Group VII Elements
Introduction
Group VII elements are called halogens. The word halogen mens “Salt Former” because a Group VII element forms salts when it reacts with metals. These halogens are found in Group VII, or from the 2nd column from the right in the Periodic table. The members of group VII are Fluorine (F), chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At). These halogens are reactive non-metals and always exist in the compound form with other elements. In this article, we discuss trends, displacement reactions, extraction of Chlorine through electrolysis of Brine, as well as extraction of Bromine, in addition to the physical and chemical properties of group VII elements.
Trends in Group VII
Trends are the possible predictions of the elements based on the properties of the other elements. One trend would be the colour of the halogens. Going down the group, the colour darkens. For example, iodine is below bromine in group VII so it will have a darker colour than Iodine because the color tends to get darker when we move down the group. So bromine is reddish brown and iodine is black.
Properties of Group VII elements
The halogens exist as simple molecules. Each molecule has 2 atoms joined by a single covalent bond. Halogens exhibit similar physical properties. They share the same chemical properties.
Physical Trends
Color, solubility, state at room temperature, melting points, and boiling points form the physical properties of an element. These physical properties change from top to bottom of the Group.
Melting and boiling Points
Halogens typically have low melting and low boiling point. This is because they exist as simple molecules held together by weka intermolecular forces of attraction. The melting and boiling points of the halogens increase as we go down the group. Fluorine has the lowest melting and boiling point and Astatine has the highest melting and boiling point in the group. As we move down the group, molecules become larger, there is strong surface area of contact between molecules, resulting in stronger intermolecular forces of attraction between molecules, thus more energy is needed to overcome the forces.
State at room temperature
The state of these elements at room temperature is different from each other. The greater the molecular mass, and intermolecular forces, the higher the melting point, and hence they would change from being a gas for fluorine and chlorine, to a liquid for bromine and a solid for iodine and astatine.
Color
The color also tends to get darker as we move downward in the group. Fluorine is pale yellow-green, Chlorine is pale yellow, Bromine is brown, Iodine is purple and Astatine is black.
Solubility
Solubility in water
Fluorine reacts vigorously with water to give hydrogen fluoride gas or a mixture of hydrofluoric acid. Chlorine, Bromine, and Iodine mix with water to some extent. The solubilities of Chlorine, Bromine, and Iodine in water are 0.091 mol dm– 3, 0.21 mol dm-3, and 0.0013 mol dm-3 respectively.
Chemical Trends
Reactivity of the group VII elements decrease when you go down the group. These elements are strong oxidizing agents and undergo halogen displacement reactions (redox reactions). Electronegativity decreases down the group.
Displacements reactions
When more reactive halogens displace a less reactive halogen in an aqueous solution of its halide, it is called a displacement reaction.
Example 1: If you add chlorine to a colorless potassium solution, bromine is formed and the color is changed to red.
Cl2(aq)+2KBr(aq)→ 2KCl(aq)+Br2(aq)
Example 2: If you add chlorine to an aqueous solution of Sodium, iodine is formed.
Cl2+2NaI(aq)→NaCl(aq)+I2(s)
Extraction of Chlorine
Industrial electrolysis of Brine
Electrolysis is a process in which electric current is used to bring chemical change for the formation of new chemical products. Electrolysis of Brine (seawater with a higher concentration of NaCl) is a large scale process that manufactures chlorine from salt. Sodium hydroxide (NaOH) and H2 are other byproducts of this procedure. Electrolysis of NaCl is an important industrial process.
What is Brine?
Brine is a solution of Sodium chloride (NaCl) and water (H2O). We use the electrolysis process of brine to extract chlorine. Along with chlorine, Sodium hydroxide is also obtained. It is important to separate both the products because they both are highly reactive with each other.
Procedure of Brine
The process involves the passing of electric current from salt (NaCl) and water (H2O). Two charged electrodes are immersed in salt and water solution. Then the current is passed through them. When the power supply is switched on, the bubbles appear around both the electrodes which is a sign that gases are being produced. For the collection of gas, the test tube filled with brine, or concentrated sodium chloride (NaCl) is placed on each electrode. Gases displace the solution, and start collecting at the top of the tube. What gases are they? To determine each gas, a simple test is done.
These gases have different colours. To determine the obtained gas, the gas collected at the positive electrode is checked using blue Litmus paper. Litmus paper is quickly bleached and it turns red. This shows the obtained gas at positive electrode is Chlorine. The gas collected at negative electrode is tested by a lightning wooden splint. It pops indicating hydrogen gas.
The negative and positive charges in the compounds roam freely into the mixture of NaCl and H2O. When an electric current is passed, the positive ions of Hydrogen in H20 are attracted to the negative electrode, and negativeions of chlorine are attracted into the positive electrode. They both lose their charges and transform into gases leaving behind sodium and hydroxide ions which combine to form NaOH. These left behind NaOH is collected and reused for industrial purposes.
In old days, mercury was used to separate NaOH and Chlorine but it used high power to bring about the purification process. It was also toxic and it needed to be separated from the brine which required costly procedures. However, using membrane was a better substitute found for purification as it was not toxic and had low maintenance cost.
Extraction of Bromine
Extraction of bromine from seawater includes the following process.
Oxidation of Bromide ions into Bromine
Cl2(aq)+H2O(aq)→HCl(aq)+HClO(aq)
A small amount of chlorine is added to the acidified water to displace bromine by a redox reaction.
Cl2(aq)+2Br– (aq)→2Cl–(aq) + Br2(aq)
Half Equation:
2Br–(aq)→Br(2)+2e:
Cl2(aq)+2e→2Cl– (aq)
Removal of Bromine vapors
Treated bromine water then passes into the blowing out tower. Bromine is highly volatile, which is removed from the air. The bromine vapors are then condensed to produce bromine liquid.
Reduction of Bromine into hydrobromic Acid
Sulfur dioxide is added to the blowing out tower. Bromine is reduced to bromic acid in the presence of sulphuric acid.
Br2 (aq) + SO2 (g) + 2H2O (l) → 2HBr (aq) + H2SO4 (aq)
Then condensation occurs in the tower. The mixture now contains 13% of bromine.
Oxidation of Hydrobromic acid to Bromine.
The acidic solution now passes into a steaming out tower.
Chlorine regenerates the bromine using redox reactions.
2HBr(aq)+Cl2(g) → Br2(g)+2HCl(aq)
Chlorine oxidizes the bromide in HBr into Bromine.
Hot vapor is condensed to form an aqueous layer and a lower layer of bromine.
Spent seawater is released and the remaining Br2 and Cl2 are destroyed suing Sulphur dioxide.
