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Summary Chemistry OCR A Chapter 9-10
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Summary of Chapter 9-10 in Module 3: Periodic table and Energy of Chemistry OCR A A level. Great for revision
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Chapter 9: Enthalpy9.1 Enthalpy changes
Enthalpy - measure of heat energy in a chemical system
System - the atoms, molecules, ions making up the chemical compound being studied
o Enthalpy - energy stored within bonds
o Cannot be measured, enthalpy changes can
In chemical reaction, reactant and products likely to have different enthalpies
o Difference in enthalpies is enthalpy change ΔH
o ΔH = H(products) - H(reactants)
Law of conservation of energy - energy cannot be created or destroyed
Chemical reaction involving enthalpy change
o Heat energy transferred between system and the surrounding
System - chemicals such as the reactants and products
Surrounding - apparatus, laboratory, things that are not the chemical
system
Enthalpy change can be determined experimentally
o Measure energy transfer between the system and surroundings
Exothermic
Energy transferred from system to surroundings
o Energy loss by system balanced by same energy gain by surroundings
o ΔH is negative
o Temp of surrounding increase as they gain energy
Endothermic
Chemical system takes in heat energy from the surroundings
, o Energy gain by system balanced by same energy loss by the surroundings
o ΔH is positive
o Temp of surroundings decreases - lose energy
Activation energy
Atoms ions held together by chemical bonds
Chemical reaction
o Bonds in reactants needs to be broken by an input of energy
o New bonds in products can form to complete reaction
o Ea - energy input required to break bonds
Act as energy barrier to reaction
Min energy required for reaction to take place
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