5.2.1&2 Enthalpy & Entropy
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bond dissociation enthalpy
standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms Cl2—>2Cl
first electron affinity
the enthalpy change that occurs when 1 mole of gaseous atoms gain 1 mole of electrons to form 1 mole of gaseous -1 ion O + e- —-> O-
lattice enthalpy
the standard enthalpy change that occurs when 1 mole of an ionic crystal lattice is formed from its constituent ions in gaseous form Na+ + Cl- —-> NaCl
enthalpy of hydration
one mole of gaseous ions become aqueous ions
enthalpy of solution
standard enthalpy change when 1 mole of ionic solid dissolves in a large enough amount of water to ensure the dissolved ions are well separated and do not interact with one and other
enthalpy of atomisation
enthalpy change when 1 mole of gaseous atoms are formed from its elements in its standard states
enthalpy of neutralisation
standard enthalpy change when 1 mole of water is formed in a reaction between an acid and an alkali under standard conditions
enthalpy of formation
standard enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions
enthalpy of combustion
enthalpy when 1 mole of a substance is completely burned in oxygen with all reactants and products in standard states under standard conditions
in hess’ law when using formation data which way do the arrows point
upwards
in hess’ law when using combustion data which way do the arrows point
downwards
give the calculation for delta H
delta H = sum of products - sum of reactants
when calculating bond enthalpy what equation is used
enthalpy change = sum of reactants - sum of products
give the equation that calculates q
q=mcdeltaT /1000 m= mass of substance heated c= specific heat capacity (4.18)
give the equation to find enthalpy change per mole
enthalpy change = q/n
which two factors contribute to a greater lattice enthalpy
• sizes of ions - the larger the ion the less negative the lattice enthalpy, the charges become further apart so have a weaker attractive force between them • charges of the ions- the bigger the charge the greater the attraction between the ions so the stronger the lattice enthalpy
what is the perfect ionic model
where the ions are completely ionic and spherical and the attractions are purely electrostatic
when is there tendency towards covalent character in ionic substances
• the positive ion is small • the positive ion has multiple charges • the negative ions is large • the negative ions has multiple negative charges
what is polarisation
when the negative ion becomes distorted and more covalent the metal cation is more polarising if it polarises the negative ion
what is entropy
the number of ways atoms can share quanta of energy, high entropy means the system is disorders as there are more ways of arranging it
what is the substance with the lowest entropy and why
perfect crystal of diamond because there is only 1 type of atom 1 regular giant structure
give the order of entropy of solids, liquids and gases
highest gas liquid solid lowest
what is the effect of moles on entropy
if there is a significant increase in the number of moles from reactants to products then entropy increases
what is the unit of entropy
JK^-1 mol^-1