# Chemistry- Energetics

Energetics

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## Standard Enthalpy of Formation

This is the enthalpy change when one mole of a compound is formed from it's elements in their standard states and under standard conditions.

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## Standard Enthalpy of Combustion

This is the enthalpy change of one mole of a substance burned completely in oxygen under standard conditions.

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## Standard Enthalpy Change of Reaction

This is the enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions and states.

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## Enthalpy Change

This is the heat transferred in a reaction at constant pressure. The units are kJmol-1.

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## Calculating Enthalpy Change

q=mcΔT       q= Heat lost or gained, J.         m= Mass of solution, g.

c= Specific heat capacity, 4.18 (Jg-1k-1).       ΔT= Change in temperature.

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## Experiments

In experiments mearsuring enthalpy change, use a polysyrene beaker as a container so that not much heat is gained or lost through the sides.

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## Hess's Law

The total enthalpy change of a reaction is always the same, no matter which route is taken.

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## Average Bond Enthalpy

Enthalpy needed to break one mole of the bond, averaged over different molecules to produce gaseous atoms.

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## Enthalpy Formation of an Element

Enthalpy formation for an element is always zero.

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## Enthalpy changes of Formation

- You need to know the enthaply formation for all reactants and products that are compounds.

ΔHr + sum of ΔHf (reactants) = sum of ΔHf (products)
So, ΔHr = sum of ΔHf (products) - sum of ΔHf (reactants)

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