The halogens for AQA A-level Chemistry
This page covers the following topics:
1. Melting and boiling points of halogens
2. Electronegativity of halogens
3. Oxidising and reducing abilities of halogens
4. Uses of halogens
5. Solubility of silver halides
Halogens form small covalent molecules with the formula X₂ held together by van der Waals forces. Larger molecules can have stronger van der Waals interactions, increasing the energy needed to separate them from other molecules. Therefore, the melting and boiling points of halogens increase with an increasing atomic radius and the size of molecules as you travel down the group 7 of the periodic table. Due to different melting and boiling points, fluorine and chlorine at room temperature are gases, bromine - liquid and iodine - solid.
Electronegativity is the ability of an element to attract a pair of shared electrons in a bond. For example, if an atom of a less electronegative element, carbon, were to form a bond with a more electronegative fluorine, electrons that participate in bonding between them would be mainly surrounding the fluorine atom.
Going down the group 7 atomic radius increases. The shared electrons in covalent bonds with larger atoms get progressively further away from the nucleus and the atoms have a weaker attraction to them. Since electronegativity is the ability of an element to attract a pair of shared electrons in a bond, going down the group 7 electronegativity decreases.
The neutral molecular halogens readily accept electrons, so they act as oxidising agents, taking electrons from other molecules. The negatively charged halide ions have spare electrons available, so they act as reducing agents, giving electrons to other molecules.
Oxidising agent is a particle that takes electrons from another molecule, oxidising it, while also being reduced itself. A reducing agent gives electrons to another molecule, reducing it, while also being oxidised itself. OIL RIG abbreviation presented in the image can help remember the definitions.
Going down the group 7 of the periodic table valence electrons are further away from the nucleus and are more easily removed to reduce another molecule. Thus, reducing ability of halide ions increases going down the group.
Going down the group 7 electrons from other particles are further away from the nucleus and have a weaker attraction to it. Thus, the oxidising ability of halogens increases going up the group. In fact, fluorine is such a strong oxidising agent that it can oxidise water.
A halogen can oxidise a less reactive halide. For example a reaction between bromide ions and chlorine would produce chloride ions and bromine:
Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂.
Fluorine is highly reactive, but is used to make teflon, a famously unreactive material. Teflon has a very high melting point thus is used for heat shields on spacecrafts and pans.
Chlorine compounds can kill bacteria. Thus, they are used in swimming pool cleaning products, bleach. Chlorine is also a part of PVC, a plastic polymer with many applications, from raincoats to indoor plumbing.
Chlorine and fluorine were used in the production of CFC refrigerant gases. It was discovered that they were reacting with the ozone layer that protects living organisms on Earth from harmful UV radiation and their production has been subsequently banned in many countries.
Bromine is toxic, thus has applications in insecticides. It may harm farm workers during their application or contaminate food produce if it is not washed properly.
Iodine is used in the synthesis of pharmaceuticals as well as in antiseptic solutions used to clean an area of incision.
The solubility of silver halides decreases going down the group 7 of the periodic table. A common experiment to identify halide ions is the addition of silver nitrate. By the characteristic solubility properties of silver halides and the colour of precipitate formed the halide ions can be identified. The addition of silver nitrate to a halide solution produces a corresponding silver halide. A corresponding ionic equation for such reaction is Ag⁺ + X⁻ → AgX.
Some silver halides react with diluted or concentrated ammonia to form a complex [Ag(NH₃)₂]X that is soluble in water, thus in some cases the precipitate dissolves and can be explained using equation AgX + 2NH₃ → [Ag(NH₃)₂]X. Silver chloride precipitate is white, silver bromide - pale yellow, silver iodide - yellow.
Chlorine and fluorine were used in the production of CFC refrigerant gases. Explain why their production was banned.
It was discovered that CFC refrigerant gases were reacting with the ozone layer that protects living organisms on Earth from harmful UV radiation and their production has been subsequently banned in many countries.
CFCs react with ozone layer that protects us from harmful UV radiation.
An F—Cl bond is formed and a diagram representing the distribution of electron density for it is shown in the image. Assign F and Cl to X and Y. Explain your answer
Fluorine has a greater electronegativity than chlorine, thus any shared electrons between fluorine and chlorine atoms are closer in a F—Cl bond to the fluorine atom.
X = F, Y = Cl, fluorine is more electronegative than chlorine.
Which halide is the strongest reducing agent: F⁻, Br⁻, Cl⁻ or I⁻?
Going down the group 7 of the periodic table electrons are further away from the nucleus and are more easily removed to reduce another molecule. Thus, reducing ability of halide ions increases going up the group. The strongest reducing agent out of the ions provided is F⁻.
Explain why during reduction reaction of a chlorine molecule becoming a chloride ion, electrons need to be considered.
Chlorine molecule is neutral, while chloride ion is negative. For the neutral atoms to change into negatively charged particles, they have to gain electrons.
Neutral molecules need electrons to become negatively charged ions.
Explain how does the melting point change going down the group 7 of the periodic table.
Going down the group 7 of the periodic table elements form larger molecules that can have stronger van der Waals interactions, increasing the energy needed to separate them from other molecules. Therefore, the melting point of halogens increases with an increasing atomic radius and the size of molecules as you travel down the group.
larger molecules → stronger van der Waals interactions → higher melting point
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