• Metallic bonding is the strong electrostatic attraction between positive metal ions and negative delocalised electrons in a metal lattice • Metals have a fixed lattice structure of positive ions. The outer shell of electrons is delocalised. • Metals are good electrical conductors, due to the delocalised electrons free to move and carry the current. • Metals have high melting and boiling points due to the large amounts of energy needed to overcome the metallic bonds. Melting point is influenced by: ◦ Charge of metal ion - Metal ions of greater charge contribute more electrons to the delocalised sea, therefore there are more electrostatic forces of attraction between the ions and electrons, resulting in a higher melting point ◦ Size of metal ion - smaller cations are closer to the delocalised electrons, resulting in a stronger metallic bond, and a higher melting point • Metals are malleable and ductile. The ions can slide and move past each other as there are no bonds holding specific ions together Simple Molecules • The shape of a molecule or ion is dictated by the number of electron pairs around the central atom and whether they are bonding or lone pairs. • Bonding pairs and lone pairs of electrons are charge clouds that repel each other • Electron pairs in the outer shell arrange themselves as far apart as possible to minimise repulsion • Bonding pairs repel each other equally. While lone pairs repel other pairs more because they are more electron dense. • Each lone pair reduces the bond angle by about 2.5o. • A normal line represents the bond is in plane of the paper. A dotted wedge is a bond going into the paper and a bold wedge is a bond coming out of the paper. Electron Pairs Bonding Pairs Lone Pairs Bond Angles Example Linear 2 2 0 180° CO 2 Trigonal Planar 3 3 0 120° BF 3 Bent 3 2 1 <120° SO 2 Tetrahedral 4 4 0 109.5° CH 4 Trigonal Pyramidal 4 3 1 107° NH 3 V-Shaped 4 2 2 104.5° H 2 O Trigonal Bipyramidal 5 5 0 90°, 120° PCl 5 Seesaw 5 4 1 87°, 102° SF 4 T-Shaped 5 3 2 88° ClF 3 Linear 5 2 3 180° XeF 2 Octahedral 6 6 0 90° SF 6 Square Pyramidal 6 5 1 <90° BrF 5 Square Planar 6 4 2 90° XeF 4
snaprevise.co.uk ENTHALPIES, CALORIMETRY & HESS’S LAW CHEAT SHEET Enthalpy • Enthalpy, H , is the thermal energy that is stored in a system • Enthalpy change is the heat energy change measured under conditions of constant pressure. • Enthalpy change values are usually given under standard conditions (100 kPa, 298K & the standard state of the substance). Standard enthalpy changes are denoted by ΔH Ɵ . • total enthalpy of products ( kJ mol –1 ) total enthalpy of reactants ( kJ mol –1 ) • The standard enthalpy of formation ( Δ f H Ɵ ) is the enthalpy change when one mole of a substance is formed from its constituent elements, with all reactants and products being in their standard states and under standard conditions • The standard enthalpy of combustion ( Δ c H Ɵ ) is the enthalpy change when one mole of a substance is completely burnt in oxygen, with all reactants and products being in their standard states and under standard conditions Exothermic & Endothermic Reactions
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