygen produced at STP by the complete dissociation of one volume of H2O2 solution. The calculation starts from the formula that 68 grams of H2O2 produces 22400 ml of O2 at STP. The relationship between volume strength, percentage strength, molarity, and normality is discussed.