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- Practice Exercises
anion - a negatively charged ion.
bond dissociation enthalpy β the energy required to break a particular bond. A cycle used to calculate the enthalpy of formation of an ionic compound. The formation is considered to be broken down into 5 steps: atomization of the metal, atomization of the non-metal, ionization of the metal, electron affinity of the non-metal, and lattice formation.
cation - a positively charged ion.
covalent character β covalent character occurs in ionic compounds where some of the electron density is shared between the two ions.
ΞG β βDelta Gβ, the symbol used to represent Gibbs free energy change.
ΞH β βDelta Hβ, the symbol used to represent enthalpy change.
ΞS β βDelta Sβ, the symbol used to represent entropy change.
ΞGsol β the Gibbs free energy change for dissolving.
endothermic reaction β a chemical reaction during which heat energy is taken in from the surroundings. ΞH for the reaction is positive.
enthalpy - a measure of the energy stored in (or heat content of) a system. Given the symbol H.
enthalpy change (for a reaction) β the heat energy exchange between the reacting system and its surroundings at constant pressure. Given the symbol ΞH.
entropy β a measure of disorder, given the symbol S. The units of S are: J Kβ1 molβ1
entropy change (for a reaction) β the total entropy of the products of a reaction minus the total entropy of the reactants.
exothermic reaction β a chemical reaction during which heat energy is given out to the surroundings. ΞH for the reaction is negative.
Gibbs free energy change β a quantity taking into account the thermodynamic factors affecting whether or not a reaction takes place spontaneously. ΞG = ΞH β TΞS and a reaction is spontaneous if ΞG < 0.
Hess cycle β a diagram showing a reaction taking place by two or more alternative routes, all of which involve the same enthalpy change, according to Hess's law. Also called an enthalpy cycle.
Hess's Law β a rule stating that the overall enthalpy change for a reaction is independent of the route the reaction takes.
lattice dissociation enthalpy β the enthalpy changes to break apart one mole of solid ionic compound into its constituent gaseous ions. This quantity has the same magnitude but opposite sign to the lattice formation enthalpy.
lattice formation enthalpy β the enthalpy changes for the formation of one mole of solid ionic compound from its constituent gaseous ions. This quantity has the same magnitude but opposite sign to the lattice dissociation enthalpy.
mean bond enthalpy β the average (mean) bond dissociation enthalpy for a particular bond in a range of different compounds. Precisely, it is the average enthalpy change for breaking one mole of a particular bond in a range of different compounds in the gas phase.
polarizable β the more easily polarizable an anion is, the more easily its electron density can be distorted by positive charge. Large ions are more polarizable than smaller ions.
polarizing β a cation is polarizing if it is able to draw electron density away from an anion, distorting the anion's electron density. Small and highly charged cations are the most polarizing.
spontaneous reaction β a reaction that tends to occur of its own accord.
standard conditions β the conditions under which standard enthalpy changes are measured. These are 298 K and 100 kPa.
standard enthalpy of atomization β the enthalpy changes when 1 mole of gaseous atoms of a substance are formed from the substance in its standard state under standard conditions.
standard enthalpy of combustion β the enthalpy changes when 1 mole of a substance undergoes complete combustion under standard conditions, with all substances being in their standard states.
standard enthalpy of electron affinity β the standard enthalpy of first electron affinity is the enthalpy change for the conversion of one mole of gaseous atoms into one mole of gaseous anions, each atom gaining one electron, under standard conditions, with all substances being in their standard states. Subsequent electron affinities represent the addition of further electrons to the gaseous anions.
standard enthalpy of formation β the enthalpy changes when 1 mole of a substance is formed from its elements under standard conditions, with all substances being in their standard states.
standard enthalpy of hydration β the enthalpy changes when one mole of gaseous ions is converted to one mole of aqueous ions under standard conditions, with all substances being in their standard states.
standard enthalpy of solution β the enthalpy changes when one mole of an ionic compound is dissolved in water to produce aqueous ions under standard conditions, with all substances being in their standard states.
standard ionization enthalpy β the first standard ionization enthalpy is the enthalpy change for the conversion of 1 mole of gaseous atoms to 1 mole of gaseous cations, each atom losing an electron, under standard conditions with all substances in their standard states. Subsequent ionization enthalpies represent removing further electrons from the gaseous cations.
standard enthalpy of electron affinity β the standard enthalpy of first electron affinity is the enthalpy change for the conversion of one mole of gaseous atoms into one mole of gaseous anions, each atom gaining one electron, under standard conditions, with all substances being in their standard states. Subsequent electron affinities represent the addition of further electrons to the gaseous anions.
thermodynamics β a branch of study concerned with the transfer of heat and work and the conversion between heat and other forms of energy. In its modern form it is based on four laws, the zeroth, first, second and third laws of thermodynamics.