AC15: Lattice energy
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When heat is released, is the reaction endo or exo thermic
Exothermic
For an exothermic reaction, is the enthalpy change positive or negative
Negative
When heat is absorbed is the reaction endo or exo thermic
Endothermic
For an endothermic reaction, is the enthalpy change positive or negative
Positive
What are the standard conditions for enthalpy changes
• 1 atm • 298 K / 25 degrees
Define the standard enthalpy change of a reaction
The enthalpy change when the number of moles of reactants shown in an equation react together under standard conditions, all reactants being in their standard states
Define the standard enthalpy of formation of a compound
The enthalpy change when 1 mole of a compound is formed form its ELEMENTS in their standard states, under standard conditions
Define bond enthalpy
The enthalpy change required to break 1 mole of bonds between 2 atoms in the gaseous state
Define first ionisation energy of an element
The energy needed to remove 1 electron from every atom in 1 mole of gaseous atoms of an element
Define second ionisation energy of an element
The energy needed to remove 1 electron from every ion in 1 mole of gaseous +1 ions of an element
Are ionisations endo or exo thermic
Endothermic
Why are all ionisations endothermic
Because energy is needed to overcome the attraction between the outer electrons and the nucleus
Is the first or second ionisation more endothermic
Second ionisation
Why is the second ionisation more endothermic
Because the number of protons remain the same but 1 less electron, so there is less repulsion between the remaining electrons, allowing them to move in closer to the nucleus, where they are more strongly attracted
What does Hess’s law state about the enthalpy change in a chemical reaction
Independent of the route taken by the reaction, provided the initial and final conditions are the same
Define enthalpy change of atomisation of an element
The enthalpy change when 1 mole of gaseous atoms of an element are formed from the element in its standard state
Why does aluminium have a greater enthalpy change of atomisation
Since the number of out shell electrons increases, so there are more electrons contributing to the metallic bond which requires more energy
Define the first electron affinity of an element
The enthalpy change when 1 electron is added to each atom in a mole of gaseous atoms of an element
Is the first electron affinity of an element that commonly forms negative ions positive or negative
Negative
Why is the first electron affinity of an element that commonly forms negative ions negative
Because the electron that is added is attracted to the nucleus more strongly than it is repelled by the other electrons
Are second electron affinities positive or negative
Positive
Why are second electron affinities always positive
Energy must be put in to overcome the repulsion between the negatively charged X- ion and the e- that is added
Define lattice enthalpy
The enthalpy change when 1 mole of a sold ionic compound is formed from its gaseous ions
What is lattice enthalpy a measure of
The strength of ionic bonding in a compound