Chapter 4 Thermochemistry

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Information about Chapter 4 Thermochemistry

Published on July 10, 2014

Author: MadihahRamly


Chapter 4 Thermochemistry madihahramly_2013

A. ENERGY CHANGES IN CHEMICAL REACTIONS Thermochemistry: The study of changes in heat energy which take place during chemical reactions STEP TO CONSTRUCT ENERGY LEVEL DIAGRAMS Step 1 Identify whether the reaction is exothermic or endothermic Step 2 Draw and label the energy axis Step 3 Draw the energy level for reactants and products Step 4 Draw an arrow from reactants level to the products level Step 5 Write the reactants and products based on the balanced chemical equations Step 6 Label ∆H as positive or negative Exothermic reaction Endothermic reaction Definition A chemical reaction that gives out heat to the surroundings A chemical reaction that absorbs heat from the surroundings What happen?  During exothermic reaction, temperature of the surrounding increases.  This is because heat given out from the reaction is transferred to the surroundings.  During endothermic reaction, temperature of the surrounding decreases.  This is because the reactants absorb heat energy from the surroundings. Heat of reaction, ∆H The change in the amount of heat in a chemical reaction. ∆H negative: heat is given out ∆H positive: heat is absorbed Energy level diagram The energy of the products is lower than the total energy of the reactants The energy of the products is higher than the energy of the reactants Reactants Products ∆H = negative Energy Energy Reactants Products ∆H = positive

Energy change during formation and breaking of bonds  Usually a chemical reaction involves bond breaking and bond formation.  Bond breaking : always requires energy  Bond formation : always releases energy Type of Reaction Energy Change Sign of ∆H Exothermic Energy absorb for bond breaking is less than energy released from bond formation ∆H negative Endothermic Energy absorb for bond breaking is more than energy released from bond formation ∆H positive Application of exothermic and endothermic reaction Cold packs Contain chemicals that react to absorb heat from surroundings.  Help to reduce high temperature  Help to reduced swelling Hot packs Contain chemicals that react to release heat.  Help to warm up something  Help to lessen the pain of aching muscles Heat of Reaction  The change in the amount of heat in a chemical reaction.  Symbol: ∆H Different types of reactions Types of Reaction Heat of Reaction Precipitation Heat of Precipitation Displacement Heat of Displacement Neutralization Heat of Neutralization Combustion Heat of Combustion Heat of reaction Definition Heat of Precipitation The heat change when one mole of a precipitate is formed from their ions in aqueous solution Heat of Displacement The heat change when one mole of a metal is displaced from its salt solution by a more electropositive metal Heat of Neutralization The heat change when one mole of water is formed from reaction between an acid and an alkali Heat of Combustion The heat change when one mole of a substance is completely burnt in oxygen under standard conditions The Ways to Calculate Heat of Reaction Step 1: Calculate how many moles of reactant Number of moles =

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