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Enthalpy Changes:

An Enthalpy Change is the heat energy exchange with surroundings at constant pressure

Energy Pathway Diagrams.

Exothermic




= POSITIVE

This diagram represents an exothermic reaction.

During an exothermic reaction, heat energy is released to the surroundings. An energy loss from the chemicals is
balanced by the same energy…

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Any energy gain to the chemicals provided by the same energy loss from surroundings, which fall in temperature.

Standard Enthalpy Changes

Standard Conditions are:

A Pressure of 100 kPa/1 Atmosphere
A stated temperature: 298k(25 degrees c)
A concentration of 1 mol dm-3(for aqueous solutions)



Example : The state of water…

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e.g. H2(g) + ½ O2(g) => H20(l)

NOTE: for an element, the standard enthalpy change of formation is defined as Zero

Standard Enthalpy Change of Neutralisation

The standard enthalpy change of neutralisation is the energy change when an acid and base form 1 mole of water
under standard conditions of…

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