Chemistry - Energetics

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  • Created by: umar
  • Created on: 26-05-13 15:35

Enthalpy Change

The Enthalpy, H, of a system is a measure of the stored energy (or heat content in) a system. It cannot be measured directly.

The energy either given out or taken in is known as the Enthaply change.

"The enthalpy change of a reaction is the heat energy exhange with its surroundings at constant pressure"

Standard Enthalpy changes are measured at a standar pressure of 100kPa and a temperature of 298K. Standard enthalpy changes are represented by ΔHø.

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ΔHfø and ΔHcø

ΔHfø = The Standard Enthalpy of Formation

This is the enthalpy change when one mole of substance is formed from its elements under standard conditions with all substances being in their standard states.

ΔHcø = The Standard Enthalpy of Combustion

This is the enthalpy change when one mole of substance undergoes complete combustion under standard conditions. ΔHcø is usually measured by using a technique known as calorimetry.

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How can Enthalpy Change be calculated?

Enthalpy change can be calculated using the following equation :

q = mcΔT


q = Enthalpy Change (J)

m = mass of substance heated (g)

c = Specific heat capacity in J per K per g (4.18)

ΔT = change in temperature (K)

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Hess's Law

Hess's Law States:

"The overall enthalpy change for a reaction is independant of the route the reaction takes"

This then went on to form the basis of the first law of thermodynamics which states:

"Energy cannot be created or destroyed, it can only change form"

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Bond Enthalpies

A chemical bond is a force of attraction between two atoms. Breaking the bond requires input of energy. it is therefore an endothermic process.

The energy change required to break a bond depends on the atoms that form the bond. The energy required to break a particular bond is the dissociation enthalpy.

Bond formation is a process that where energy is released. It is therefore an exothermic process.

The amount of energy released is when a particular bond is broken is the same magnitude as the energy taken in when the bond is broken, just the opposite sign.

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