Thermochemistry Summary PDF

Thermochemistry Exothermic - A reaction in which heat is Car Engine given out is exothermic. Induction, Compression, Ignition, Exhaust: ∴ ∆H will be negative Auto-ignition is ignition before the spark is Endothermic - A reaction in which heat is produced. taken in is endothermic. The octane number i...

Thermochemistry Exothermic - A reaction in which heat is Car Engine given out is exothermic. Induction, Compression, Ignition, Exhaust: ∴ ∆H will be negative Auto-ignition is ignition before the spark is Endothermic - A reaction in which heat is produced. taken in is endothermic. The octane number is a measure of the ∴ ∆H will be positive tendency of a fuel to resist auto-ignition. ∆ The heat of reaction ( H) of a chemical reaction is the heat in kilojoules released or absorbed when the number of moles of reactants indicated in the balanced equation describing the reaction, react The octane scale is based on 2 compounds: 2,2,4-trimethylpentane octane no. = 100 Factors affecting the octane number of a molecule include: completely. 1. Chain length: Bond energy is the amount of energy in kJ 2. Degree of Branching: needed to break one mole of bonds of the 3. Cyclic Structure: same type, all species being in the gaseous state. The heat of combustion of a substance is the heat change in kilojoules when one mole of the substance is completely burned in excess oxygen. The kilogram calorific value is the amount of heat energy produced per kg of fuel. The heat of formation is the heat change in kilojoules when one mole of a substance is formed from its elements in their standard states. The Law of Conservation of Energy states that energy cannot be created or destroyed, but can change from one form to another. Hess’s Law states that the heat change of a reaction depends only on the initial & final states of a reaction and is independent of the route by which the reaction may occur. Heptane octane no. = 0 Improving Octane Number 1. ISOMERISATION: When certain compounds are heated in the presence of a suitable catalyst, a more branched isomer is formed 2. CATALYTIC CRACKING: The large molecules are broken down into smaller ones, with higher octane numbers. 3. DEHYDROCYCLISATION: When heated in the presence of a suitable catalyst, straight compounds can be converted to ring compounds. Thermochemistry Hydrogen can be produced by 2 main processes; 1. Steam reforming : →CO + 3H₂ CO +H₂O→ CO₂ + H₂ Fractions Produced Number of Carbons CH₄ + H₂O Domestic gas 2. Electrolysis of water: This involves passing electricity through acidified water. Hydrogen is produced at the cathode Refinery Gas C1-4 Light Gasoline/ Petrol C5-10 & oxygen at the anode. LPG is a mixture of propane and butane (liquified petroleum gas) Used to make petrol For petrochemicals + to make petrol Naphtha Kerosene C7-10 C10-14 Petrochemicals = chemicals made from crude oil (medicines, plastics, detergents) For central heating oil For aircraft fuel Thermochemistry Fractions Produced Gas oil/ diesel Lubricating oil Fuel oil Bitumen Number of Carbons C14-19 To make diesel for trucks, buses, trains Used as a lubricant to prevent wear + tear in engines C19-35 To make diesel for trucks, buses + trains C30-40 > C35 Fuel for ships + power stations Used as road resurfacing material Material for waterproof ing roofs