Energetics and Enthalpy Change


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.

Delta symbol (http://www.bbc.co.uk/staticarchive/45d56e2ef237bb9cef77fb457f5def93ac7b2e00.gif)H = H(products) - H(reactants). 

For exothermic reactions, Hproducts < Hreactants 

For endothermic reactions, Hproducts > Hreactants

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Heat Capacities

Delta symbol (http://www.bbc.co.uk/staticarchive/45d56e2ef237bb9cef77fb457f5def93ac7b2e00.gif)H = mcDelta symbol (http://www.bbc.co.uk/staticarchive/45d56e2ef237bb9cef77fb457f5def93ac7b2e00.gif)

Energy Transferred (J) = Mass (kg) x Specific heat capacity (J kg-1 K-1) x Temperature change (K). 

Specific heat capacity, 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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