15: Enthalpy changes Flashcards
If heat energy (enthalpy) is evolved is the reaction exothermic or endothermic
Exothermic
If heat energy (enthalpy) is absorbed is the reaction exothermic or endothermic
Endothermic
What is enthalpy change of atomisation of an element
The enthalpy change when 1 mol of gaseous atoms of an element are formed from the element in its standard state
What is the enthalpy change of atomisation related to for a metal
The strength of metallic bonding in the solid element
What is enthalpy change of atomisation related to for atoms that already exist as gaseous molecules
The bond enthalpy
But for diatomic molecules like Cl₂, ΔHatomisation will be half the bond enthalpy as only 1 mol of Cl atoms need to be produced
What is the first electron affinity of an element
The enthalpy change when one electron is added to each atom in a mol of gaseous atoms of an element
X(g) + e¯ → X¯(g)
Why is the first electron affinity negative for an element that commonly forms negative ions
The electron that is added is attracted to the nucleus more strongly than it is repelled by the other electrons
Why are all second electron affinities positive
Energy must be put in to overcome the repulsion between the negatively charged X¯ ion and the e¯ that is added
What is the lattice enthalpy of an ionic compound
The enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions
What is lattice enthalpy a measure of (in terms of bonding)
The strength of ionic bonding in a compound
Why are all lattice enthalpies exothermic
When ions come together to ionic bonds, heat energy is always released
If a compound has a stronger ionic bonding is its lattice enthalpy more or less exothermic
More exothermic
What factors effect lattice enthalpy
The size of the ions
The charge of the ions
How does the size of the ions affect lattice enthalpy
The smaller the ionic radii of the ions, the greater their charge density, hence the stronger the attraction between them and the more exothermic the lattice enthalpy
How does the charge on the ions affect lattice enthalpy
The greater the charge on the ions, the greater their charge density, hence the stronger the attraction between them, and the more exothermic the lattice enthalpy
How does the strength of ionic attraction change going down a group
Strength decreases
The charge on the ion stays the same but its radius increases. This decreases the charge density, so attraction to oppositely charged ions becomes weaker and lattice enthalpies become less exothermic.
How does the strength of ionic attraction change going across a period
Strength increases
The charge on the ion increases and the radius of the ion decreases.
Greater attraction to other ions, so lattice enthalpies become more exothermic
How do melting points relate to lattice enthalpies
The more exothermic the lattice enthalpy, typically the higher the melting point.
This is because both depend on how much energy is needed to disrupt the ionic lattice
What would the steps be for the Born-Haber cycle for ΔfH of NaCl starting from Na(s) +½Cl₂(g)
1) ΔatomisationH of Na
2) 1st IE of Na
3) ΔatomisationH of Cl
4) 1st electron affinity of Cl
5) Lattice enthalpy of NaCl
What is enthalpy change of solution (ΔsolH)
The enthalpy change when 1 mol of a compound dissolves in water
What is enthalpy change of hydration (ΔhydH)
The enthalpy change when 1 mole of gaseous ions dissolve to form one mole of hydrated ions
What 2 steps can you separate the dissolving of an ionic solid into
1) The lattice breaks up, overcoming the attraction between the oppositely charged ions
2) The separate ions are hydrated, interacting with water molecules through ion-dipole forces
Factors that affect the enthalpy change of hydration of an ion
The greater the ionic charge, the stronger the attraction between the ion and the water molecules, so the more exothermic the enthalpy change of hydration
The greater the ionic radius, the weaker the attraction between the ion and the water molecules, so the less exothermic the enthalpy change of hydration
