Chemical Energetics Definitions

enthalpy defintions

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Enthalpy 1

Standard enthalpy of formation - enthalpy change when one mole of substance is formed from its constituent elements in their standard states under standard conditions of 25ºC and 1 atmosphere pressure.

Standard enthalpy of atomisation - enthalpy change when one mole of gaseous atoms are formed from the element in its standard state (EXO)

Standard enthalpy of reaction - enthalpy change when one mole of a specified reactant in its atandard state reacts to form products under standard conds.

Standard enthalpy of combustion - enthalpy change when one mole of a substance in its standard state is completely burnt in excess oxygen under standard conds. (EXO)

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Enthalpy 2

Standard enthalpy of neutralisation - enthalpy change when one mole of water is formed in the reaction between an acid and alkali ander standard conds. (EXO)

Bond dissociation energy -enthalpy change when one mole of (covalent) bonds are broken in the gaseous state to from the isolated gaseous atoms. (EXO = forming bonds, ENDO = breaking bonds)

Average bond enthalpy is the energy needed to break one mole of bonds in the gas phase, averaged out over many different compounds, therefore it provides a less accurate answer than the actual bond dissociation energies for each individual bond.

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IE

1st ionisation energy - enthalpy change when one mole of isolated gaseous atoms loses one mole of electrons to form one mole of singly positively charged gaseous ions. (ENDO)

2nd ionisation energy - enthalpy change when one mole of isolated singly positively charged gaseous ions loses one mole of electrons to form one mole of doubly positively charged isolated gaseous ions. (MORE ENDO than 1st IE because the proton:electron ratio has increased and the remaining electrons are held more strongly by the nucleus therefore more energy has to be put in to break the bond)

Factors affecting ionisation energy are nuclear charge, distance from nucleus (atomic radius) and shielding. Remember all of these factors when explaining why IE increases down a group and increases across a period

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EA and LE

1st electron affinity - enthalpy change when one mole of isolated gaseous atoms gains one mole of electrons to form one mole of singly negatively charged isolated gaseous ions. (EXO)

2nd electron affinity - enthalpy change when one mole of singly negatively charged isolated gaseous ions gains one mole of electrons to form one mole of doubly negatively charged isolated gaseous ions. (ENDO)

Lattice energy - the exothermic enthalpy change that occurs when one mole of crystalline solid in its standard state is formed from its constituent isolated gaseous ions. (EXO)

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Misc.

Hess' Law states that 'the enthalpy change during a chemical reaction depends only on the initial and final states of the chemical system and not on the path followed in getting from reactants to products.

Therefore the total enthalpy change in a system must be zero!

enthalpy change = energy absorbed to break bonds - energy released in making bonds

The stronger the atraction between the atoms, the higher the bond dissociation energy and the shorter the bond length. Therefore bonds that require a larger energy to break will create a slower reaction at room temperature because more energy has to be taken from the surroundings to break the reactant bonds.

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