AQA AS Physical Chemistry Bonding PDF
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2024
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This document provides an overview of bonding types in chemistry (Physical Chemistry). Includes details on covalent, ionic, metallic bonding, and intermolecular forces. It also discusses the properties of these types of bonding.
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AS CHEMISTRY 3.1.3 BONDING AN OVERVIEW OF STRUCTURE & BONDING e.g. Diamond, Graphite. SiO2 Giant Covalent bonds must be broken in order to melt / boil. Covalent This requ...
AS CHEMISTRY 3.1.3 BONDING AN OVERVIEW OF STRUCTURE & BONDING e.g. Diamond, Graphite. SiO2 Giant Covalent bonds must be broken in order to melt / boil. Covalent This requires huge amounts of energy. e.g. NaCl, MgBr2 etc Range depends on: Ionic - charge on the ions. The greater the charge, the stronger the electrostatic attractions. - ionic radius. The smaller the ions, the stronger the electrostatic e.g. sodium, magnesium, copper etc Range depends on: - charge on the ions. The greater the charge, the stronger the Metallic electrostatic attractions. - ionic radius. The smaller the ions, the stronger the electrostatic attractions. e.g. NH3, H2O, HF, Alcohols, Carboxylic Acids, Amines, Amides. Hydrogen Compounds must have a hydrogen atom DIRECTLY Bonding bonded to a highly electronegative element: Nitrogen, Oxygen or Fluorine Molecules must be POLAR OVERALL. Simple Permanent - Anything with a lone pair Covalent Dipole - Anything that is asymmetrical in terms of polar Molecules bonds Intermolecular ALL molecules have these. Forces However, these are the only IMFs between NON- POLAR molecules. Induced The strength of these forces relates DIRECTLY to the Dipole number of electrons within a molecule. The greater the number of electrons in the molecule, the greater the strength of the IMF. e.g. Group 8. Molecular elements, Alkanes, Alkenes AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING HINTS | TIPS | HACKS All bonding types are based on ELECTROSTATIC ATTRACTIONS. These are the attractive forces between something positive and something negative. When any new bond or intermolecular force is formed, energy is released (exothermic). When any bond or intermolecular force is broken, energy is absorbed (endothermic) Learn to describe the bonding in each of the substances. You can then use this to explain why they have the chemical properties that they do (electrical conductivity, melting & boiling point etc) How To Answer Questions About Physical Properties AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING IONIC BONDING An ionic substance is formed when a transfer of one or more electrons occurs between atoms. This occurs when there is a large difference in electronegativity* between the elements. You may be asked to show the formation of the ions using a dot and cross diagram. e.g. calcium chloride, CaCl2 X Cl 2+ Cl XX Ca Ca X Cl Cl CATION ANIONS An ionic substance is defined as: “A giant lattice of alternating cations and anions, held together by electrostatic attractions” + - e.g. NaCl - + - + + - HINTS | TIPS | HACKS Cations tend to be smaller than anions as they have lost the electrons from their outer energy level. The formula of ionic substances reflects the ratio between the cations and anions. e.g. MgBr2 = ONE Mg2+ ion for every TWO Br- ions. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING COVALENT BONDING Covalent bonds form between non-metal atoms. As the atoms are more stable with a full outer energy level, electrons are shared to achieve this. An covalent bond is defined as: “A shared pair of electrons between two atoms, one donated by each atom” The electrons in a covalent bond have opposite spins ↼ ↼ ORBITAL OVERLAPS XX It is possible for atoms to share electrons as the orbitals in their outer energy levels overlap. The electrons in X X X + )))))))))) (((((((((( + these orbitals are now free to be shared between both atoms. e.g. Two ’s’ orbitals overlapping in Cl2. XX Where the overlap occurs is an area of high electron density. This means that thIs area is negative. The )))))))))) represent the electrostatic attractions between the positive nuclei and the negative overlap. This is what holds the bond together. Of course, there will be repulsion between the shared pair of electrons as they are both negative. Likewise, the two nuclei repel each other. However, the forces of attraction between the nuclei and the shared pair of electrons overcome these. It is possible for DOUBLE or even TRIPLE bonds to form, which involve 2 PAIRS or 3 PAIRS of electrons respectively. BOND LENGTH Vs BOND STRENGTH Covalent bonds are the strongest form of bonds possible between two atoms. However, their strengths very depending on the SIZE of the atoms involved. The larger the atomic radius of the atoms, the greater the distance between the nuclei and the overlap. This means the electrostatic forces of attraction are weaker and the bond is easier to break. So, the LONGER a covalent bond is, the weaker it is, and vice versa. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING COVALENT SUBSTANCES There are two types of covalently bonded substance. Simple Molecular / Simple Covalent These are covalently bonded discrete (individual) molecules. The INTRAmolecular bonds (the covalent bonds inside the molecule) are very strong, but the INTERmolecular forces between the molecules are very weak. e.g. Cl2, CO2, H2O, CH4 Giant Covalent These are giant solid structures, entirely made up of large numbers of atoms that are covalently bonded together. They do not exist as individual molecules. e.g. diamond, graphite, silicone dioxide DATIVE COVALENT BONDS Dative covalent bonds (AKA coordinate bonds) are a sub-type of covalent bonds that are formed when the BOTH of the electrons in the shared pair are donated by the same atom. e.g. Hydrogen ion H+ + H XX XX X X X X H N H H N H X X H H Ammonia Ammonium ion Ammonia, NH3, has 2 electrons in its outer energy level that aren’t being used for bonding. This is known as a lone pair. The H+ ion has space for 2 electrons in its outer energy level. The H+ ion bonds to the N atom using this lone pair. This is the coordinate bond. Once a coordinate bond is formed, it has the exact same properties as a regular covalent bond. The only difference is where the electrons came from to form the bond. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING METALLIC BONDING Metallic Crystal Structure / Bonding: A regular lattice of cations (positive ions) surrounded by a sea of delocalised electrons. This is an example of Group 1 metal structure as the cations have a 1+ charge. If you were asked to draw a Group 2 metal, they would be 2+, and Group 3 would be 3+. )))) )))) )))) In their elemental state, metals lose the electrons from their outer energy level and become positively charged. These electrons become delocalised, and form non-directional electrostatic attractions with the surrounding cations, holding the structure together. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING PHYSICAL PROPERTIES OF METALS Electrical Conductors All metals conduct electricity as the delocalised electrons are free to move and carry a charge through the structure. Malleable All metals are malleable (can be bent into shape). This is the opposite of brittle. This is because the delocalised electrons move with the cations when they move, maintaining the electrostatic attractions and, therefore, its shape. Melting point Melting point is dependant on the strength of electrostatic attractions between the cations and the delocalised electrons. Li+ Down a Group: Melting point decreases. This is because shielding between the nuclei and the delocalised electrons increases, weakening the Na+ attraction, so less energy is needed to break them. Across a Period: Melting point increases. This is because the charge on the ion increases, but shielding remains constant. This means a stronger attraction K+ between the nuclei and the delocalised electrons. Na+ Mg2+ Al3+ AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING GIANT COVALENT STRUCTURES | CARBON Carbon exists as two different forms of giant covalent structure. These are known as allotropes. Diamond Each carbon atom is covalently 109.5o bonded to 4 others. 109.5o bond angle (Tetrahedral shape). Extremely hard / strong due to strong covalent bonds. Very high melting point due to strong covalent bonds that require a lot of energy to break. Does not conduct electricity as there are no delocalised electrons. All outer electrons are used in covalent bonds. Graphite Each carbon atom uses 3 electrons to covalently bond to 3 others creating a layered, hexagonal structure. 120o 120o bond angle (trigonal Planar Top View shape). The 4th electron form each atom is delocalised across the structure. This means that graphite conducts electricity. Soft due to weak induced dipole (VdW’s) forces between the layers so they are able to slide over one another. Very high melting point (like diamond) due to strong covalent bonds that Side View require a lot of energy to break. Weak Induced Dipole Forces between the layers. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING IODINE Iodine exists as a non-polar diatomic molecule (I2) with a single covalent bond between the atoms. Induced dipole (VdW’s) IMF’s exist between the molecules which create a crystal structure. However these are relatively strong as each iodine molecule contains a large number of electrons (106). This means that iodine is a solid a room temperature. When iodine melts (114oC), the induced dipole forces are broken, but the covalent bonds remain intact. It is brittle and does not conduct electricity as it has no delocalised electrons to carry a charge. I I Weak Induced Dipole Forces between molecules AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.3 BONDING ICE Like iodine, water exists as a covalent molecule (H2O) which is V-shaped (104o) However, unlike iodine, water is highly polar. The molecules have strong hydrogen bonding IMFs between them. When water freezes (0oC), the molecules arrange themselves into a 3D hexagonal crystal structure. This leaves empty space between the molecules. This is the reason why water expands when it freezes and why ice is less dense than liquid water. It is brittle and does not conduct electricity as it has no delocalised electrons to carry a charge. 𝜹+ H 𝜹- O H 𝜹+ 𝜹- 𝜹+ 𝜹+ 𝜹- 𝜹- 𝜹+ 𝜹+ 𝜹- 𝜹- 𝜹+ 𝜹+ 𝜹- Hydrogen bonding between the molecules creating a hexagonal crystal structure in ice. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY BONDING & PHYSICAL PROPERTIES Melting / Boiling Electrical Hardness Solubility in Water Points Conductance Very low N/A Low Insulator I.D. / VdW's (Weakest IMF) Non-polar Nothing to carry a charge Low N/A Low Insulator Permanent Dipole (Stronger IMF) Not polar enough Nothing to carry a charge High N/A High Insulator (Strongest IMF) Polar enough to Nothing to carry a Hydrogen Bonding interact with polar charge H2O molecules Low Hard Low Insulator Iodine Only weak I.D. forces Crystalline Structure Non-Polar Nothing to carry a between molecules charge Relatively high Water: Soft N/A Insulator Hydrogen Bonding Molecules free to Nothing to carry a Water / Ice move charge Ice: Hard Crystalline Structure Very High Hard Some are Soluble Insulator when solid Strong electrostatic Crystalline structure Depends on the Ions fixed in position attractions between and strong strength of the ions electrostatic electrostatic Conductor when Ionic attractions between attractions between molten or in solution ions. the ions (amongst as ions are free to other things) move and carry a charge High Hard but malleable Insoluble Conductor Strong electrostatic Strong electrostatic Water cannot disrupt Contains free- attractions between attractions between the strong moving delocalised Metallic the cations and the cations and electrostatic electrons throughout delocalised electrons delocalised electrons attractions between the structure. These cations and can carry a charge delocalised electrons Very High Hard Insoluble Insulator Diamond Strong covalent Very strong covalent Water cannot Nothing to carry a Giant Covalent bonds must be bonds between C interrupt the strong charge broken atoms covalent bonds Very High Soft Insoluble Conductor Strong covalent Weak I.D. forces Water cannot disrupt Contains free- Graphite bonds must be between layers of the strong covalent moving delocalised Giant Covalent broken carbon atoms. bonds electrons throughout the structure. These can carry a charge AQA www.chemistrycoach.co.uk © scidekick ltd 2024