A2 Aqa Chemistry Enthalpy Change Definitions

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Enthalpy Change

Back 1

Heat Energy Change under constant pressure

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Standard Conditions

Back 2

298K, 100kPa, 1moldm^-3

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Standard enthalpy of formation

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The enthalpy change when one mol of a compound is formed from it's elements in their standard states under standard conditions

H2(g) + 0.5O2 (g) ----> H2O(g)

Exothermic or Endothermic

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Standard Enthalpy of Combustion

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The enthalpy change when 1 mol of a compound is completely burnt in oxygen with all elements in their standard states under standard conditions

C4H8 (g) + 6O2 (g) ----> 4CO2 (g) + 4H20 (g)

Exothermic

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Hess' Law

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The enthalpy change for a reaction is the same is independent of the route taken

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Standard Bond Dissociation Enthalpy

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The enthalpy change when 1 mol of bonds of the same type in gaseous molecules is broken under standard conditions, producing gaseous fragments.

H-Cl (g) ----> H(g) + Cl(g)

Exothermic

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Lattice dissociation enthalpy

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The enthalpy change when 1 mol of an ionic solid is separated into 3 gaseous ions

NaCl(s) ----> Na+ (g) + Cl- (g)

Endothermic

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Lattice Formation Enthalpy

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The enthalpy change when 1 mol of an ionic solid is formed from its gaseous ions.

Na+ (g) + Cl- (g) ----> NaCl(g)

Endothermic

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Enthalpy of hydration

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The enthalpy change when 1 mol of separated gaseous ions are dissolved completely in water to form 1 mol of aqueous ions

Na+ (g) + (aq) ----> Na+ (aq)

Exothermic

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Enthalpy of solution

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The enthalpy change when 1 mol of an ionic substance is dissolved in a volume of water large enough to ensure ions are separated enough so that they do not interact with each other.

NaCl (s) + (aq) ----> Na+ (aq) + Cl-(aq)

Endothermic

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First ionization enthalpy

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The enthalpy change when the highest energy electrons are removed from a gaseous atom, or molecule to form a mol of gaseous ions, each with a single positive charge.

Na(g) ----> Na+ (g) + e-

Endothermic

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Second ionization enthalpy

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The enthalpy change when the highest energy electrons are removed from a gaseous ion with a single positive charge to form a mol of gaseous ions with two positive charges

Na+ (g) ----> Na2+ (g) + e-

Endothermic

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First Electron Affinity

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The enthalpy change when electrons are gained by a mol of gaseous atoms or molecules to form a mol of gaseous ions, each with a single negative charge.

Cl(g) + e- ----> Cl-(g)

Exothermic

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Second Electron Affinity

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The enthalpy change when electrons are gained from a mol of gaseous ions with a single negative charge to form a mol of gaseous ions, each with 2 negative charges.

O- (g) + e- ----> O2- (g)

Endothermic

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Enthalpy of Atomisation/Sublimination

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The enthalpy change when 1 mol of gaseous atoms are formed from an element or compound

0.5Cl2 (g) ---> Cl (g)

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Mean Bond Enthalpy

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The enthalpy change when 1 mol of a specified type of bond is broken, averaged over many different compounds.