Year 13 Energy Key Definitions OCR Chemistry A

Lattice Enthalpy

The energy change associated with the formation of one mole of a solid ionic lattice from its gaseous ions under standard conditions

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Standard enthalpy change of formation

The enthalpy change that accompanies the formation of one mole of a compound from its constituent elements in their standard states. Exothermic as bonds are formed

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Standard enthalpy change of atomisation

One mole of gaseous atoms is formed from its element in its standard state. Endothermic as bonds have to be broken

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First Ionisation Energy

One mole of gaseous 1+ ions is formed from gaseous atoms. Endothermic because the electron being lost has to overcomeattraction from the nucleus in order to leave the atom

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Second Ionisation Energy

One mole of gaseous 2+ ions are formed from one mole of gaseous 1+ ions. Endothermic because the electron being lost has to overcome attraction from the nucleus

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First Electron Affinity

One mole of gaseous 1- ions is formed from gaseous atoms. Exothermic as the electron is attracted into the outer shell of an atom by the nucleus

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Second Electron Affinity

One mole of gaseous 2- ions is formed from gaseous 1- ions. Endothermic because the electron is repelled by the 1- ion and this repulsion has to be overcome

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Standard Enthalpy Change of Solution

The enthalpy change that takes place when one mole of a solute is completely dissolved in water under standard conditions. Can be endothermic or exothermic. KCl(s)->K+(g) + Cl-(g)

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Standard enthalpy change of hydration

The enthalpy change that takes place when dissolving one mole of gaseous ions in water

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Entropy,S

The quantitative measure of the degree of disorder in a system

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Standard Entropy

The entropy content of one mole of the substance under standard conditions (Units:JK^-1mol^-1)

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Standard entropy change of reaction

The entropy change that accompanies a reaction in molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states. sum of standard entropy of products-sum of standard entropy of

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Standard entropy change or reaction equation

Sum of standard entropy of products-sum of standard entropy of reactants

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Free energy change (delta G)

The balance between enthalpy, entropy and temperature for a process. Gibbs equation: deltaG=deltaH-TdeltaS. A process can take place spontaneously when deltaG<0

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Oxidation

Loss of electrons or an increase in oxidation number

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Reduction

Gain of electrons, or a decrease in oxidation number

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Oxidising Agent

The species that is reduced in a reaction and cause another species to be oxidised

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Reducing Agent

The species that is oxidised in a reaction and causes another species to be reduced

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Standard Electrode potential of a half cell

The e.m.f of a half cell compared with a standard hydrogen half cell, measured at 298K with solution concentrations of 1moldm^-3 and a gas pressure of 100kPa

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Half Cell

Comprises an element in two oxidation states. The simplest half cell has a metal placed in an aqueous solution of its ions

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Non-rechargeable storage cells

Provide electrical energy until the chemicals have reacted to such an extent that the voltage falls. The cell is then 'flat' and is discarded

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Rechargeable storage cells

The chemicals in the cell react, providing electrical energy. The cell reaction can be reversed during recharging. The chemicals in the cell are regenerated and the cell can be used again eg. Ni-Cad batteries and lithium-ion and lithium-polymer

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Fuel Cells

The cell reaction uses external supplies of a fuel and an oxidant, which are consumed and need to be continuously supplied. The cell will continue to provide electrical energy so long as there is a supply of fuel and oxidant

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How a fuel cell works

Based on H or H rich fuels eg.Methanol. Uses energy from the reaction of a fuel with oxygen to create a voltage. Reactants flow in and products out while the electrolyte remains in the cell.

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Year 13 Energy Key Definitions OCR Chemistry A

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