The study of energy transfers between reacting chemicals and their surroundings.
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Exothermic and Endothermic
Exothermic is the release of heat energy.
Enothermic is the taking of heat energy.
Bond breaking requires energy whilst bond making releases energy.
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Enthalpy Change
The energy released or taken from the surrounding is known as enthalpy change of reaction.
The reaction in which the changes are happening are known as the system. - Everything else is known as the surroundings.
Energy cannot be created or destroyed. The total energy of the universe is constant.
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Measuring energy changes
Enthalpy is represented by the letter H.
H = H(products) - H(reactants).
For exothermic reactions, Hproducts < Hreactants
For endothermic reactions, Hproducts > Hreactants
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Heat Capacities
H = mcT
Energy Transferred (J) = Mass (kg) x Specific heat capacity (J kg-1 K-1) x Temperature change (K).
Specific heat capacity, c is the amount of energy in joules (J) needed to raise the temperature of one kilogram (kg) of a particular substance by 1 kelvin (K).
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Bomb Calorimeter
Most accurate.
HOWEVER, some heat is still transferred to the surroundings. - Leads to inaccuracy and unreliability.
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Standard Enthalpy of Combustion
The enthalpy change when one mole of a substance is completley burnt in oxygen under standard coniditions.
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Standard Enthalpy of Formation
The enthalpy change when one mole of the compound is formed from its elements, under standard conditions.
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Standard Enthalpy of Atomisation
The enthalpy change when one mole of its atoms in the gaseous state is formed from the element under standard conditions.
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Standard Enthalpy of Neutralisation
The enthalpy change when an acid and a base react to form one mole of water under standard conditions.
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Hess Law
The total enthalpy change for a reaction is independant of the route taken.
A + B → C + D
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Bond Enthalpy
Energy change in a reaction involving covalent bonds.
The energy needed to break a particular covalent bond, or energy released when the bond is broken is bond dissociation energy.
Mean bond enthalpy - the average value of the bond dissociation enthalpy of a particular bond over a wide range of compounds.
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